Chapter 1 Introduction to Workshop Safety Regulations Chapter Outline 1.1 Introduction 1.2 General workshop safety rules 1.3 Personal Protective Equipment (PPE) 1.4 Fire Safety Learning outcome When you complete this chapter you should be able to: 1. Acquire knowledge about safety precautions while working in workshop. 2. Use the protective wear. 3. Classify and be able to understand the safety equipment : e.g fire extinguisher.
1.1 : INTRODUCTION Before you can use equipment and machines or attempt practical work in a workshop you must understand basic safety rules. These rules will help keep you and others safe in the workshop. Free access to the workshop areas is restricted to authorised personnel only. No other person may enter the workshop without permission. Always listen carefully to the workshop supervisor and follow instructions. Safety in a machine shop may be divided into two broad categories: Those practices that will prevent injury to workers. Those practices that will prevent damage to machines and equipment. Too often damaged equipment results in personal injuries.
1.2 : GENERAL WORKSHOP SAFETY RULE Any person working in the mechanical workshop must familiarise themselves with equipments, tools, etc used. Personal Protective Equipment (PPE) is provided and must be used where necessary. Lab. coats, safety glasses/goggles and safety shoes are to be used as the work dictates. Report any defective equipment to the workshop supervisor. Do not run in the workshop, you could ‘bump’ into another pupil and cause an accident. Always be patient, never rush in the workshop. The gangway through the workshop must be kept clear. Any oil spillage, grease etc. must be cleaned up immediately. Tools/equipment must be cleaned after use. Any materials, tools or equipment used must be tidied away. Precision measuring equipment, drills, etc. must be replaced in their appropriate cabinets after each working day. When carrying tools, sharp edges should be pointed downwards. Tools and equipment must not be removed from the workshop without permission from the workshop supervisor. In the event of a fire, leave the building immediately and proceed to the assembly point. All unusual incidents (cuts. burns, direct chemical exposure, etc.) and emergencies whether personal injury resulted or not, must be reported to the workshop supervisor. The person witnessing the occurrence, the person directly involved and the supervisor workshop may fill out the report. The report must be signed by the person reporting the incident. Safety rules for MACHINE used in the workshop : When learning how to use a machine, listen very carefully to all the instructions given by the workshop supervisor. Ask questions, especially if you do not fully understand. Use the appropriate machine for the work to be done.
When working with machine tools or other equipment with rotating spindles, jewellery, loose clothes etc. are prohibited and long hair must be completely covered. No machine may be used or work undertaken unless the workshop supervisor is satisfied that the person is capable of doing so safely. If equipment is fitted with guards these must be used. Equipment should never be used if the safety guards have been removed.
1.3 : PERSONAL PROTECTIVE EQUIPMENT (PPE) For the operation of the work, PPE must be worn in the workshop. It is the individual’s responsibility to maintain PPE in good condition. PPE is used to increase individual safety while performing potentially hazardous tasks. Eye Protection Where a potential for flying particles or any damage to the eyes exists, eye protection must be worn by all persons who may be affected. Face shields or goggles must be worn when operating a grinding wheel. Examples of operation requiring the use of eye protection include chipping, impact drilling, welding or grinding.
Hearing protection Damage to hearing can occur from both impact noise and exposure to lower intensity steady state noise over long period of time. Both hand and power tools are capable of producing damage to hearing. It is recommended that hearing protection must be worn if average noise levels exceed 85BA (decibels) over an 8 hour period. Dust Masks Make sure that the appropriate masks are worn in dusty conditions. Foot wear Make sure that appropriate solid covered footwear is worn to protect feet from falling objects. NO slippers are allowed!!! Gloves Wear the appropriate work gloves as needed to protect your hands from injuries caused by handling sharp or jagged objects, wood, or similar hazard-producing materials.
Laboratory coats Lab coats provide additional protection and it is recommended that they must be worn at all times in a workshop. Body protection Loose fitting clothing, neckties, rings, bracelets, or other apparel that may become entangled in moving machinery will not be worn by machine operators.
1.4 : FIRE SAFETY Workshop must be equipped with a fire extinguisher. In selecting the appropriate extinguishers, the type of combustible material must be considered. Fire extinguishers Class A : Combustible materials such as wood, cloth, paper, rubber and many plastic (RED). Class B : Flammable liquid and gases, oils, greases, tars, oil-base paints, lacquers and some plastics (LIGHT YELLOW). Class C : Involve Class A and/or B materials in the presence of live electrical equipment, motors, switches and wires (BLACK/GREEN). Class D : Combustible materials such as Magnesium, Titanium, Sodium, Potassium, Zirconium, Lithium and any other finely-divided metals which are oxidizable (LIGHT BLUE). Fire prevention precautious : Know the location and the operation of every fire extinguisher in the workshop. Know the location of the nearest fire exit from the building. Know the location of the nearest fire-alarm box and its operating procedure. When using a welding or cutting torch, be sure to direct the sparks away from any combustible material. Always dispose of oily rags in proper metal containers. Be sure of the proper procedure before lighting a gas furnace. Storage of Flammables Flammable liquids shall be handled and stored away from sources of heat, spark,or open flame.
Chapter 2.0: Introduction to Bench Fitting Chapter Outline 2.1 Introduction to bench fitting 2.2 Overall safety in bench fitting 2.3 Application of hand tools a) File b) Hacksaw c) Fixing tools 2.4 Application of layout tools a) Steel rule b) Caliper c) Scriber d) Try square e) Surface gauge 2.5 Application of simple manual machines a) Drilling b) Tapping Learning outcome When you complete this chapter, you should be able to: 1. Understand common operations in bench fitting workshop 2. Describe various types of hand tools used. 3. Explain the application of various hand tools. 4. Understand the safety procedure/practice in bench fitting workshop
2.1 INTRODUCTION TO BENCH FITTING Bench Fitting is an operation where hand tool are use (which are held by hand to perform the operation and not required any machine). Hand tools are used to remove small amounts of material, usually from small areas of the workpiece. Bench fitting usually conducted due to : i.
No machine is available
ii. The workpiece is too large to go on a machine iii. The shape is too complicated or simply that it would be too expensive to set up a machine to do work Since the use of hand tools is physically tiring, it is important that the amount of material to be removed by hand is kept to an absolute minimum and thus that the correct tool is chosen for the task. Common bench fitting operation 1. Marking
6. Drilling
2. Sawing
7. Threading
3. Filing
8. etc
4. Scrapping 5. Chipping COMMON HAND TOOL 1. Hammer
10. Scrappers
2. Hacksaw
11. Chisels
3. Files
12. Trammel
4. Vices
13. Screw driver
5. V-block
14. Drills
6. C-Clamp
15. Spanner
7. Try Square
16. Pliers
8. Scribers
17. Tap
9. Punch
2.2
OVERALL SAFETY IN BENCH FITTING 1. Don’t used pliers for cutting hardened steel wire 2. Never used bent, dented, crack, chipped or worn-out chisel 3. Used right tool for job 4. Keep tool at their proper place 5. Never play with tools 6. Don’t used pipe or other improvised leverage extensions on handles 7. Plastic handles are only for comfort and don’t act as insulation 8. Always used eye protection equipment 9. Ensure that files have handles 10. Don’t put sharp or pointed tools in your pocket
2.3
APPLICATIONS OF HAND TOOLS
2.3a
File
A file is a metalworking and woodworking tool used to cut fine amounts of material from a workpiece. Prior to the industrialization of machining and the development of interchangeable parts, filing is much more important in the construction of mechanisms. component parts were roughly shaped by forging, casting or machining. These components were then individually hand-fit for assembly by careful and deliberate filing. Files come in a wide variety of sizes, shapes, cuts, and tooth configurations. The crosssection of a file can be flat, round, half-round, triangular, square, knife edge or of a more specialized shape. File features Most files are made from high carbon steel where the Length has been hardened and tempered, but the Tang has been left soft. Files are manufactured in a variety of shapes and sizes. They are known either by their cross-section, the general shape, or by their particular use. Which file you use is dependent on the type of work you are doing and the material you are using. Files are used to square ends, file rounded corners, remove burrs from metal, straighten uneven edges, file holes and slots, smooth rough edges, etc. Files have three distinguishing features, and are classified by these features: •
Length (measured without the Tang)
•
Cross-section or Shape
•
Grade of Cut
Figure 2.1 : File features •
The Point is sometimes called the Top.
•
The Shoulder can also be called the Heel.
•
Files have cutting teeth on both Faces. In the case of the Hand File, only one of the Edges has teeth on it and the other is smooth, and called the Safe Edge. The Safe Edge allows you to rub the File up against a surface without wearing any material away.
•
Always make sure that the Handle is securely attached to the Tang, otherwise you could give yourself a nasty injury.
•
The Length of the File, (measured in millimeters), is measured from the Shoulder to the Point.
Grade of Cut Files are usually made in two types of cuts : •
Single Cut; and
•
Double Cut
The Single Cut File has a single row of teeth extending across the face at an angle of 65° to 85° for the length of the file. It is often used with light pressure to produce a smooth surface or to put a keen edge on knives, shears and saws. The Double Cut File has two rows of teeth which cross each other. For general work, the angle of the first row is 40° to 45°, and the angle of the second row can be anywhere between 30° and 87°. First set of teeth is know as the overcut, second is known as upcut and upcut is finer then overcut. This type of file usually used with heavier pressure than the single cut and removes material faster from the workpiece.
Figure 2.2 : Single cut file and double cut file Files are also classified by the coarseness of the teeth. The bigger the teeth the rougher the feel of the File, and the quicker the File will remove material when you are using it. There are four main levels of coarseness that you may come across in the metalwork room and they are : 1. Rough file (8 teeth per cm) 2. Coarse middle file (10 teeth per cm) 3. Bastard file (12 teeth per cm) 4. Second cut file (16 teeth per cm) 5. Smooth file (20 - 24 teeth per cm) 6. Dead file (40 or more teeth per cm)
Type of files
Figure 2.3 : Types of file Care of file 1. Store the files in wooden rack and must not contact each other 2. If not using for long time – applied lubricant for preventing from rust. When it going to used, lubricant have to be wash out 3. Applied chalk to the file before use – prevent chip stick on file surface (holes between teeth) 4. Clean files before and after used -- file cleaner or file brush 5. If the file have been used for filling hard material never used it for filling soft material
2.2b
HACKSAW
•
Used for cutting metal rods, bars, pipes etc.
•
Job piece held in a vice and hacksaw move forward and backward
•
Cutting operation occur in forward stroke (required pressure) and in return stroke, no cutting action takes place
•
Hacksaw blade are made from high carbon steel and high speed steel
A hacksaw is a fine-tooth saw with a blade in a frame, used for cutting materials such as metal or bone.
Hacksaw features Hand-held hacksaws consist of a metal arch with a handle, usually a pistol grip, with pins for attaching a narrow disposable blade. A screw or other mechanism is used to put the thin blade under tension. The blade can be mounted with the teeth facing toward or away from the handle, resulting in cutting action on either the push or pull stroke. On the push stroke, the arch will flex slightly, decreasing the tension on the blade.
Figure 2.4 : Hacksaw features Frame : There are two types of Hacksaw Frame, a fixed and an adjustable. The fixed frame can only take one length of Blade, but is more rigid that the adjustable type, which can take Blades of different lengths ( typically handles 25 mm and 30 mm blades, some can accept blades ranging from 20 mm to 40 mm). Blade & teeth : Blades are available in standardized lengths, usually 25 mm or 30 mm for a standard hand hacksaw. The blade chosen is based on the thickness of the material being cut, with a minimum of three teeth in the material.
Hacksaw blade are made from Low Tungsten Steel or Carbon Steel, however the more expensive blades are made from High Speed Steel.
Different hacksaw blades have different number of teeth ranging from 14 to 32 teeth per 25 mm. Blades having lesser number of teeth per cm are used for cutting soft materials like aluminum, brass and bronze. Blades having larger number of teeth per cm are used for cutting hard materials like steel and cast iron. Table 2.1. Selection of hacksaw blade Teeth per 25mm
Use
14
Soft thick materials. Aluminium, Copper, Mild Steel
18
General use. Soft materials in thin sections. Hard materials in thick sections.
24
Thin section hard materials.
32
Very thin materials such as thin tubing and sheet metal.
No of teeth per 25 mm Material thickness (mm) Hard materials Soft materials Up to 3 32 32 3 to 6 24 24 6 to 13 24 18 13 to 25 18 14 Thin stock calls for finer teeth; thicker metal requires fewer teeth per 25 mm.
Handle :There are three types of Hacksaw Handle used. The most commonly used handles are the File Handle and the Pistol Grip Handle.
Figure 2.5 : Types of hacksaw handle Sawing Procedure i.
The hacksaw blade is fixed in the hacksaw frame through.
ii.
When fitting a new blade to a hacksaw, point the teeth forward, (away from the handle).
iii.
Tighten the wing nut and then as a general rule tighten three turns. It is important to have the correct tension on the blade, if it is too loose then the blade will buckle and not cut straight, and if it is too tight damage to the blade ends or the frame may result.
Figure 2.6 : Types of hacksaw a) Adjustable frame [pistol grip], b) Fixed frame, c) Tubular adjustable frame 1. Groove
2. Pistol
3. Pin 4. Pin 6. Lock screw
5. Thumb rest
Care of hacksaw 1.
Choose the correct blade for the material being cut.
2.
Secure the blade with the teeth pointing forward.
3.
Keep the blade rigid and the frame properly aligned.
4.
Cut using strong, steady strokes directed away from you.
5.
Use the entire length of the blade in each cutting stroke.
6.
Keep saw blades clean, and use light machine oil on the blade to keep it from overheating and breaking.
7.
Cut harder materials more slowly than soft materials.
8.
Clamp thin, flat pieces that require edge cutting.
2.2c
FIXING TOOLS
i] Vice A vice is a mechanical screw apparatus used for holding or clamping a work piece to allow work to be performed on it with tools such as saws, drills, screwdrivers, etc. Vises usually have one fixed jaw and another, parallel, jaw which is moved towards or away from the fixed jaw by the screw. There are three types of vice : •
Bench vice
•
Drilling vice
•
Hand vice
Bench vice
Moveable jaw
Fixed jaw
Spindle with buttress thread
Handle
Figure 2.7 : Bench vice Because the bench vise is fitted on the bench, generally it is not possible to change the height of the vice or the vice itself. Thus, care is needed when fitting a vice on the bench so that it is at a suitable and convenient height. The height of the top of the bench vice should be at level with the elbow of the worker. For this purpose, the height of an average man is selected as height of elbow of individual varies.
Figure 2.8 : A vice must be mounted so that when you are standing next to it, the top surface of its jaws is level with your bent elbow.
Figure 2.9 : If the bench top is too low, the vice can be raised with a hardwood block.
Drilling vice It is used to hold the job on the drilling machine for drilling holes in the workpiece. Drilling vice generally consists of different parts like handle, fixed jaw, screw, moveable jaw, nut, washer, etc but its base is provided with slots through which it is fitted on the drilling machine.
Figure 2.10 : a) Machine vice with swivel base 1. Moveable jaw
2. Fixed handle
b) Parallel jaw machine vice 3. Body fixed on base
4. Swivel base
Hand vice It is a very small machine which is used for gripping very small jobs.
Figure 2.11 : Hand vice
Figure 2.12 : Holding of flat, small workpieces in the hand vice Care of vice While using a vice, the following precautions should always be ensured : 1. Always set the workpiece in the centre of the jaw.
Figure 2.13 2. If it is necessary to hold the job on one side of the jaws, then use a piece of steel or wood on the other side to keep jaws parallel and free from excessive strain.
3. Length of the handle of the vice must be sufficient to tighten the job. Never use a hammer to tighten the workpiece. 4. Clean the vice and its moving parts regularly before use for smooth working. 5. Fixed the vice rigidly to the work bench with the help of nuts and bolts. 2.2c
FIXING TOOLS
ii] Clamp It is used to hold different jobs together to perform fitting operations like assembly and marking. A c clamp is a form of a clamp used to hold metal or wood items. As the name implies, they are shaped in the letter C which offers a good grip to the metal and wooden items. Many people also refer this clamp to be a G clamp as the screw part beneath the revolving rod gives it a shape of the letter G. These tools are considered as the most recognizable clamps and are available in sizes varying from 1 inch to 8 inches. Every size denotes the maximum opening width of the tool. Although, there are larger sizes available as well however, the smaller ones are most commonly used.
Figure 2.14 : Types of C-clamp
2.2c
FIXING TOOLS
iii] V block Basic function of V block is to mark and drill. The bar length is placed longitudinally in the V groove and the screw of U-clamp is tightened. It grips the rod firmly with its axis parallel to the axis of the V groove. It is used in conjuction with a U-clamp for holding round bars for marking, centre drilling, for holding in the centre of a lathe and for drilling holes.
Figure 2.15 : V block
Figure 2.16 : V blocks hold round stock securely
2.4 APPLICATION OF LAYOUT TOOLS
i. ii. iii. iv. v. vi.
Steel rule Caliper Scriber Try square Surface gauge Surface flat
i] Steel rule •
The most basic of the graduated measuring devices is the rule (made of steel, and often called a steel rule), used to measure linear dimensions.
•
Rules are available in various lengths. Metric rule lengths include 150, 300, 600, and 1000 mm, with graduations of 1 or 0.5 mm.
•
Steel rules may be flexible or non flexible, but the thinner the rule, the more accurately it measures, because the division marks are closer to the work.
Figure 2.17 : Steel rule •
Beside measuring the length, a steel rule can also be used to assess the flatness of a surface.
Procedure to measure the flatness of a surface :
1. Place the rule on edge across the surface then hold the work and the rule up to the light (Figure 2.18) 2. If the surface is flat, no light will be able to penetrate between the edge of the rule and the work piece.
Figure 2.18 : Measuring flatness of a material by using a steel rule
There are two important aspects to keep in mind when using a steel rule : 1) The first is that the rule should be placed so that the graduations are as close to the work as possible (Figure 2.19). This will eliminate parallax and other errors which might result because of the thickness of the rule. For example, where possible, the rule should be placed at 90° to the work so that the graduations actually touch the work surface.
Figure 2.19
2) Secondly, a habit to avoid is measuring a work piece from the beginning of the rule – the very end markings of a steel rule may have been damaged. Read the measurement at the graduation that coincides with the distance to be measured. However, when you use the rule in this way, remember to subtract the initial numbered amount from the total reading.
Figure 2.20 ii] Caliper •
The calipers are opened (either by being pulled open against a friction screw or having an adjustment knob unwound) until the jaws of the caliper are a gentle push-fit over the work.
•
This manual calipers do not have a scale to read the measurement. Therefore, the calipers are then transferred to steel rule and a reading made of the distance between the manual caliper’s jaws. With care, this process can be highly accurate.
•
Three types of spring calipers : •
Inside
•
Outside
•
Hermaphrodite
Figure 2.21 : Inside Caliper
Figure 2.22 : Outside Caliper
Figure 2.23 : Hermaphrodite Caliper iii] Scriber •
A scriber is a piece of hardened steel rod having a needle like a point on one or both sides.
•
It is used in metalworking to mark lines on workpiece, prior to machining.
•
The process of using a scriber is called scribing and is just part of the process of marking out.
•
It is used instead of pencil or pen, because the marks are hard to see, easily erased, and inaccurate due to their wide mark; scribe lines are thin and semipermanent.
•
For proper use, the end of scriber must be kept sharp. If the end of the scriber gets blunt, it can be sharpened with an oil stone.
Figure 2.24 : Types of scriber
Using the Scriber a.Sharpness. Make sure the point of the scriber is sharp. To sharpen, rotate the scriber between the thumb and forefinger while moving the point back-and forth on an oilstone. b.Work Surface. Clean work surfaces of all dirt and oil. c.Steel Rule. Place a steel rule or straight edge on the work beside the line to be scribed. d.Holding the Scriber. Use the fingertips of one hand to hold the rule in position and hold the scriber in the other hand as you would a pencil. e. Scribing the Line. Scribe the line by drawing the scriber along the edge of the rule, at a 45 degree angle and tipped outward slightly in the direction it is being moved.
Figure 2.25 : Scribing the line Care of scriber 1. Place a soft wood over the point of scriber when not in use. 2. Coat scriber with rust prevention compound before storage. 3. Do not store scribers in drawer with other tools. This practice can cause damage to scribers and injury to personnel. 4. Rack properly and store in a suitable box. Do not use scribers for purposes other than those intended.
iv] Try square It is used for scribing straight lines at right angles to each other, for testing trueness of a surface and for testing mutually perpendicular surface. Try squares are made from hardened alloy steel blades. More accurate try squares have properly ground and leveled edges with a fine finish. Both inner and outer surfaces of the blade are kept true at right angles to the corresponding surfaces of the stock.
Figure 2.26 : Try square v] Surface gauge •
It is also called scribing block. A surface gauge is used for layout work.
•
It consists of a heavy flat cast iron black fitted with a vertical steel rod.
•
Surface gauge is used for locating the centre of round bars held in V block by drawing straight lines and by tilting the job through different angles.
Figure 2.27 : Surface gauge is also used to the diameter of round bars
Figure 2.28 : Surface gauge is used to scribe layout lines •
The spindle may be adjusted to any position with respect to the base and tightened in place with the spindle nut.
•
The scriber can be positioned at any height and in any desired direction on the spindle by adjusting the scriber.
Figure 2.29: Surface gauge
•
Normally, surface gauge is used in conjunction with either a surface plate or a marking table which is needed to set the surface gauge to the correct dimension (Figure 2.30).
Figure 2.30
2.5 i. ii.
APPLICATION OF SIMPLE MANUAL MACHINES Drilling Tapping
i] Drilling •
Drilling is an operation of producing circular holes o different sizes with the help of drills.
•
It cuts by applying pressure and rotation to the workpiece, which forms chips at the cutting edge.
•
Twist drills are generally used drills for drilling holes in workpieces.
Figure 2.31 : Twist drill
Figure 2.32 : A drill bit enters the workpiece axially and cuts a hole with a diameter equal to that of the tool ii] Tapping •
This tool is used for cutting internal threads in cylindrical holes.
•
A tap cuts a thread on the inside surface of a hole, creating a female surface which functions like a nut.
Figure 2.33 : Tap
Figure 2.34 : Parts of a tap •
Two types of taps :
1) Hand operated taps 2) Machine operated taps
Hand operated taps
Figure 2.35 : Hand operated taps •
By using a set of taps : starting tap, intermediate tap and finishing tap.
•
Different tap set are required for cutting different threads of different diameter.
Figure 2.36 : Hand tap
Figure 2.37 : Set of taps Starting tap (a.k.a taper tap) : * The number of tapered threads typically ranges from 8 to 10. * A very gradual cutting action that is less aggressive than that of the plug tap. * Most often used when the material to be tapped is difficult to work (e.g., alloy steel) or the tap is of a very small diameter and thus prone to breakage. Intermediate tap (a.k.a plug tap or second tap) : * The number of tapered threads typically ranges from 3 to 5. * It has tapered cutting edges, which assist in aligning and starting the tap into an untapped hole. Finishing tap (a.k.a bottoming tap) : * It has a continuous cutting edge with almost no taper — between 1 and 1.5 threads. * This feature enables a bottoming tap to cut threads to the bottom of a blind hole. * A bottoming tap is usually used to cut threads in a hole that has already been partially threaded using one of the more tapered types of tap.
Wrench is a tool for holding the tap during the hand tapping process.
Figure 2.38 : A tap and T-wrench. Procedure (using wrench-hand operated taps): 1. A hole must be drilled to the tapping size for the thread. 2. After drilling a hole of the required size, the taper or fixed in the wrench and screwed into the hole. 3. When starting the cutting, the tap must be perpendicular in all planes to the work. 4. Excessive force must not be used, as this will result in breaking the tap. 5. Cutting fluid should be used to improve the surface finish of threads.. 6. Start the cutting action keeping in mind that the tap is turned continuously but after every half turn, it should be reversed slightly to clear the threads. 7. Therefore, threads must be cleared as often as is necessary to prevent the flutes from clogging. 8. Proceed until the taper tap is through the hole. 9. Repeat the above operations with the intermediate and finishing tap to finish the hole.
Figure 2.39 : Using Tap Wrench to create a thread for a screw
Machine operated taps * By using lathe, radial drilling machine, vertical milling machine, etc. * Machine tapping is faster and generally more accurate because human error is eliminated.
Figure 2.40 : Machine operated taps
Care of tap 1. Care must be taken not to damage the cutting edges. A chipped tap must never be used. 2. When not in use, taps should be kept clean and stored in a rack (Figure 2.34).
Figure 2.41
References Books : 1. B.J.Black, Workshop processes, practices and materials, 2nd Edition, Arnold, 1997. 2. F.W.Turner, O.E.Perrigo, H.P.Fairfield, Machine Shop Work 3. S.K. Yadav, Workshop Practice : Principles and Applications, IBS Buku Sdn Bhd. Website : 1. http://www.wikipedia.com 2. http://home.howstuffworks.com 3. http://homerepair,about.com 4. http://www.practicalstudent.com 5. http://autospeed.co.nz 6. http://www.technologystudent.com 7. http://www.tpub.com 8. http://books.google.com.my 9. http://chestofbooks.com 10. http://www.mechlook.com 11. http://www-mdp.eng.cam.ac.uk 12. http://www.cdxetextbook.com 13. http://www.custompartnet.com
Chapter 3 Measuring instrument, gauges and marking out tools Chapter Outline 3.1 Introduction 3.2 Errors In Measurement 3.3 Vernier Scale 3.3.1 Vernier Caliper 3.3.2 Vernier Height Gauge 3.3.3 Vernier Depth Gauge 3.4 Micrometers 3.4.1 External Micrometer 3.4.2 Internal Micrometer 3.4.3 Depth Micrometer 3.5 Gauges Learning outcome When you complete this chapter you should be able to: 1. Describe common measuring instruments, gauges and marking out tools in mechanical workshop 2. Select suitable measuring instruments, gauges and marking out tools in engineering application. 3. Use accurately common measuring instruments, gauges and marking out tools in engineering application.
3.1 INTRODUCTION There are three reasons why we need measurement. 1. To make things, whether the things we make are of our own designs or somebody else’s. 2. To control the way other people make things. This applies to ordering an engagement ring, fencing a yard or producing a million spark plugs. 3. For scientific description. It would be impossible to give definite information to someone about aircraft design, electron mobility, etc without measurements. In some application, not only measurements are required but precise measurement is necessary especially where parts are to be fit together. To achieve any degree of precision, the measuring equipment used must be precisely manufactured with reference to the same standard of length. Having produced the measuring equipment to a high degree of accuracy, it must be used correctly. 3.2
ERRORS IN MEASUREMENT
Any measurement made with a measuring device is approximate. If you measure the same object two different times, the two measurements may not be exactly the same. The difference between two measurements is called variation in the measurement. Another word for this variation - or uncertainty in measurement is "error". This "error" is not the same as a "mistake." It does not mean that you got the wrong answer. The error in measurement is a mathematical way to show the uncertainty in the measurement. It is the difference between the result of the measurement and the true value of what you were measuring Errors can be minimized, by choosing an appropriate method of measurement, but they cannot be eliminated. Causes of error : 1) Reading value Digital : Record all the digits shown.
Figure 3.1
Non digital : Record all the figures that are known for certain
Figure 3.2 2) Parallex Error The error that occurs when the pointer on a scale is not observed along a line normal to the scale
Figure 3.3 3) Rounding off If the last figure is between 5 and 9 inclusive round up If the last figure is between 0 and 4 inclusive round down
Should not be done after each step of a calculation (it causes rounding errors)
Should only be done at the end of a calculation
4) Errors in procedure The accuracy of a final result also depends on the procedure used. Ways to improve accuracy in measurement : 1. Make the measurement with an instrument that has the highest level of precision. The smaller the unit, or fraction of a unit, on the measuring device, the more precisely the device can measure. The precision of a measuring instrument is determined by the smallest unit to which it can measure. 2. Know your tools! Apply correct techniques when using the measuring instrument and reading the value measured. Avoid the error called "parallax" -- always take readings by looking straight down (or ahead) at the measuring device. Looking at the measuring device from a left or right angle will give an incorrect value. 3. Repeat the same measure several times to get a good average value. 4. Measure under controlled conditions. If the object you are measuring could change size depending upon climatic conditions (swell or shrink), be sure to measure it under the same conditions each time. This may apply to your measuring instruments as well. 3.3
VERNIER SCALE
3.3.1 VERNIER CALIPER The vernier caliper is a precision instrument that can be used to measure internal, external, step and depth measurement. It is used to take measurements that are accurate to within 0.001 of an inch or 0.02 of a millimeter, depending whether the vernier is metric or imperial. Vernier Caliper are important in tool room die making, model making and similar applications. They provide long measurement ranges (6 to 80 inch) and are economical. A vernier caliper has an L-shaped design with a movable arm. The movable arm can be slid out to allow an object to fit between the arms, and a measurement can be taken. Vernier calipers have two scales. The main scale is fixed, while the vernier, the secondary scale, slides along the main scale as the movable arm is shifted. Measurements are taken by looking for the mark on the main scale which is just to the left of the zero on the vernier caliper for the first
measurement, and then looking to see which mark on the vernier caliper comes most closely into alignment with a mark on the main scale. This yields a secondary measurement. Features Parts of a vernier caliper: 1. 2. 3. 4. 5. 6. 7.
External jaws: used to measure external diameter or width of an object Inside jaws: used to measure internal diameter of an object Depth probe: used to measure depths of an object or a hole Matric scale: gives measurements of up to one decimal place(in cm). Imperial scale: gives measurements in fraction(in inch) Vernier scale gives measurements up to two decimal places(in cm and inch) Locking screw(Retainer): used to block movable part to allow the easy transferring a measurement
Figure 3.4 : Vernier caliper Measuring with Vernier Caliper Example 1
Example 2
Exercise
DIAL CALIPER Accomplish with a dial for the least count readout. Normally dial caliper is accurate to 20 µm per 150 mm (0.001 in. per 6 in.) of travel. In this instrument, a small gear rack drives a pointer on a circular dial. Typically, the pointer rotates once every inch, tenth of an inch, or 1 millimeter, allowing for a direct reading without the need to read a vernier scale (although one still needs to add the basic inches or tens of millimeters value read from the slide of the caliper).
Figure 3.5 : Dial caliper
DIGITAL CALIPER Another extension of vernier caliper scale reading is used in the electronic digital caliper. These battery operated devices not only count the distance travelled by the moveable jaw and display the count on a digital readout, but they also provide parts for computer cables, so that the data can easily be used in statistical process control. Because the digital display makes the instrument even easier to read, electronic digital caliper is very useful for less experienced users. It also use a floating zero which allows users to make any point within the scale ranges the references, setting it to zero. Some digital calipers can be switched between metric and inch units.
Figure 3.6 : Digital caliper
3.3.2 VERNIER HEIGHT GAUGE This is also a sort of vernier caliper, equipped with a special base block and other attachments which make the instrument suitable for height measurements. It follows the principle of a vernier caliper and also follows the same procedure for linear measurement. The vernier height gauge is mainly used in the inspection of parts and layout work. With a scribing attachment in place of measuring jaw, this can be used to scribe lines at certain distance above surface.The vernier height gage may be used to measure or mark work off vertical distances to + or - .001 inch (.02 mm) accuracy.
Figure 3.7 : Vernier height gauge features
The vernier height gauge consists of a vertical graduated beam or columnon which the main scale is engraved. The vernier scale can move up and down over the beam. The bracket carries the vernier scale which slides vertically to match the main scale. The bracket also carries a rectangular clamp used for clamping a sciber blade. The whole arrangement is designed and assembled in such a way that when the tip of the sciber blade rests on the surface plate, the zero of the main scale and vernier scale coincides. The scriber tip is used to scribed horizontal lines for preset height dimensions.
The height gauges can also be provided with dial gauges or electronic readout instead of a vernier. The dial height gauge reads to 20 µm (0.001 in). Electronic height gauge is even more precise which reads to 10 µm (0.0001 in)
Figure 3.8 : Electronic and dial height gauge 3.3.3 VERNIER DEPTH GAUGE A depth gauge is a precision measuring instrument, designed specifically to measure the depth of holes, recesses, cavities and distances from a plane surface to a projection. In other word, a depth gauge is a variation of the ruler. The depth gauge consists of a ruler—usually a narrow one to measure the depth of holes, counter bores, etc. There is a base through which the ruler can slide up and down, and a locking screw on the cover to clamp the ruler in place.
base ruler
Locking screw
Figure 3.9 : Vernier depth gauge
Depth gauges comes in various configurations, depending on the specific application. Depth gauge types include digital tire thread depth gauges, digital depth gauges, single hook type digital depth gauges, needle digital depth gauges, double hooks digital depth gauges, digital depth gauges with adjustable base, vernier depth gauges, and dial depth gauges. The digital version is capable to output measurements to a wide variety of peripherals and data collection devices. These simple, inexpensive tools are typically about 150mm long, and they can even measure up to longer range, such as 1000mm.
Figure 3.10 : Digital depth gauge Caution 1) All depth gauges should be calibrated at least once a year, depending on how frequent it is used. If a gauge had an impact, it should be tested before use. If a gauge is not calibrated, it can be checked against an accurately marked shot line. 2) When the display keeps flashing or does not appear, take off the battery cover as the arrow shows and replace the battery with a new one (SR44, 1.55V). Note that the positive pole of the battery must be facing out. If the battery bought from the market does not work properly, it might be power lost due to long shelf life.
3.4
MICROMETER
A micrometer allows a measurement of the size of a body. It is one of the most accurate mechanical devices in common use. Three most common types of micrometer; the names are based on their application:
External micrometer, used to measure the diameter of holes. Internal micrometer (aka micrometer caliper), typically used to measure wires, spheres, shafts and blocks. Depth micrometer, measures depths of slots and steps.
Figure 3.11 : a) External micrometer b) Internal micrometer c) Depth micrometer The micrometer is a precision measuring instrument, used by engineers. Each revolution of the rachet moves the spindle face 0.5mm towards the anvil face. The object to be measured is placed between the anvil face and the spindle face. The rachet is turned clockwise until the object is ‘trapped’ between these two surfaces and the rachet makes a ‘clicking’ noise. This means that the rachet cannot be tightened anymore and the measurement can be read. Reading an inch-system micrometer The spindle of an inch-system micrometer has 40 threads per inch, so that one turn moves the spindle axially 0.025 inch (1 ÷ 40 = 0.025), equal to the distance between two graduations on the frame. The 25 graduations on the thimble allow the 0.025 inch to be further divided, so that turning the thimble through one division moves the spindle axially 0.001 inch (0.025 ÷ 25 = 0.001). To read a micrometer, count the number of whole divisions that are visible on the scale of the frame, multiply this number by 25 (the number of thousandths of an inch that each division represents) and add to the product the number of that division on the thimble which coincides with the axial zero line on the frame. The result will be the diameter expressed in thousandths of an inch.
Figure 3.12 : Micrometer thimble showing 0.276 inch where 0.2000 + 0.075 + 0.001 Example : -
The spindle has 40 threads per inch.
-
Distance between two graduations on the sleeve = 1/40 = 0.025
-
25 graduations on the thimble allow the 0.025 inch to be further divided (0.025 ÷ 25 = 0.001).
Sleeve = 0.025 inch Thimble = 0.001 inch Sleeve read (full)
= 0.3
Sleeve read (sub-division) = 1 * 0.025 = 0.025 Thimble read = 1* 0.001 Total Measurement
= 0.001 = 0.326 inch
Reading a metric micrometer The spindle of an ordinary metric micrometer has 2 threads per millimeter, and thus one complete revolution moves the spindle through a distance of 0.5 millimeter. The longitudinal line on the frame is graduated with 1 millimeter divisions and 0.5 millimeter subdivisions. The thimble has 50 graduations, each being 0.01 millimeter (one-hundredth of a millimeter). To read a metric micrometer, note the number of millimeter divisions visible on the scale of the sleeve, and add the total to the particular division on the thimble which coincides with the axial line on the sleeve.
Figure 3.13 : Micrometer thimble showing 5.78 mm where 5.00 + 0.5 + 0.28 = 5.78 mm
Example :
The accuracy of micrometers is checked by using them to measure gauge blocks, rods, or similar standards whose lengths are precisely and accurately known.
3.4.1 EXTERNAL MICROMETER
Figure 3.14 : External micrometer features
Frame The C-shaped body that holds the anvil and barrel in constant relation to each other. It is thick because it needs to minimize flexion, expansion, and contraction, which would distort the measurement.
Anvil The shiny part that the spindle moves toward, and that the sample rests against.
Sleeve / barrel / stock The stationary round part with the linear scale on it. Sometimes vernier markings.
Lock nut / lock-ring / thimble lock The knurled part (or lever) that one can tighten to hold the spindle stationary, such as when momentarily holding a measurement.
Screw (not seen) The heart of the micrometer, as explained under "Operating principles". It is inside the barrel.
Spindle The shiny cylindrical part that the thimble causes to move toward the anvil.
Thimble The part that one's thumb turns. Graduated markings.
Ratchet stop Device on end of handle that limits applied pressure by slipping at a calibrated torque.
3.4.2 INTERNAL MICROMETER The normal procedure for using inside micrometers is to set them across diameters or between inside surfaces, remove them, and then read the dimension. For this reason, the thimble on an inside micrometer is much stiffer than the one on a micrometer caliper. Thus, it holds the dimension better. It is good practice to verify the reading of an inside micrometer by measuring it with a micrometer caliper.
Figure 3.15 : Internal micrometer 3.4.3
DEPTH MICROMETER The depth micrometer is used to measure the precise depths of holes, grooves, and recesses by using interchangeable rods to accommodate different depth measurements. The ratchet is turned clockwise until the spindle face touches the bottom of the blind hole. The scales are read in exactly the same way as the scales of a normal micrometer.
Figure 3.16 : Depth micrometer
Care of micrometer 1. Coat metal parts of all micrometers with a light coat of oil to prevent rust. 2. Store micrometers in a separate container provided by manufacturer. 3. Keep graduations and markings on all micrometers clean and legible. 4. Do not drop any micrometer. Small nicks or scratcthes can cause inaccurate measurements. 3.5
GAUGES
In engineering, a gauge or gage, is used to make measurements. 1. Bore gauge 2. Center gauge 3. Dial indicator 4. Feeler gauge 5. Block gauge 6. Pressure gauge 7. Radius gauge 8. Ring gauge 9. Thread pitch gauge Types of gauge Bore gauge
Purpose A device used for measuring holes.
Picture
Center gauge
It is a tool used in machining to check the angle of tool bits used to cut screw threads
(a.k.a fishtail gauge) The center gauge helps ensure that the tool bit is the correct dimensions to cut these threads.
Dial indicator
To check the variation in tolerance during the inspection process of a machined parts.
(a.k.a dial Used in industrial. test indicator, dial gauge or probe indicator) Feeler gauge
Used to measure gap widths. Mostly used in engineering to measure the clearance between two parts. They consist of a number of small lengths of steel of different thicknesses with measurements marked on each piece. In a metric set of feeler gauges the thickness ranges from 0.05 mm to approximately 1 mm in varying steps. When using the thinner gauges care should be taken to pull the gauge through a gap rather than push, as by pushing, the gauge will tend to bend and wrinkle or possibly if a sideway movement is used the gauge will tear.
Block gauge
Used as a reference for the setting of measuring equipment used in machine shops, such as micrometers, calipers, and dial indicators
Pressure gauge
Used for pressure measurement of a gas or liquid.
(a.k.a vacuum gauge)
Radius gauge
E.g. : used to measure the pressure difference between a system and the surrounding atmosphere.
Used to measure the radius [internal and external] of an object.
The gauges are a set of thin blades with (a.k.a fillet a convex (external) and concave gauge) (internal) radius of the same size on each blade. The size of the radius is marked on each blade. It requires a bright light behind the object to be measured. The gauge is placed against the edge to be checked and any light leakage between the blade and edge indicates a mismatch that requires correction.
Inside radius
Outside radius
Ring gauge
Used for checking the external diameter of a cylindrical object.
Thread pitch Used to measure the screw pitch gauge threads. (treading gauge, pitch It is a series of thin marked blades gauge, screw which have different pitched teeth. Each set of screw pitch gauges has the thread gauge) form stamped on it. Thread pitch gauges also come in the standard thread forms of metric, Whitworth, BSF, UNF, and UNC which allows both the pitch of the thread to be gauged and the form or shape of the thread, to be checked.
References 1. http://www.knockhardy.org.uk 2. http://www.me.iitb.ac.in 3. http://www.regentsprep.org 4. http://www.splashmaritime.com.au 5. http://www.technologystudent.com 6. http://www.tresnainstrument.com 7. http://www.wikihow.com 8. http://www.wisegeek.com 9. Anand K Bewoor, Vinay A. Kulkarni. Metrology & measurement. McGraw Hill.
Chapter 4 Industrial materials used in workshop and identification Chapter Outline 4.1 Steel and its alloys 4.2 Cast Iron 4.3 Copper and its alloys 4.4 Aluminum and its alloys 4.5 Bearing Metals
Learning outcome When you complete this chapter you should be able to: 1) Acquire knowledge on basic engineering materials in real world industry. 2) Differentiate types of engineering materials, basic characteristics and its application. 3) Application of engineering materials especially in workshop practice.
4.0 : INTRODUCTION Generally, engineering materials can be divided into two categories; metal and nonmetal. The major characteristics of metallic materials are their crystallinity, conductivity to heat and electricity and relatively high strength and toughness. In workshop practices, all materials we use are metallic material. Metal mainly comprises as ferrous and nonferrous. It means classification of metal based on ferrous (iron) content or non ferrous content. There are five types of metal covered in this topic such as : 4.1 4.2 4.3 4.4 4.5
Steel and its alloys Cast Iron Copper and its alloys Aluminum and its alloys Bearing Metals
Table below show classification of engineering material based on manufacturing engineering reference’s book by Serope Kalpakjian.
Figure 4.1 : Enginering materials
4.1 : STEEL AND ITS ALLOYS Steel mainly consist of iron that contains carbon ranging by weight between 0.022% and 2.14% It often includes other alloying ingredients: Manganese Chromium Nickel Molybdenum Steel can be grouped into the following categories: Steel
Plain carbon steels Low carbon steels
Medium carbon steels
High carbon steels
Low alloy steels Stainless steels Tool steels
(1) PLAIN CARBON STEELS Contain carbon as the principal alloying element; with only small amounts of other elements (about 0.5% manganese is normal). Designation scheme : American Iron and Steel Institute (AISI) & the Society of Automotive Engineers (SAE) First digit indicates the family to which the steel belongs : 1- Carbon steels; 2- Nickel steels; 3- Nickel-chromium steels; 4- Molybdenum steels; 5- Chromium steels;
6- Chromium-vanadium steels; 7- Tungsten-chromium steels; 9- Silicon-manganese steels. Second digit indicate % of major alloying elements (1 means 1%). Last two digits (3rd and 4th number) indicate amount of carbon in steel (10 means 0.10% C). Example : Plain carbon steels are specified by a four-digit number system: 10XX where , 10 indicates that the steel is plain carbon, and XX indicates the percent of carbon in hundredths of percentage points. For example, 1020 steel contains 0.20% C. The plain carbon steels can be classified into three groups according to their carbon content: TYPES OF CARBON PLAIN PROPERTIES/ CONTENT APPLICATION CARBON CHARACTERISTICS STEEL Relatively soft and weak, but possess high ductility and toughness Automobile sheet metal Good formability, parts Good weldability Plate steel for Low carbon Contain less Low cost fabrication than 0.25% C steels Tin can Rated at 55-60% machinability Nails Bolt and nut Easy to form in which high strength is not required. Applications requiring higher strength than Machinery Range in low carbon steel components and carbon Medium engine parts Machinability is 60between carbon steel i.e : crankshafts, 70% 0.25% and gears and 0.60% Good toughness and connecting rods. ductility
Fair formability
Hardest, strongest Least ductile of the carbon steel High wear resistance
High carbon steel
Carbon in amounts greater than 0.60% but less than 1.4%
Springs, cutlery Cutting tools & blades
Increasing carbon content : Strengthens and hardens the steel Ductility is reduced. Can be heat treated - improved properties of steel (hard and strong) (2) LOW ALLOY STEELS Low alloy steels are iron-carbon alloys that contain additional alloying elements in amounts totaling less than about 5% by weight. Owing to these additions, low alloy steels have mechanical properties that are superior to those of the plain carbon steels for given applications. Superior properties usually mean; Higher strength Higher hardness High hot hardness High wear resistance Higher toughness Heat treatment is often required to achieve these improved properties. The effects of the principal alloying ingredients as follows: ALLOYING ELEMENTS Chromium (Cr)
Manganese (Mn) Molybdenum (Mo)
EFFECTS Improves : Strength, Hardness, Wear resistance Hot hardness. Improves Strength Hardness of steel Increases Toughness
OTHERS
Effective alloying ingredients for increasing hardenability Cr improves corrosion resistance.
when the steel is heat treated, hardenability is improved with increased manganese. improves hardenability and forms carbides for wear resistance.
Nickel (Ni)
Vanadium (V)
Hot hardness improves Strength Toughness Enhances strength toughness of steel
Increases hardenability but not as much as some of the other alloying elements It improves corrosion resistance Forms carbides that increase wear resistance.
(3) STAINLESS STEELS A group of highly alloyed steels. Differ from carbon steel by the amount of chromium present. Called stainless because in the presence of oxygen (air), they develop a thin, hard, adherent film of chromium oxide that protect the metal from corrosion. Also the protective film builds up when the surface is scratched. Designed to provide high corrosion resistance. Stainless steel does not stain, corrode, or rust as easily as ordinary steel, but it is not stain-proof. It is also called corrosion-resistant steel, particularly in the aviation industry. Stainless steel is used where both the properties of steel and resistance to corrosion are required. The principal alloying element in stainless steel are as follow: ALLOYING ELEMENTS
EFFECTS
Chromium, (usually above 15%)
Contain sufficient chromium to form a passive film of chromium oxide, which prevents further surface corrosion and blocks corrosion from spreading into the metal's internal structure.
Nickel
Increase corrosion protection.
Used to strengthen and harden the metal;
Carbon
However, increasing the carbon content has the effect of reducing corrosion protection Why? Because chromium carbide forms to reduce the amount of free Cr available in the alloy.
Common used of stainless steel is kitchen cutlery, watch strap, piping and fitting.
Figure 4.2 : Applications of stainless steel (4) TOOL STEELS Tool steel refers to a variety of carbon and alloy steels that are particularly well-suited to be made into tools. Their suitability comes from their distinctive hardness, resistance to abrasion, their ability to hold a cutting edge, and/or their resistance to deformation at elevated temperatures (red-hardness). A class of high-carbon alloy steels used to forming, machining or cutting other materials. It has an excess carbides (carbon alloys) which make them hard and wear resistant. Its characteristics: high strength impact toughness wear resistance Commonly has two types: High speed steel(HSS) - The most highly alloyed tool and die steels. - Two basic types of HSS, the molybdenum type (M-series) contain about 10% molybdenum with other materials or the tungsten type (T-series) contain 1218% tungsten with other alloying elements. It can be coated with titanium nitride and titanium carbide for improving wear resistance.
-
Die steel Are designed for use at elevated temperatures. It has high toughness and high resistance to wear and cracking. Alloying elements are tungsten, molybdenum, chromium and vanadium.
4.2 : CAST IRON Cast iron is a family of ferrous alloys composed of iron, carbon (ranging from 2% 4.5%) and silicon (about 3.5%) Mainly act as casting raw material in metal casting industry. There are several types of cast iron :
TYPES OF CAST IRON
DESCRIPTIONS * The structure causes the surface of the metal to have a grey color when fractured; hence the name gray cast iron.
Grey Cast Iron
* Grey cast iron accounts for the largest tonnage among the cast irons. Composition In the range 2.5% to 4% carbon and 1% to 3% silicon.
Ductile Iron
White Cast Iron
* Composition of grey iron in which the molten metal is chemically treated before pouring to cause the formation of graphite spheroids rather than flakes. * Has less carbon and silicon than gray cast
ATTRACTIVE PROPERTIES * Good vibration damping, which is desirable in engines and other machinery
TYPICAL PRODUCT
* Automotive engine blocks and heads
* Internal lubricating qualities, which makes the cast metal * Motor housings machinable. * Ductility of grey cast iron is very low; it is a relatively brittle material.
* Stronger and more ductile iron * Ductile and shock resistant
* Hard and brittle
* Machine tool bases
* Machinery components requiring high strength and good wear resistance
* Railway brake shoes
iron * Formed by more rapid cooling of the molten metal after pouring
* Excellent wear resistance * Good strength
* When fractured, the surface has a white crystalline appearance that gives the iron its name.
Malleable Iron
* When castings of white cast iron are heat treated to separate the carbon out of solution and form graphite
* Ductility (up to 20% elongation) * Strength * Shock resistant
* Pipe fittings & flanges * Certain machine components * Railroad equipment parts.
4.3 : COPPER AND ITS ALLOYS Copper first produced in about 4000 B.C. Properties of copper : Reddish-pink color, Low electrical resistivity - one of the lowest of all elements. An excellent thermal conductor. Noble metals (gold and silver are also noble metals), so it is corrosion resistant. Low strength and hardness of copper. Easily process by various forming, machining, casting and joining technique. Applications : Widely used as an electrical conductor. To improve strength, copper is frequently alloyed. Bronze is an alloy of copper and tin (typically, about 90% Cu and 10% Sn). Brass is another familiar copper alloy, composed of copper and zinc (typically around 65% Cu and 35% Zn). The highest strength alloy of copper is beryllium - copper (only about 2% Be). Be Cu alloys are used for springs.
4.4 : ALUMINUM AND ITS ALLOYS Among the commercial metals, aluminum is second only to iron in production and consumption. Aluminum and its alloy are the most important in nonferrous metallic materials following a few based characteristic. Attractive Properties High strength to weight ratio – allows the design and construction of strong lightweight structures. Advantages for portable equipment, vehicles and aircraft application (82% of a Boeing 747 and 70% of a Boeing 777) its structural made from aluminum. Resistance to corrosion in atmospheric environment, in fresh and salt waters and in many chemicals solution. Resistance to corrosion is excellent due to the formation of a hard thin oxide surface film. Aluminum has no toxic-suitable for processing, handling, storing and packaging of foods and beverage (aluminum can & foil). High electrical and heat conductivity, especially when light weight is important. Very ductile metal – easy to form Pure aluminum is relatively low in strength, but it can be alloyed and heat treated to compete with some of the steels, especially when weight is taken into consideration high strength but light weight
4.5 : BEARING METALS Also known as babbit metal, was invented in 1839 by Isaac Babbit (USA). Most commonly used in as a thin surface layer in a complex, multi-metal structure, but its original use was as a cast-in place bulk bearing material. Babbitt metal is soft and easily damaged, which suggests that it might be unsuitable for a bearing surface. However, its structure is made up of small hard crystals dispersed in a softer metal, which makes it a metal matrix composite. As the bearing wears, the softer metal erodes somewhat, which creates paths for lubricant between the hard high spots that provide the actual bearing surface. When tin is used as the softer metal, friction causes the tin to melt and function as a lubricant, which protects the bearing from wear when other lubricants are absent.
Figure 4.3 : Bearing metals
Chapter 5 Introduction to Welding Chapter Outline 5.1 Overall safety in welding 5.2 Filler, Flux, Electrode and shielding gases 5.3 Gas welding 5.3.1 Oxy-acetylene gas welding 5.4 Arc welding 5.4.1 SMAW 5.4.2 MIG 5.4.3 TIG 5.5 Spot welding Learning outcome When you complete this chapter you should be able to: 1. Acquire basic knowledge on arc welding and gas welding and its operating. 2. Differentiate between arc welding, gas welding and its application. 3. Justify which welding operation is relevant to certain task. 4. Acknowledge safety procedure when handling with this equipment
5.1 : OVERALL SAFETY IN WELDING
Figure 1 : Oxy-gas welding station (keep cylinders and hoses away from the flame) 1.
2.
3.
4.
5.
6.
7. 8. 9. 10.
Because of the heat sources, such as open flames, arcs, sparks, and hot metal used in welding and related operations, fire and explosion hazards are always present in the work area. Welding processes should be carried out away from all combustible materials, including flammable fluids, vapors, gases, fuel, wood and textiles. Protection of the operator’s eyes, face, and body against sparks, spatter, and infrared and ultraviolet radiation is essential. Several types of safety equipment and protective clothing are available and should be used. Welding and related methods and machinery that use electricity as a source of energy also present hazards. Proper installation and maintenance of equipment and training of personnel are essential. Proper ventilation systems must be installed and maintained. Never look at the arc with the naked eye. Stand on dry footing when welding. Keep area around welder clean.
INTRODUCTION This chapter covers permanent joining technique using welding process such as gas welding, arc welding and spot welding. Welding is a materials joining process in which two or more parts are coalescence at their contacting surfaces by a suitable application of heat and/or pressure. Oxyfuel-gas and arc welding are among the most commonly used joining operations. Gas welding uses chemical energy; to supply the necessary heat, arc welding use electrical energy instead. In all of these processes, heat is used to bring the joint being welded to a liquid state. Shielding gases are used to protect the molten-weld pool and the weld area against oxidation. Filler rods may or may not be used in oxyfuel-gas and arc welding to fill the weld area. The selection of a welding process for a particular operation depends on the workpiece material, on its thickness and size, on its shape complexity, on the type of joint, on the strength required, and on the change in product appearance caused by welding. A variety of welding equipment is available—much of which is now robotics and computer controlled with programmable features. Discontinuities can develop in the weld zone (such as porosity, inclusions, incomplete welds, tears, surface damage, and cracks). Residual stresses and relieving them also are important considerations in welding. The weldability of metals and alloys depends greatly on their composition, the type of welding operation and process parameters employed, and on the control of welding parameters. General guidelines are available to help in the initial selection of suitable and economical welding methods for a particular application. Applications of welding : Manufacture of automobile bodies, furniture, machine frames, general repair work and ship building. The weld joint is where two or more metal parts are joined by welding. The five basic types of weld joints are shown in figure below :
Figure 2 : Basic types of weld joints
Butt joint : The parts lie in the same plane and are joined at their edges. Corner joint : The parts in a corner joint form a right angle and are joined at the corner of the angle. Lap joint : Consist of two overlapping parts. Tee joint : One part if perpendicular to the other in the approximate shape of the letter T. Edge joint : A joint between the edges of two or more parallel or mainly parallel members.
5.2 : FILLER, FLUX, ELECTRODE AND SHIELDING GASES 5.2.1 Filler When welding two pieces of metal together, we often have to leave a space between the joint. The material that is added to fill this space during the welding process is known as the filler material (or filler metal). Filler metals are used to supply additional metal to the weld zone during welding. Filler metal that does not conduct an electric current during the welding process and often used for gas welding. They are available as filler rods or filler wire and may be bare or coated with flux. i) Coated Filler Metal Rods of this type of filler metal consists the coating of flux material. ii) Bare Filler Metal No coating of flux. Typically, filler metal is in the form of rod, 90 mm long and diameter ranging from 1.6 mm to 9.5 mm. Different types of welding rods are used for welding of different metals. Some examples are given below. Metal to be welded (a) Iron rich steels (b) Stainless steel (c) Copper (d) Aluminium and its alloy Composition of welding rod (a) More ‘C’ Si, Mn less ‘P’ and ‘S’ (b) Should have ‘Cr’ and ‘V’. (c) Copper rods with phorphorus. (d) Rods of same metal containing some silicon.
Filler metal composition should be same as that of the material to be welded. Sometimes additional alloying elements are added to them to improve mechanical properties of weldment. 5.2.2 Flux Flux is a substance which is nearly inert at room temperature, but which becomes strongly reducing at elevated temperatures, preventing the formation of metal oxides. The role of flux is typically dual: dissolving of the oxides on the metal surface, which facilitates wetting by molten metal, and acting as an oxygen barrier by coating the hot surface, preventing its oxidation. Before performing any welding process, the base metal must be cleaned form impurities such as oxide because this will weaken the weldment. Flux is used to convert the oxides and nitrides to slag that can be removed from welding zone easily. Fluxes come in the form of a paste, powder, or liquid. Typical flux : SiO2, TiO2, FeO, MgO, Al2O3 Flux can be supplied through coating of electrode or coating on filler metal or separately. Different fluxes are used for welding of different metals. For the welding of copper and its alloy sodium nitrate, sodium carbonates are used as flux. For welding of aluminum or its alloy chloride of sodium, potassium, lithium or barium are used.
Figure 3 : Flux used in shielded metal arc welding: (a) overall process; (b) welding area enlarged
5.2.3 Electrode Electrode is a metal rod which conducts a current from the electrode holder to the base metal. Used in electric arc welding. Two types of electrode :Consumable ; Non consumable A) Consumable electrode Consumable electrodes not only used as a conductor for the electrical current, but it provides the source of the filler metal in arc welding. These electrodes are available in two principal forms of wire and rods (also called sticks). These electrodes are available in two principal forms: rods (also called sticks) and wire. Welding rods are typically 225 to 450 mm (9—18 in.) long and 9.5 mm (3/8 in.) or less in diameter. The problem with consumable welding rods, at least in production welding operations, is that they must be changed periodically, reducing arc time of the welder Consumable weld wire has the advantage that it can be continuously fed into the weld pool from spools containing long lengths of wire, thus avoiding the frequent interruptions that occur when using welding sticks. In both rod and wire forms, the electrode is consumed by the arc during the welding process and added to the weld joint as metal.
Figure 4 : Shielded metal arc welding process (consumable electrode)
Figure 5 : Various welding electrodes and an electrode holder Consumable electrodes can further be classified into two categories : i) Coated electrode The most popular arc welding electrodes. No additional filler metal and flux are required with them. Purposes of electrode’s coating : It forms a gas shield to prevent impurities in the atmosphere from getting into the weld and form separable slag from metal impurities. ii) Bare electrode Rarely used (only in MIG) Simple rods made of filler metal with no coating over them. Flux is required additionally. B) Nonconsumable electrode Only used as a conductor for the electrical current. Electrode is made of tungsten which resist melting by the arc. A nonconsumable electrode is gradually depleted during the welding process. For arc welding processes that utilize nonconsumable electrodes, any filler metal used in the operation must be supplied by means of a separate wire that is fed into the weld pool. 5.2.4 Shielding gases Form a protective envelope around the weld area prevent oxidation Shielding gases fall into two categories : i) Inert shielding gases Example : Argon, helium ii) Semi-inert shielding gases Carbon dioxide, oxygen, nitrogen, and hydrogen. Most of these gases, in large quantities, would damage the weld, but when used in small, controlled quantities, can improve weld characteristics.
No flux is required Used in MIG and TIG 5.3 : GAS WELDING 5.3.1 Oxy-acetylene gas welding Oxyfuel-gas welding (OFW) is a general term used to describe any welding process that uses a fuel gas combined with oxygen to produce a flame. This flame is the source of the heat that is used to melt the metals at the joint. Fuel gases (such as hydrogen and methylacetylene propadiene) can be used in oxyfuelgas welding. But, the most common fuel gas welding process uses acetylene (C2H2). This process is known as oxyacetylene-gas welding (OAW) and is used typically for structural sheet-metal fabrication, automotive bodies and various repair work. The apparatus used in gas welding consists basically of an oxygen source and a fuel gas source (usually cylinders), two pressure regulators and two flexible hoses (one of each for each cylinder), and a torch. The cylinders are often carried in a special wheeled trolley.
Figure 6 : Apparatus in oxyfuel gas welding
Advantages : 1. Simple, portable and cheap Disadvantages 1. Low welding speed 2. Not recommended for welding reactive metals such as titanium and zirconium Flame types In oxy-acetylene welding, flame is the most important tool. All the welding equipment simply serves to maintain and control the flame. The correct type of flame is essential for the production of satisfactory welds. The flame must be of the proper size, shape and condition in order to operate with maximum efficiency. The proportion of acetylene and oxygen in the gas mixture is an important factor in oxyfuel-gas welding. Three basic types of oxyacetylene flames used in oxyfuel gas welding : (a) neutral flame (b) oxidizing flame (c) carburizing or reducing flame a) Neutral flame The neutral flame as shown in figure below is produced when the ratio of oxygen to acetylene, in the mixture leaving the torch, is almost exactly one-to-one. It’s termed "neutral” because it will usually have no chemical effect on the metal being welded. It will not oxidize the weld metal and it will not cause an increase in the carbon content of the weld metal. The neutral flame is commonly used for the welding of: (i) Mild Steel (ii) Stainless steel (iii) Cast iron (iv) Copper (v) Aluminum b) Oxidizing flame The oxidizing flame results from burning a mixture which contains more oxygen than required for a neutral flame. It will oxidize or” burn” some of the metal being welded. To have this flame set carburizing flame first convert it to neutral flame and then reduce the supply of acetylene to get oxidizing flame. Its inner cone is relatively shorter and excess oxygen turns the flame to light blue color. It burns with a harsh sound. An oxidizing flame tends to be hotter than the neutral flame. This is because of excess oxygen and which causes the temperature to rise as high as 6300°F. The oxidizing flame is commonly used for the welding metals that are not oxidized readily such as : (i) Copper base metals (brass, bronze) (ii) Zinc base metals (iii) A few types of ferrous metals, such as manganese steel and cast iron
c) Carburizing flame The carburizing (or reducing) flame, is created when the proportion of acetylene in the mixture is higher than that required to produce the neutral flame. A carburizing flame has an approximate temperature of 5500°F (3038°C). A reducing flame can be recognized by acetylene feather which exists between the inner cone and the outer envelope. The outer flame envelope is longer than that of the neutral flame and is usually much brighter in color. Larger the excess of acetylene larger will be its length. The carburizing flame is commonly used for the welding of aluminum and nickel alloys. Metals that tend to absorb carbon should not be welded with reducing flame. For example, iron and steel, it produces very hard, brittle substance known as iron carbide. This chemical change makes the metal unfit for many applications in which the weld may need to be bent or stretched.
Figure 7 : (a), (b), (c) shows the three basic types of flames. (d) The principle of the oxyfuel-gas welding operation.
Figure 8 : Comparison between all three flames:
Welding practice procedure and equipment The basic steps can be summarized as follows: 1. Prepare the edges to be joined and establish and maintain their proper position by using clamps and fixtures. 2. Open the acetylene valve and ignite the gas at the tip of the torch. Open the oxygen valve and adjust the flame for that particular operation. 3. Hold the torch at about 45° from the plane of the workpiece with the inner flame near the workpiece and the filler rod at about 30° to 40°. 4. Touch the filler rod to the joint and control its movement along the joint length by observing the rate of melting and filling of the joint.
Figure 9 : (a) General view of and (b) cross-section of a torch used in oxyacetylene welding. (c) Basic equipment used in oxyfuel-gas welding. 5.4 : ARC WELDING In arc welding, developed in the mid-1800s, the heat required is obtained from electrical energy. The process involves either a consumable or a nonconsumable electrode. An arc is produced between the tip of the electrode and the workpiece to be welded, by using an AC or a DC power supply. Power supply A welding power supply is a device that provides an electric current to perform arc welding operation. Welding usually requires high current (over 80 amperes) and it can need above 12,000 amps in spot welding. Low current can also be used; welding two razor blades together at 5 amps with gas tungsten arc welding is a good example. A welding power supply can be as simple as a car battery and as sophisticated as a modern machine based on silicon controlled rectifier technology with additional logic to assist in the welding process.
Figure 10 : A constant current welding power supply capable of AC and DC Arc welding (AW) is a fusion-welding process in which coalescence of the metals is achieved by the heat from an electric arc between an electrode and the work. To initiate the arc in an AW process, the electrode is brought into contact with the work and is then quickly separated from it by a short distance. The electric energy from the arc thus formed produces temperatures of 5500°C (10,000°F) or higher, sufficiently hot to melt any metal. A pool of molten metal, consisting of base metal(s) and filler metal (if one is used) is formed near the tip of the electrode. In most arc-welding processes, filler metal is added during the operation to increase the volume and strength of the weld joint. As the electrode is moved along the joint, the molten weld pool solidifies in its wake. Types of arc gas welding : (i) Shielded metal arc welding (SMAW) (ii) Gas metal arc welding, GMAW (@ metal inert gas welding, MIG) (iii)Gas tungsten arc welding, GTAW (@ tungsten inert gas welding, TIG)
Figure 11 : Basic equipment in arc welding
5.4.1
: Shielded metal arc welding (SMAW)
A process that use a coated consumable electrode to lay the weld. SMAW is by far the most widely used arc welding process. Applications : Shipbuilding, repair work, pipelines, construction. Principle : 1. During operation, the bare metal end of the welding stick (opposite the welding tip) is clamped in an electrode holder that is connected to the power source. 2. The holder has an insulated handle so that it can be held and manipulated by a human welder. 3. The heat of the welding process melts the coating to provide a protective atmosphere and slag for the welding operation. It also helps to stabilize the arc and regulate the rate at which the electrode melts. 4. An electric arc is produced between the end of a coated metal electrode and the steel components to be welded. 5. As the electrode melts, the (flux) coating disintegrates, giving off shielding gases that protect the weld area from atmospheric gases and provides molten slag which covers the filler metal as it travels from the electrode to the weld pool. The slag floats to the surface and protects the weld from contamination as it solidifies. Once hardened, the slag must be chipped away to reveal the finished weld. 6. This process may be performed manually. Advantages : 1. Equipment is portable, cheap & easy to use most widely used of AW process 2. Can be used on carbon steels, low alloy steels, stainless steels, cast irons, copper, nickel, aluminum Disadvantages 1. Low productivity 2. Operator dependant 3. Electrode must periodically be changed
Figure 12 : SMAW 5.4.2
: Gas metal arc welding, GMAW (@ metal inert gas welding, MIG)
Also known as MIG welding Most widely used arc welding process for aluminum alloys. A process in which a continuous and consumable wire electrode and a shielding gas are fed through a welding gun. Applications : Used in fabrication operations in factories for welding a variety of ferrous and non-ferrous metals, automated welding using robots. Principle : 1. A continuous wire (known as consumable electrode) is fed into the welding gun. 2. The wire melts and combines with the base metal to form the weld. 3. The molten metal is protected from the atmosphere by a gas shield which is fed through a conduit to the tip of the welding gun. 4. Gases used for shielding : Argon, Helium, Carbon Dioxide (CO2) inert gas 5. The inert gas shield eliminates slag and allows cleaner and stronger weld. 6. This process may be automated.
Advantages : 1. Continuous weld may be produced. 2. High level of operator’s skill is not required. 3. Slag removal is not required. Disadvantages : 1. Expensive and non-portable equipment is required. 2. Outdoor applications are limited because of effect of wind, dispersing the shielding gas. 3. GMAW guns can be bulky and difficult-to-reach small areas or corners.
Figure 13 : MIG 5.4.3
: Gas tungsten arc welding, GTAW (@ tungsten inert gas welding, TIG)
Also known as TIG welding A process that use a nonconsumable electrode [tungsten, Tm = 3410°C] to produce a weld. The weld area is protected from atmospheric contamination by a shielding gas (argon, helium) and a filler metal that is fed manually is usually used. Applications : Used extensively in the manufacture of space vehicles, when joining pipes for offshore applications, applied to thin materials, frequently employed to weld small-diameter and thin-wall tubing such as those used in the bicycle industry
Principle : 1. The torch holding the tungsten electrode is connected to a shielding gas cylinder as well as one terminal of the power source. This will allow the welding current from the power source to enter the electrode. 2. The workpiece is connected to the other terminal of the power source through a different cable. 3. The shielding gas goes through the torch body and is directed by a nozzle toward the weld pool to protect it from the air. 4. Protection from the air is much better in TIG than in SMAW because an inert gas such as argon or helium is usually used as the shielding gas and because the shielding gas is directed toward the weld pool. 5. When a filler metal is used, it is added to the weld pool from a separate rod or wire, being melted by the heat of the arc. 6. This process may be performed manually or by machine and automated methods.
Figure 14 : TIG Advantages : 1. High quality weld 2. Little or no postweld cleaning 3. Can be used to weld reactive metals, such as titanium, zirconium, aluminum and magnesium. Disadvantages : 1. Slow process
5.5 : SPOT WELDING Used to join two sheet of metal by lap joint form a small nugget at the interface. Thickness of workpiece = up to 3 mm (1/8 inch). Applications : In mass production of automobiles, appliances, metal furniture. Principle : 1. Pressure are applied to workpiece. 2. Current [1000A to 100,000A] is passed through the joint for certain period of time, so that just enough heat generated to melt the joint. 3. A current stop but the pressure is maintained for certain time until solidification complete.
Figure 15: Spot welding Advantages : 1. Very little skill required to operate 2. High production rate 3. Heating involve a small portion (less distortion) 4. Possible to weld dissimilar metal with different thickness Disadvantages 1. Complex equipment (expensive) 2. Limited to lap joint only References : http://www.asminternational.org https://eis.hu.edu.jo http://www.ignou.ac.in
Chapter 6 Introduction to Metal Casting Chapter Outline 6.1 Overall safety in metal casting 6.2 Melting practice 6.3 Sand casting 6.3.1 Process cycle 6.3.2 Elements of gating system 6.3.3 Molding material 6.3.4 Pattern 6.3.5 Core 6.4 Die casting 6.5 Basic design rule in casting 6.6 Cleaning and casting defects Learning outcome When you complete this chapter you should be able to: 1. Have a basic understanding on casting terminology and processes 2. Have acquired knowledge on casting defects and design rules
6.1
: Overall safety in metal casting 1. Aprons, gloves and leggings should be leather as this offers the most protection is a spillage of molten metal occurs. 2. Wear protective clothing and devices like safety glasses and full face shields. 3. Cloths made of natural fibers like cotton are to be preferred over synthetics that can melt and stick to the skin. 4. Strong, leather shoes should be worn at all times in the workshop as they offer the best protection for feet. Do not distract anybody during a pour. 5. Do not look into the furnace without appropriate eye safety gear for splattering and infrared radiation. 6. Always operate in a well-ventilated area. Fumes and dusts from combustion and other foundry chemicals, processes and metals can be toxic. 7. Spilled molten metal can travel for a great distance. Operate in a clear work area.
6.2 : Melting Practices Furnaces are charged with melting stock consisting of liquid and/or solid metal, alloying elements and various other materials such as flux and slag forming constituents. Furnace choice is based on the type of metals that are to be melted. Factors for furnace selection 1. Economic consideration (less cost during operation) 2. Composition and melting point of the metal/alloy 3. Capability to control furnace atmosphere 4. Capacity 5. Environmental consideration (environment pollution) 6. Maintenance (less) 7. Safety (high) Metals or alloys in casting process are melted and prepared in a furnace which may be of: 1. Electric arc 2. Induction 3. Crucible 4. Cupola When the metal is heat up in the furnace, this molten metal is poured into the assembled mold either via a ladle or directly from the furnace. When the metal has cooled, the mold and the core materials are removed and the casting is cleaned. Certain castings may require welding, heat treatment or painting.
1. Electric arc furnace : The furnace is charged with ingots, scrap, alloy metals and fluxing agents. An arc is produced between three electrodes and the metal charge, melting the metal. A slag with fluxed covers the surfave of the molten metal to prevent oxidation, to refine the metal and to protect the furnace roof from excessive heat. When ready, the electrodes are raised and the furnace tilted to pour the molten metal into the receiving ladle. Other characteristics : High rate of melting, much less pollution and the ability to hold the molten metal for any length of time for alloying purposes.
Figure 1 : Electric arc furnace 2. Induction furnace : It melts the metal by passing a high electric current through copper coils on the outside of the furnace, including an electric current in the outer edge of the metal charge that heats the metal because of the high electrical resistance of the metal charge. Melting progresses from the outside of the charge to the inside. Special hazards include metal fumes. Other characteristics : Used in smaller foundries, produce composition controlled smaller melts, do not introduce dust and noise emissions in operation. Heating is safe and efficient with no open flame to endanger the operator or obscure the process. Production rates can be maximized because induction works so quickly.
Figure 2 : Induction furnace
3. Crucible furnace : The crucible or container holding the metal charge is heated by a gas or oil burner. When ready, the crucible is lifted out of the furnace and tilted for pouring into the molds. Other characteristics : Used for ferrous and nonferrous metals. Special hazards include carbon monoxide, metal fumes, noise and heat.
Figure 3: Crucible furnace 4. Cupola : Furnace is a tall, vertical furnace, open at the top with hinged doors at the bottom. It is charged from the top with alternate layers of coke, limestone and metal. The molten metal is removed at the bottom. Other characteristics : They operate continuously, have high melting rates and produce large amounts of molten metal. Special hazards include carbon monoxide and heat.
Figure 4 : Cupola furnace
6.3 :
Sand Casting Form complex metal parts that can be made of nearly any alloy (aluminum, cast iron, stainless steel). Involves the use of a furnace, metal, pattern and sand mold. Produce automobile components, such as engine blocks, engine manifolds, cylinder heads, and transmission cases
Advantages : It can create big components with complex shapes; it can be used for variety of metals; the cost associated with tools and equipment involved in the casting process is relatively lower; the scrap metal can be used again and the preparation time is relatively shorter. Disadvantages : The materials created by sand casting are not as strong as those created by other techniques. The rate of production is relatively low for sand casting because one mold only can be used once. The preparation of the final product requires additional machinery and labor which adds to the cost.
Figure 5 : Sand casting
Figure 6 : Sand casting products
Figure 7 : Cast parts in a typical automobile 6.3.1 Process Cycle 1. Mold making 2. Clamping 3. Pouring 4. Cooling 5. Removal 6. Trimming
- Shape of casting was perform with pattern & core - To prevent any loss of casting - Enough molten metal must be poured - Shorter cooling time may exhibit severe shrinkage - Sand mold can simply be broken and the casting removed - Excess material must be trimmed from the casting
Figure 8 : Process cycle in sand casting 6.3.2 Elements of gating system Mold cavity : Mold cavity is formed by packing sand around the pattern. Flask : Consist of upper half & bottom half which meet at ‘parting line’ Core : To formed internal surfaces (e.g. : holes, passages) Sprue : Carry the molten metal from basin down to the main channel Runner : Carry the molten metal into the cavity
Riser Vent sand.
: Supply an additional source of metal during solidification : Carry off gases produced when the molten metal contacts with
Figure 9 : Elements of gating system used in sand casting 6.3.3 Molding material Molding is the process whereby a pattern is pressed into sand so as to form the desired “impression” or shape. After the pattern has been carefully extracted, molten metal is poured into the cavity thus formed and the metal on cooling solidifies. The sand is then removed and the resulting product formed by the metal is known as a “casting”. Molding materials (depend on type of mold) : 1. Sand (silica sand, zircon sand, etc) 2. Clay (as a binder: Bentonite, kaolin, etc) 3. Water (active the clay) 4. Additives Table 1 : Additives used in casting process
Molding is performed by several methods such as green sand and dry sand. A) Green sand mold : Mixture of sand (90%), water (3%) and a clay (7%) Widely used Cheap Surface finish of the castings obtained by this process is not good and machining is often required to achieve the finished product B) Skin dried mold (also known as dried green sand) : Additional bonding materials (or binder) are added Cavity surface is dried by a torch or heating lamp directed to the mold surface to increase mold strength. This improves the dimensional accuracy and surface finish. More expensive and require more time, thus lowering the production rate. 6.3.4 Pattern In casting, pattern is a replica of the object to be cast, used to prepare the cavity into which molten material will be poured during the casting process. Therefore, the first step in making a sand casting is the design and construction of a pattern. The pattern material is determined primarily by the number of castings to be made. Wood patterns are relatively easy to make and are frequently used when small quantities of castings are required. However, it is not very dimensionally stable, as it may warp or swell with changes in humidity and it tends to wear out fairly rapidly. Metal patterns are more expensive but are more dimensionally stable and longer lasting. Types of pattern :
Figure 10 : Types of pattern a) Single piece, b) Split, c) Match-plate, d) Cope and Drag pattern a) Single piece pattern : This is the simplest type of pattern, exactly like the desired casting. For making a mould, the pattern is accommodated either in cope or drag. Used for producing a few large castings, for example, stuffing box of steam engine. This is generally used for casting
simple shapes and the productivity rate is low by using single piece pattern and the removal of the pattern from the mold cavity is difficult when complex pattern shapes are inserted. Used for producing a few large castings, for example, stuffing box of steam engine. b) Split pattern : The disadvantage of single piece pattern can be avoided by using split piece pattern by splitting the complex shape in to two parts that is one of them is attached to the cope part and the other to the drag part and both of them are joined together by using dowel pin (dowel pin is a temporary fastener used for joining of two parts). Gated system is incorporated after the cope and drag part are joined together. Example, taps, bushings, gears, flywheels. c) Match plate pattern : A match plate pattern is a split pattern having the cope and drags portions mounted on opposite sides of a plate (usually metallic), called the "match plate" that conforms to the contour of the parting surface. The gates and runners are also mounted on the match plate, so that very little hand work is required. This results in higher productivity. This type of pattern is used for a large number of castings. Because the moulding is done on machines, match plate patterns produce castings which are more accurate than those produced by hand moulding. Example, piston rings, engine. d) Cope and drag pattern : A cope and drag pattern is a split pattern having the cope and drag portions each mounted on separate match plates. These patterns are used when in the production of large castings. The complete molds are too heavy and cannot be handled by a single worker. 6.3.5 Core For castings of the hollow type, “cores” are employed which form the inner pattern thereby providing for molten metal to run into the cavity formed between the core outer face and the inner sand formation of the mold, similarly to that for a solid casting. Cores are made from sand (silica sand) and binder (linseed oil, core oil, resins, etc). The mixture of sand will be compacting in a specially formed box composed of two or more parts.
Figure 11 : Examples of sand cores showing core prints and chaplets to support cores
6.4 Die casting Die casting is a process, in which the molten metal is injected into the mold cavity at an increased pressure up to 30,000 psi (200 MPa). The die casting method is especially suited for applications where many small to medium sized parts are needed with good detail, a fine surface quality and dimensional consistency. The reusable steel mold used in the die casting process is called a die. The dies are fabricated from tool and die steels. The die life is determined the ability of the material to withstand wear caused by the molten alloys and fatigue caused by multiple heating and expansion. he cores are made of refractory ceramic materials. Sand based cores are not applicable due to their insufficient strength under pressure applied in die casting. The following parts are manufactured by die casting method : automotive connecting rods, pistons, cylinder beds, electronic enclosures, toys, plumbing fittings.
Figure 12 : An engine block with aluminium and magnesium die castings. In this process, the molten metal injection is carried out by a machine called die casting machine. Most die castings are made from non-ferrous metals Zn, Cu, Al, Mg. The selection of a material for die casting is based upon several factors including the density, melting point, strength, corrosion resistance, and cost. The material may also affect the part design. For example, the use of zinc, which is a highly ductile metal, can allow for thinner walls and a better surface finish than many other alloys. The material not only determines the properties of the final casting, but also impacts the machine and tooling. Materials with low melting temperatures, such as zinc alloys, can be die cast in a hot chamber machine. However, materials with a higher melting temperature, such as aluminum and copper alloys, require the use of cold chamber machine. The melting
temperature also affects the tooling, as a higher temperature will have a greater adverse effect on the life of the dies. Advantages : 1. Near net shaped components 2. High production rate 3. Design can reduce machining and assembly operations (welding, fastening, etc.) 4. Good dimensional control 5. Good surface finish 6. Can cast thin walled components (<0.050”) Disadvantages : 1. Not applicable for high melting point metals and alloys (e.g. : steel) 2. Long wait for first production parts (too long lead time) 3. Casting defects (e.g. : air entrapment, gas and shrinkage porosity can be prevalent causing lower mechanical properties) 4. Component size limitations 5. High die cost There are two principal die casting methods: i) Hot chamber machine Used for alloys with low melting temperature, which are chemically inert to the material of the plunger and other parts of the casting machine : zinc alloys (except zinc alloys containing more than 10% of aluminum), tin alloys and magnesium alloys. In the hot chamber die casting machines, the pressure chamber (cylinder) and the plunger are submerged in the molten metal in the pot (crucible). Hot chamber machines have short casting cycle (about 1 sec.). They are capable to cast thin wall casting with good filling the cavity under precise temperature control of the molten metal. Maintenance of hot chamber machines is more expensive as compared to the cold chamber process. Process : It relies upon a pool of molten metal to feed the die. At the beginning, the plunger goes up allowing the melt to fill the cylinder space. The die is closed at this stage. The plunger goes down forcing the melt to flow through the gooseneck into the die cavity. After the die has been filled with the molten metal, the plunger is held under a pressure until the solidification is completed. The die opens. The casting stays in the die part equipped with ejectors. The plunger goes up and the melt residuals return through the gooseneck back to the pot. The ejectors push the casting out of the die.
Figure 13 : Hot chamber machine used in die casting
ii) Cold chamber machine Used for alloys with high melting temperature include aluminum alloys, magnesium alloys, copper alloys and zinc alloys (including zinc-aluminum alloys). Process : The process for these machines starts with melting the metal in a separate furnace. Then a precise amount of molten metal is transported to the coldchamber machine. When the pressure chamber is filled with a molten metal, the plunger starts traveling forward and builds up a pressure forcing the metal to flow to the die cavity. After the metal has solidified, the plunger returns to its initial position allowing a new portion of the molten metal to fill the pressure chamber. The die then opens and the ejector pins removes the casting from the die. The casting cycle now may be repeated.
Figure 14 : Cold chamber machine used in die casting
Comparison between sand casting and die casting Table 2 : Sand Casting Startup Time
A few days
Initial Expense Inexpensive Labor Costs
Die Casting Several weeks Expensive
Higher labor costs on long runs Lower labor costs on long runs
Finish
Pebbly
Smooth
Alloys
High temperatures
High fluidity materials; Better life with lower temperatures (e.g. zinc)
Unlimited
Casting weight must be between 30 grams (1 oz) and 10 kg (20 lb). Casting must be smaller than 600 mm (24 in.).
Product Size
Wall Thickness Thicker than die casting
Thinner than sand casting
6.5 :
Basic Design Rule in Casting
i) Wall thickness Uniform wall thickness will ensure uniform cooling and reduce defects. A thick section, often referred to as a hot spot, causes uneven cooling and can result in shrinkage, porosity or cracking.
Figure 15 : Wall thickness ii) Pattern draft Draft is the taper allowed in vertical faces of a pattern to permit its removal from the sand or other molding medium without tearing the mold cavity surfaces. Apply a draft angle of 2° - 3° to all walls parallel to the parting direction to facilitate removing the part from the mold.
Figure 16 : Taper on patterns for ease of removal from the sand mold iii) Corner Round corners to reduce stress concentrations and fracture. Inner radius should be at least the thickness of the walls
Figure 17 : Corner
6.6 : Cleaning and casting defects 6.6.1 Cleaning The complete process of the cleaning of castings, called “fettling”. It involves the removal of the cores, gates and risers, cleaning of the casting surface and chipping of any of the unnecessary projections on surfaces.
Figure 18 : Aluminum piston for an internal combustion engine: (a) as cast and (b) after machining. The part on the left is as cast, including risers, sprue, and well, as well as a machining allowance; the part on the right is the piston after machining.
6.6.2 Casting defects Several types of defect in casting, include : 1. 2. 3. 4. 5. 6. 7. 8.
Misrun Shrinkage cavity Cold shut Microporosity Sand wash Scab Sand blow Shift (mold shift, core shift)
1) Misrun
Figure 19 : Misrun
Also known as cold sheet or short run This defect is incomplete cavity filling. The reasons can be: - inadequate metal supply, too low mold or melt temperature, improperly designed gates, .or length to thickness ratio of the casting is too large. When molten metal is flowing from one side in a thin section, it may loose sufficient heat resulting in loss of its fluidity, such that the leading edge of the stream may freeze before it reaches the end of the cavity.
2) Shrinkage cavity
Figure 20 : Shrinkage cavity
A shrinkage cavity is a depression or an internal void in a casting that results from the volume contraction that occurs during solidification.
3) Cold shut
Figure 21 : Cold shut
Occur when two fronts of liquid metal do not fuse properly in the mold cavity, leaving a weak spot. Solution: avoid narrow cross-section
4) Microporosity
Figure 22 : Microporosity
Caused by air entrained in the melt/mold Solution: Proper foundry practices, e.g. : melt preparation and mold design
5) Sand wash
Figure 23 : Sand wash
It is a projection on the drag face of a casting that extends along the surface, decreasing in height as it extends from one side of the casting to the other end. It usually occurs with bottom gating castings in which the molding sand has insufficient hot strength and when too much metal is made to flow through one gate into the mold cavity.
6) Scab
Figure 24 : Scab
Occurs when a portion of the face of a mold lifts or breaks down and the recess thus made is filled by metal. When the metal is poured into the cavity, gas may be disengaged with such violence as to break up the sand which is then washed away and the resulting cavity filled with metal. The reasons can be : Too fine sand, low permeability of sand, high moisture content of sand and uneven molds ramming.
7) Sand blow
Figure 25 : Sand blow
Blow is relatively large cavity produced by gases which displace molten metal form.
8) Shift (mold shift, core shift)
Figure 26 : Mold shift and core shift
Mold shift refers to a defect caused by a sidewise displacement of the mold cope relative to the drag, the result of which is a step in the cast product at the parting line. Core shift is similar to mold shift, but it is the core that is displaced, and (he displacement is usually vertical. Core shift and mold shift are caused by buoyancy of the molten metal
References http://www.custompartnet.com http://www.ghinduction.com http://www.ilo.org http://mechanicalinventions.blogspot.com http://mechanical-info.blogspot.com http://www.rotblattsculpture.com http://www.substech.com http://www.westcoastcastings.com http://en.wikipedia.org
Workshop Practice(MEM 160) : Introduction To Metal Removal Processes FKM,UiTM Pulau Pinang. _______________________________________________________________________________________________
Chapter 7.0: Introduction to Metal Removal Processes Chapter Outline 7.1 : Overall Safety in Metal Removal Processes 7.2 : Turning 7.3 : Milling 7.4 : Shaping 7.5 : Grinding 7.6 : Drilling
Learning outcome When you complete this chapter, you should be able to: 1. Understand common machine tools used in workshop 2. Describe various types of machine operations 3. Understand the safety procedure/practice in machine shop workshop
1
Workshop Practice(MEM 160) : Introduction To Metal Removal Processes FKM,UiTM Pulau Pinang. _______________________________________________________________________________________________
7.1
: OVERALL SAFETY IN METAL REMOVAL PROCESSES • • • • • • •
7.1.1
Correct dress is important, remove rings and watches, roll sleeves above elbows. Always stop the machine before making adjustments. Always wear protective eye protection. Know where the emergency stop is before operating the machine. Use pliers or a brush to remove chips and never use your hand. Never lean on the machine. Never attempt to measure work while the machine is operated.
:TURNING
Lathes generally are considered to be the oldest machine tools. Lathe is a machine which removes the metal from a piece of work to the required shape and size. One of the most basic machining processes is turning, meaning that the part is rotated while it is being machined. It is a versatile machine tool, manually operated, and widely used in low and medium production also capable of variety of shapes and operations. The job must be softer than tool material. There are several types of lathe machines, but the most popular is engine lathe. No machine shop is seen without this type of lathe. The good thing about engine lathes is that it can be used in various materials, aside from metal. Moreover, the set-up of these machines is so simple that they are easier to use. Its main components include the bed, headstock, and tailstock. These engine lathes can be adjusted to variable speeds for the accommodation of a wide scope of work. In addition, these lathes come in various sizes. Engine lathes all have the same general functional parts, even though the specific location or shape of a certain part may differ from one manufacturer. Basic parts in a lathe machine consist of:
Figure 1 : Diagram of an engine lathe, indicating its principal components.
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Workshop Practice(MEM 160) : Introduction To Metal Removal Processes FKM,UiTM Pulau Pinang. _______________________________________________________________________________________________
Bed Carriage Apron Cross slide Lead screw Head stock Tail stock
Ways
: To support the whole machine. It act as the foundation of the machine, must resist vibration and deflection. : Assembly that moves the tool post and cutting tool along the ways : The front part of the carriage assembly on which carriage hand wheel is mounted : A platform that moves perpendicular to the lathe axis under control of the cross slide hand wheel : A precision screw that runs the length of the bed. It is used to drive the carriage under power for turning and thread cutting operations. : Contain the drive or power element, remains fixed, always on the left end (facing the lathe) of the bed. : It is a cast iron assembly that can be slide along the ways and be locked in place. Used to hold long work in place or mount a drill chuck for drilling into end of the work : A precision ground surfaces along top of the bed on which saddle rides. the ways are precisely aligned with the centerline of the lathe.
The cutting tool is held in a tool post fastened to the cross-slide, which is assembled to the carriage. By moving the carriage, the tool can be fed parallel to the work axis to perform straight turning or by moving the cross-slide, the tool can be fed radially into the work to perform facing, form turning or cut-off operations. Workholding devices Workholding devices are important, particularly in machine tools and machining operations, as they must hold the workpiece securely. There are four common methods used to hold workparts in turning : 1. 2. 3. 4.
Mounting the work between centers Chuck Usually is equipped with three or four jaws. Collet Basically a longitudinally-split, tapered bushing. Face plate Used for clamping irregularly shaped workpieces. The plates are round and have several slots and holes through which the workpiece is bolted or clamped.
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Workshop Practice(MEM 160) : Introduction To Metal Removal Processes FKM,UiTM Pulau Pinang. _______________________________________________________________________________________________
Figure 2 : Four workholding methods used in lathes a) Mounting the work between centers using a “dog” b) Three jaw chuck c) Collet d) Face plate for noncylindrical workparts Turning operations perform on lathe machine : (a) Facing : The tool is fed radially into the rotating work on one end to create a flat surface on the end. (b) Taper turning :Instead of feeding the tool parallel to the axis of rotation of the work, the tool is fed at an angle, thus creating a tapered cylinder or conical shape. (c) Contour turning: Instead of feeding the tool along a straight line parallel to the axis of rotation as in turning, the tool follows a contour that is other than straight, thus creating a contoured form in the turned part. (d) Form turning : In this operation, sometimes called forming, the tool has a shape that is imparted to the work by plunging the tool radially into the work. (e) Chamfering : The cutting edge of the tool is used to cut an angle on the corner of the cylinder, forming what is called a “chamfer.” (f) Cutoff : The tool is fed radially into the rotating work at some location along its length to cut off the end of the part. This operation is sometimes referred to as parting.
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Workshop Practice(MEM 160) : Introduction To Metal Removal Processes FKM,UiTM Pulau Pinang. _______________________________________________________________________________________________
g) Threading : A pointed tool is fed linearly across the outside surface of the rotating workpart in a direction parallel to the axis of rotation at a large effective feed rate, thus creating threads in the cylinder. (h) Boring : A single-point tool is fed linearly, parallel to the axis of rotation, on the inside diameter of an existing hole in the part. (i) Drilling : Drilling can be performed on a lathe by feeding the drill into the rotating work along its axis. Reaming can be performed in a similar way. (j) Knurling : This is not a machining operation because it does not involve cutting of material. Instead, it is a metal forming operation used to produce a regular crosshatched pattern in the work surface.
Figure 3 : Machining operations other than turning that are performed on a lathe a) Facing, b) Taper turning, c) Contour turning, d) Form turning, e) Chamfering, f) Cut-off, g) Threading, i) Drilling and j) Knurling
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Workshop Practice(MEM 160) : Introduction To Metal Removal Processes FKM,UiTM Pulau Pinang. _______________________________________________________________________________________________
7.2 : MILLING MACHINE Milling is the process of cutting away material by feeding a work-piece past a rotating multiple tooth cutter. The cutting action of the many teeth around the milling cutter provides a fast method of machining. The machined surface may be flat, angular or curved. The surface may also be milled to any combination of shapes. The machine for holding the work-piece, rotating the cutter and feeding it is known as the milling machine.
Milling is a machining operation in which a workpart is fed past a rotating cylindrical tool with multiple cutting edges The axis of rotation of the cutting tool is perpendicular to the direction of feed. This orientation between the tool axis and the feed direction is one of the features that distinguish milling from drilling. In drilling, the cutting tool is fed in a direction parallel to its axis of rotation. The cutting tool in milling is called a milling cutter and the cutting edges are called teeth.
Figure 4 : Various types of milling cutter
The machine tool that traditionally performs this operation is a milling machine. There are two basic types of milling operations; (a) Peripheral milling (b) Face milling
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Workshop Practice(MEM 160) : Introduction To Metal Removal Processes FKM,UiTM Pulau Pinang. _______________________________________________________________________________________________
Peripheral Milling In peripheral (or slab) milling, the milled surface is generated by teeth located on the periphery of the cutter body. The axis of cutter rotation is generally in a plane parallel to the work-piece surface to be machined.
Also called as plain milling, the axis of the tool is parallel to the surface being machined, and the operation is performed by cutting edges on the outside periphery of the cutter. Several types of peripheral milling a) Slab milling—The basic form of peripheral milling; the cutter width extends beyond the workpiece on both sides. b) Slotting, also called slot milling—The width of the cutter is less than the workpiece width, creating a slot in the work. When the cutter is very thin, this operation can be used to mill narrow slots or cut a workpart in two, called saw milling. c) Side milling—The cutter machines the side of the workpiece. d) Straddle milling—The same as side milling, only cutting takes place on both sides of the work.
Figure 5 : a) Slab milling, b) Slotting, also called slot milling, c) Side milling, d) Straddle milling Face milling In face milling, the cutter is mounted on a spindle having an axis of rotation perpendicular to the work-piece surface. The milled surface results from the action of cutting edges located on the periphery and face of the cutter. Conventional face milling—The diameter of the cutter is greater than the workpart width, so that the cutter overhangs the work on both sides. Partial face milling—the cutter overhangs the work on only one side. End milling—the cutter diameter is less than the work width, so a slot is cut into the part. Profile milling—this is a form of end milling in which the outside periphery of a flat part is cut. Pocket milling—another form of end milling, this is used to mill shallow pockets into flat parts.
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Workshop Practice(MEM 160) : Introduction To Metal Removal Processes FKM,UiTM Pulau Pinang. _______________________________________________________________________________________________
Surface contouring—a ball-nose cutter (rather than square-end cutter) is fed back and forth across the work along a curvilinear path at close intervals to create a three-dimensional surface form. The same basic cutter control is required to machine the contours of molds and dies, in which case the operation is called die sinking.
Figure 6 : a) Conventional face milling, b) Partial face milling, c) End milling, d) Profile milling, e) Pocket milling, f) Surface contouring Types of Milling Machine Milling machine can be broadly classified into the following types. a) Column & Knee Type Milling Machines • Used for general purpose milling operations, • Most common milling machines. The spindle to which the milling cutter is may be horizontal (slab milling) or vertical (face and end milling). • The basic components are: – Work table, on which the work-piece is clamped using the T-slots. The table moves longitudinally with respect to the saddle. – Saddle, which supports the table and can move transversely. – Knee, which supports the saddle and gives the table vertical movements for adjusting the depth of cut. – Overarm in horizontal machines, which is adjustable to accommodate different arbor lengths. – Head, which contains the spindle and cutter holders. In vertical machines the head may be fixed or vertically adjustable.
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Workshop Practice(MEM 160) : Introduction To Metal Removal Processes FKM,UiTM Pulau Pinang. _______________________________________________________________________________________________
Figure 7 : Schematic illustration of (a) a horizontal-spindle column-and-knee type milling machine and (b) vertical-spindle column-and-knee type milling machine. b) Bed type Machine In bed type machine, the work table is mounted directly on the bed, which replaces the knee, and can move only longitudinally. These machines have high stiffness and are used for high production work. c) Rotary Table Machine Rotary table machines are similar to vertical milling machines and are equipped with one or more heads to do face milling operations. d) Tracer Controlled Machine Tracer controlled machines reproduce parts from a master model. They are used in the automotive and aerospace industries for machining complex parts and dies.
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Workshop Practice(MEM 160) : Introduction To Metal Removal Processes FKM,UiTM Pulau Pinang. _______________________________________________________________________________________________
7.4
: SHAPING MACHINE
Shaping is the operations involving the use of a single-point cutting tool moved linearly relative to the workpart. In conventional shaping, a straight, flat surface is created by this action. In shaping, the speed motion is accomplished by moving the cutting tool.
Figure 8 : Shaping
Shaping is performed on a machine tool called a shaper.
Figure 9 : Shaping machine known as shaper
The motion of the ram consists of a forward stroke to achieve the cut and a return stroke during which the tool is lifted slightly to clear the work and then reset for the next pass.
Figure 10 : Shaping operations
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Workshop Practice(MEM 160) : Introduction To Metal Removal Processes FKM,UiTM Pulau Pinang. _______________________________________________________________________________________________
7.5 :
GRINDING MACHINE
Grinding is a machining operation that also known as abrasive machining. One of the best methods for producing accuracy and fine finishing on parts is abrasive machining. An abrasive is a small, hard particle having sharp edges and an irregular shape. Abrasives are capable of removing small amounts of material from a surface through a cutting process that produces tiny chips.
The grinding wheel is usually disk-shaped, and is precisely balanced for high rotational speeds. Cutting speeds in grinding are much higher than in milling A grinding wheel consists of abrasive particles and bonding material.
Figure 11 : Grinding
a)
b)
Figure 12 : Types of grinder a) Bench grinder, b) Hand grinder
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Workshop Practice(MEM 160) : Introduction To Metal Removal Processes FKM,UiTM Pulau Pinang. _______________________________________________________________________________________________
7.6 : DRILLING Hole making is among the most important operations in manufacturing and drilling is a major and common hole-making process.
Drilling is a machining operation used to create a round hole in a work- part. This contrast with boring, which can only be used to enlarge an existing hole. Drilling is usually performed with a rotating cylindrical tool which has two cutting edges on its working end. The tool is called a drill or drill bit. The rotating drill feeds into the stationary workpart to form a hole whose diameter is equal to the drill diameter.
Drilling machine Drilling machine is used for drilling holes, tapping, reaming, and small-diameter boring operations.
Figure 13 : Schematic illustration of the components of a vertical drill press. • •
Drilling machines with multiple spindles (gang drilling) are used for high production- rate operations. Workholding devices for drilling are essential to ensure that the workpiece is located properly.
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Workshop Practice(MEM 160) : Introduction To Metal Removal Processes FKM,UiTM Pulau Pinang. _______________________________________________________________________________________________
Operations related to drilling a) Reaming : It is used to slightly enlarge a hole to provide a better tolerance on its diameter and to improve its surface finish. The tool is called a reamer and it usually has straight flutes. b)
Tapping : It is performed by a tap and is used to provide internal screw threads in an existing hole.
c)
Counterboring : It provides a stepped hole, in which a larger diameter follows a smaller diameter partially into the hole. A counterbored hole is used to seat bolt heads into a hole so the heads do no protrude above the surface.
d)
Countersinking : This is similar to counterboring, except that the step in the hole is cone-shaped for flat head screws and bolts.
e)
Centering (a.k.a. centerdrilling) : This operation drills a starting hole to accurately establish its location for subsequent drilling. The tool is called a centerdrill.
Figure 14 : Machining operations related to drilling; (a) reaming, (b) tapping, (c) counterboring, (d) countersinking, (e) centerdrilling and (f) spot facing.
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Workshop Practice(MEM 160) : Introduction To Metal Removal Processes FKM,UiTM Pulau Pinang. _______________________________________________________________________________________________
Reference : 1. http://www.brighthub.com
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Chapter 8 Introduction to Sheet Metal Chapter Outline 8.1 Overall safety in sheet metal working 8.2 Layout 8.3 Sheet metal processes : 8.3.1 Shearing 8.3.2 Forming 8.3.3 Finishing 8.4 Joints
Learning outcome When you complete this chapter you should be able to: 1. Describe common tools and equipment used in sheet metal shop. 2. Understand common operations in sheet metal shop. 3. Select suitable tools and equipment in sheet metal application. 4. Execute accurately basic skills in sheet metal practice with appropriate tools and equipment. 5. Understand the safety procedure/practice in sheet metal shop.
8.1
: Overall Safety in Sheet Metal Shop
Working in a sheet metal shop can be a physically demanding job and requires careful attention to safety procedures. Cuts, back injury and repetitive motion injuries can be avoided by using safe work practices. Machinery Any machinery in the shop should be used with caution and carefully examined before use. If a machine is damaged, it should be locked out and not used until repaired. When working, do not wear baggy clothing, jewelry or long hair. These can get caught in moving parts. Physical Injuries Working with sheet metal involves tasks that use the same physical motions over and over. Students should be rotated between different jobs to prevent repetitive motion injuries. Sheet metal edges can be extremely sharp, so students should wear gloves to protect their hands from cuts. Work Environment The sheet metal shop is an area that should have adequate lighting and proper ventilation. This is extremely important when the shop space is small. If any fumes will be present, the students should wear a respirator and safety glasses. Fitness Maintain your overall health and fitness level because sheet metal work can require crawling into tight spaces and areas for installations. Standing, climbing, bending, and squatting may be required for long periods. Keep your work close to you and rotate your tasks as much as possible to avoid fatigue Sheet metal is simply metal formed into thin and flat pieces. It is one of the fundamental forms used in metalworking, and can be cut and bent into a variety of different shapes. Thicknesses can vary significantly, although extremely thin thicknesses are considered foil or leaf, and pieces thicker than 6 mm (0.25 in) are considered plate. Sheet metal is available as flat pieces or as a coiled strip. There are many different metals that can be made into sheet metal, such as aluminum, brass, copper, steel, tin, nickel and titanium. For decorative uses, important sheet metals include silver, gold, and platinum. Sheet metal has applications in car bodies, airplane wings, medical tables, roofs for buildings and many other things.
8.2
: Layout
The layout of metal is the procedure of measuring and marking material for cutting, drilling, or welding. Accuracy is essential in layout work. Most cases use shop drawings, sketches, and blueprints to obtain the measurements required to fabricate the job being laid out. Ability to read and work from blueprints and sketches is paramount in layout work.
Figure 1 : This type of fitting commonly used in gravity furnace leads where a rectangular-to-round transition is required to connect cold air registers. Layout tools are used for laying out fabrication jobs on metal. Some of the more common layout tools that will use in performing layout duties are as follows: a. b. c. d.
Mallets (short-handled hammer) Stake or forming support Cutting tools Marking and measuring equipment : Steel Rule Try Square Scriber Divider Protractor
1. 2. 3. 4. 5. 6. 7. 8. 9.
Figure 2 : Other marking and measuring tools used in sheet metal work a. Mallet A mallet is a kind of hammer, usually of wood, smaller and usually with a relatively large head. It has round or rectangular cross section. The striking face is made flat to the work. It is used to give light blows to the sheet metal in bending and finishing.
Figure 3 : Mallet b. Stake or forming support Used as supporting and forming tools. Help in bending operations. Types of stake : 1. Half moon stake used for working the edges on disc 2. Hatchet stake widely used for forming, bending and seaming 3. Bick iron used for forming long tapered cylindrical components 4. Funnel stake used for forming long tapered conical components 5. Horse head stake used for bending, holding and supporting the component
6. 7.
Convex stake used for forming spherical shapes Pipe stake used for forming pipes and hollow cylindrical surfaces
Figure 4 : Stakes used in sheet metal work c. Cutting tools Types of cutting tool used to cut sheet metal : 1. Hollow punch – used for producing small circular holes 2. Groove punch – used for making grooves 3. Straight shear – used for cutting along a straight lines 4. Bent shear – used for cutting along a curvature
a) Straight shear
b) Bent shear Figure 5 : Shears used in sheet metal 5. Hand shear – light application for metal < 0.8 mm thickness
Figure 6 : Hand shear
6. Throatless shear -A throatless shear is a cutting tool used to make complex straight and curved cuts in sheet metal. The throatless shear takes its name from the fact that the metal can be freely moved around the cutting blade (it does not have a throat down which metal must be fed), allowing great flexibility in shapes that can be cut.
Figure 7 : Throatless shear d. Marking and measuring equipment Include : 1. 2. 3. 4. 5.
Steel Rule Try Square Scriber Divider Protractor
1. Steel rule Used to set out dimensions
Figure 8 : Steel rule
2. Try square Used for making and testing angles of 90°
Figure 9 : Try square 3. Scriber Used to mark lines on sheet metal. To obtain the best results in scribing, hold the scale or straightedge firmly in place, and set the point of the scriber as close to the edge of the scale as possible by tilting the scriber outward. Then exert pressure on the point and draw the line, tilting the tool slightly in the direction of movement.
Figure 10 : Scribing a line 4. Divider Used for scribe arc and circles, bisecting lines, to transfer measurements from a scale to a layout and to transfer measurements from one part of the layout to another, etc. To scribe a circle or an arc, grasp the dividers between the fingers and the thumb, as shown in figure below. Place the point of one leg on the center, and swing the arc. Exert enough pressure to hold the point on center, slightly inclining the dividers in the direction in which they are being rotated.
Figure 11 : Setting the divider
Figure 12 : Scribing an arc or circle with divider 5. Protractor Protractor makes it possible to create angles other than 45° or 90°.
Figure 13 : Protractor
8.3
: Sheet metal processes
Typically, sheets of metal are sold as flat, rectangular sheets of standard size. If the sheets are thin and very long, they may be in the form of rolls. Therefore the first step in any sheet metal process is to cut the correct shape and sized ‘blank’ from larger sheet. Sheet metal processes can be broken down into two major classifications and one minor classification Shearing processes -- processes which apply shearing forces to cut, fracture or separate the material. Forming processes -- processes which cause the metal to undergo desired shape changes without failure, excessive thinning, or cracking. For example, this includes bending and stretching. Finishing processes 8.3.1 : Shearing processes Three principal operations in press working that cut sheet metal: a) Shearing b) Blanking c) Punching Shearing Shearing is a sheet metal cutting operation along a straight line between two cutting edges by means of a power shear. Typically, it is used to cut large sheets.
Figure 14 : Shearing operation (1) As punch first contacts sheet and (2) After cutting
Engineering analysis : The shearing action is illustrated in the figure:
Figure 15 : (1) Just before the punch contacts work (2) Punch begins to push into work, causing plastic deformation (3) Punch compresses and penetrates into work causing a smooth cut surface (4) Fracture is initiated at the opposing cutting edges which separates the sheet Blanking and punching Blanking and punching are similar sheet metals cutting operations that involve cutting the sheet metal along a closed outline. If the part that is cut out is the desired product, the operation is called blanking and the product is called blank. If the remaining stock is the desired part, the operation is called punching. Both operations are illustrated on the example of producing a washer:
Figure 16 : Steps in production of washer
Figure 17 : Shearing Operations which include punching and blanking 8.3.2 : Forming processes Involve: a) Bending b) Stretching c) Drawing d) Roll forming a. Bending Forming process causes the sheet metal to undergo the desired shape change by bending without failure. Bending is typically performed on a machine called a press brake, which can be manually or automatically operated. For this reason, the bending process is sometimes referred to as press brake forming. Press brakes are available in a range of sizes (commonly 20-200 tons) in
order to best suit the given application. A press brake contains an upper tool called the punch and a lower tool called the die, between which the sheet metal is located. The sheet is carefully positioned over the die and held in place by the back gauge while the punch lowers and forces the sheet to bend. In an automatic machine, the punch is forced into the sheet under the power of a hydraulic ram. The bend angle achieved is determined by the depth to which the punch forces the sheet into the die. This depth is precisely controlled to achieve the desired bend.
Figure 18 : Press brake [open]
Figure 19 : Press brake [closed]
While using a press brake and standard die sets, there are still a variety of techniques that can be used to bend the sheet. The most common method in bending operations involve the processes of :
V-bending - Sheet metal is bent along a straight line between a V-shape punch and die Edge bending [also known as wipe bending] - It requires the sheet metal to be held against the die by a pressure pad. The punch then presses against the edge of the sheet metal that extends beyond the die and pad. The sheet will bend against the radius of the edge of the die.
Figure 20 : (above) V bending and (below) Edge bending; (1) Before (2) After bending b. Stretching Stretch forming is a metal forming process in which a piece of sheet metal is stretched and bent simultaneously over a die in order to form large contoured parts. Stretch forming is performed on a stretch press, in which a piece of sheet metal is securely gripped along its edges by gripping jaws. The gripping jaws are each attached to a carriage that is pulled by pneumatic or hydraulic force to stretch the sheet. The tooling used in this process is a stretch form block, called a form die, which is a solid contoured piece against which the sheet metal will be pressed. The most common stretch presses are oriented vertically, in which the form die rests on a press table that can be raised into the sheet by a hydraulic ram. As the form die is driven into the sheet, which is gripped tightly at its edges, the tensile forces increase and the sheet plastically deforms into a new shape. Horizontal
stretch presses mount the form die sideways on a stationary press table, while the gripping jaws pull the sheet horizontally around the form die.
Figure 21 : Stretching or also known as stretch forming c. Drawing Forming process causes the sheet metal to undergo the desired shape change by drawing without failure. A tool pushes downward on the sheet metal, forcing it into a die cavity in the shape of the desired part. The tensile forces applied to the sheet cause it to plastically deform into a cup-shaped part. These parts can have a variety of cross sections with straight, tapered, or even curved walls, but cylindrical or rectangular parts are most common. Deep drawing is most effective with ductile metals, such as aluminum, brass, copper, and mild steel. Examples of parts formed with deep drawing include automotive bodies and fuel tanks, cans, cups, kitchen sinks, and pots and pans. The deep drawing process requires a blank, blank holder, punch, and dies. The blank is a piece of sheet metal, typically a disc or rectangle, which is pre-cut from stock material and will be formed into the part. The blank is clamped down by the blank holder over the die, which has a cavity in the external shape of the part. A tool called a punch moves downward into the blank and draws, or stretches, the material into the die cavity. The movement of the punch is usually hydraulically powered to apply enough force to the blank. After a part is completely drawn, the punch and blank holder can be raised and the part removed from the die. The portion of the sheet metal that was clamped under the blank holder may form a flange around the part that can be trimmed off.
Figure : Drawing
Figure 22 : Drawing sequence d. Roll forming Roll forming is a process by which a metal strip is progressively bent as it passes through a series of forming rolls. The material to be rolled is drawn by means of friction into the two revolving roll gap. The compressive forces applied by the rolls reduce the thickness of the material or changes its cross sectional area. The geometry of the product depends on the contour of the roll gap. Roll materials are cast iron and forged steel because of high strength and wear resistance requirements.
Figure 23 : Roll forming 8.3.3 : Finishing processes Processes which are used to improve the final surface characteristics. 8.4 : Joints There are numerous types of joints used in sheet metal work such as : 1. Lap joint 2. Seam joint 3. Locked seam joint 4. Wired edge joint 5. Hem joint 6. Cap joint 7. Flaged joint 8. Angular joint
Figure 24 : Various types of joints used in sheet metal work
8.4.1 : Edges Edges are formed to : Enhance the appearance of the work To strengthen the piece To eliminate the cutting hazard of the raw edge. 1) Single hem edge This edge can be made in any width. In general, the heavier the metal, the wider the hem is made.
Figure 25 : Single hem edge 2) Double hem edge It is used when added strength is needed and when a smooth edge is required inside as well as outside.
Figure 26 : Double hem edge 3) Wire edge Used to strengthen and stiffen the jobs and to eliminate sharp edges. The usage of lanced tabs is needed to attach the wire or shaft to sheet metal parts.
Figure 27 : Wire edge
Figure 28 : Lanced tabs to attach the wires or shafts to sheet metal 8.4.2 : Seam Many kind of seams are used to join sheet-metal sections. Seaming is based on the simple principle of folding two thin pieces of material together much like the joining of two pieces of paper by folding them at the corner. The metal used in seaming must be ductile in order for the bending to be feasible, as shown in figure below. The material should be capable of undergoing bending and folding at very small radii otherwise, they will crack.
Figure 29 : Single lock seaming 8.4.3
: Lap Joint
A lap joint is one of the many joints used to join two pieces of sheet metal together. With a lap joint, this is done by overlapping the metal and fastening them with rivet together. If two pieces of metal are joined without any material being removed, a joiner will have made a full lap joint. The thickness of this joint will be the sum of the thickness of both metal pieces.
Rivet Figure 30 : Lap Joint 8.4.4 : Rivet Rivet can be used to join thin pieces of metal. Rivets are permanent joining and used primarily for lap joints as shown in figure above. A rivet is an unthreaded, headed pin used to join two or more parts by passing the pin through holes in the parts and then forming a second head in the pin on the opposite side. A head is formed on the plain end of the pin by hammering or by direct pressure. Once the rivet has been deformed, it cannot be removed except by breaking one of the heads. The process of joining two or more plates by means of rivets is called riveting. Before being installed, a rivet consists of a smooth cylindrical shaft with a head on one end. The end opposite the head is called the buck-tail. On installation, the rivet is placed in a punched or pre-drilled hole and the tail is upset, or bucked (i.e. deformed), so that it expands to about 1.5 times the original shaft diameter, holding the rivet in place. To distinguish between the two ends of the rivet, the original head is called the factory head and the deformed end is called the shop head or buck-tail.
Figure 31 : Round head rivet
Figure 32 : The tail of rivet expands to about 1.5 times the original shaft diameter, holding the rivet in place Because there is effectively a head on each end of an installed rivet, it can support tension loads (loads parallel to the axis of the shaft). However, it is much more capable of supporting shear loads (loads perpendicular to the axis of the shaft). Bolts and screws are better suited for tension applications. Types of rivet head : 1. 2. 3. 4.
Snap head – use for structural work Pan head – maximum strength but can’t be shaped easily Conical head – less strong than snap head Counter sink – used for flush surfaces like ship building
Figure 33 : Types of rivet head
There are two types of rivet :
Solid rivet
Blind rivet
Figure 34 : Comparison between solid rivet and blind rivet 8.4.4.1 : Solid rivet Solid rivets are one of the oldest and most reliable types of fasteners. Solid rivets can be made from steel, stainless steel, brass, copper, titanium and aluminum. They are available with many head styles, including round, flat, countersunk, pan, truss, universal, brazier, acorn and mushroom. Solid rivets are used in applications where reliability and safety count such automobile brake shoes or mining and construction equipment. However, a typical application for solid rivets can be found within the structural parts of aircraft. Hundreds of thousands of solid rivets are used to assemble the frame of a modern aircraft.
Figure 35 : Solid rivet
There are several methods for installing solid rivets, such as : 1) Pneumatic riveting hammer One person holds a bucking bar (used to hold the rivet head in place while air hammer is used) against the formed head of the rivet, while the other person applies the tool to the unformed end.
Figure 36 : Riveting team working on the cockpit shell of a C-47 transport at the plant of North American Aviation. The woman on the left operates an air hammer, while the man on the right holds a bucking bar.
Figure 37 : Air hammer
2) Riveting machine
Figure 38 : Riveting machine 3) Pin hammer (rivet set) Rivets may also be upset by hand, using a ball-peen hammer. The head is placed in a special hole made to accommodate it, known as a rivet-set. The hammer is applied to the buck-tail of the rivet, rolling an edge so that it is flush against the material.
Figure 39 : Manual installation of solid rivet Generally, a method of riveting solid rivet are shown as below : 1. 2. 3. 4.
Hole both plate in proper position using punch or drill Clear all burr along the holes Place rivet in the hole (original head at the bottom) Form a second heat (by hand or by riveting machine)
Figure 40 : Method of riveting (a) Initial position, (b) Final position showing riveted head
Figure 41 : Riveting using a solid rivet 8.4.4.2 : Blind rivet Blind rivets are tubular and are supplied with a mandrel through the center. Blind rivets are the best choice for light-duty applications. The rivet assembly is inserted into a hole drilled through the parts to be joined and a specially designed tool is used to draw the mandrel into the rivet. This expands the blind end of the rivet and then the mandrel snaps off.
Figure 42 : Blind rivet
Figure 43 : Rivet gun is used to fasten rivets into two separate materials allowing the materials to clamp together Method of riveting blind rivet using rivet gun: 1. 2. 3.
Drill holes in both materials that you will be using the rivet to hold together. The holes should be adequate enough for the rivet to fit. Open handle of rivet gun completely. Insert rivet mandrel into nosepiece until rivet head is in contact with face of nosepiece. Squeeze handle until rivet mandrel breaks or snap off. If it doesn't snap off after you squeeze it, simply release the handle and squeeze again and do not try to force the pin to snap off or you may damage the material or pull the rivet through before it has completely flattened.
Figure 44 : Riveting of a blind rivet using rivet gun
References 1. 2. 3. 4. 5. 6. 7. 8.
http://www.assemblymag.com http://www.custompartnet.com http://www.doityourself.com http://www.ehow.com http://me.emu.edu.tr/ http://www.surebonder.com http://en.wikipedia.org United States Department of Labor: Sheet Metal Workers Occupational Outlook Handbook, 2010-11 Editions. 9. State Compensation Insurance Fund: Sheet Metal Worker Safety 10. S. Kalpakjian and S.R. Schmid. Manufacturing Engineering and Technology, 6th Edition in SI Units, Prentice Hall, Singapore. 11. M. P. Groover. Principle of Modern Manufacturing, 4th Edition SI Version, John Wiley and Sons, Asia.