UNESCO-NIGERIA TECHNICAL & VOCATIONAL EDUCATION REVITALISATION PROJECT-PHASE II
NATIONAL DIPLOMA IN MECHANICAL ENGINEERING TECHNOLOGY
BASIC WORKSHOP TECHNOLOGY COURSE CODE: MEC113 YEAR I- SE MESTER I THEORY Version 1: December 2008
BASIC WORKSHOP TECHNOLOGY COURSE CONTENT
Subject
Basic Workshop Technology
Year
1
Semester
1
Course Code
MEC 113
Credit Hours
3
Theoretical
1
Practical
2
CHAPTER
1
: Workshop Safety
CHAPTER 2 : Classification of Machine shop Marking out tools • Assessment 1 CHAPTER •
5
6
Week 5 – 6
Week 7 – 8
3
: Metal Joining Processes.
Week 9 – 12
4
: The Use Of Machining Processes.
Assessment
Week 3 – 4
2
: Use of drilling Machine.
Assessment
CHAPTER •
4
Assessment
CHAPTER •
: Machine Shop Measurement Tools And Type.
Assessment
CHAPTER •
3
Week 1 – 2
5
Week 13 -15
Table of Contents: CHAPTER 1 : WORKSHOP SAFETY
WEEK
1 machine gurad:....................................................................................................................................... safe lifting heavy materials .................................................................................................................... Safe use of machines and tools..........................................................................................................
WEEK 2 Safety in the Workshop ......................................................................................................................... Wearing a safety dress ........................................................................................................................... Safety shoes .......................................................................................................................................... Safety goggles ........................................................................................................................................ Correct behavior ....................................................................................................................................
CHAPTER 2 : CLASSIFICATION OF MACHINE SHOP MARKING OUT TOOLS
WEEK
3 Learning outcomes: ............................................................................................................................... Introduction: .......................................................................................................................................... Use of marking out tools: ...................................................................................................................... Production of Simple objects using bench/hand tools .......................................................................... Maintenance of hand tools: ....................................................................................................................
WEEK 4 Use of simple measuring and testing equipments:........ ……….……………………………..…….. Perform simple measuring exercises ..................................................................................................... Use of Dail Indicator: ............................................................................................................................ Carry out exercises involving flatness, squareness straithness e.t.c .....................................................
CHAPTER 3 : AND TYPES
MACHINE SHOP MEASUREMENT TOOLS WEEK 5
Learning outcomes: ............................................................................................................................... Introduction............................................................................................................................................ Practical use of different types of drilling machine ............................................................................... Carry out drilling operatio such as Counter-sink, Counter Boring ........................................................ Grinding Drill Bits ................................................................................................................................ Select Correct Drilling Speed ................................................................................................................ WEEK 6 Demonstrate the use of Grinding Machine ........................................................................................... Calculate Drill Speed .............................................................................................................................
CHAPTER 4 : USE OF DRILLING MACHINES
WEEK
7
Learning out comes : ............................................................................................................................. Demonstrate use of Reamer ................................................................................................................... Select correct Speed for Reaming Operation ........................................................................................
WEEK 8 Select Correct Tapping Drill Size .......................................................................................................... Select Correct Taps ................................................................................................................................ Carry Out Tapping operation on the bench,on drilling and Lathe Machine ......................................... Student results........................................................................................................................................
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CHAPTER 5 : THE USE OF FITTING TOOLS
WEEK
9
Learning out comes ............................................................................................................................... Fabrication Of Metal Container ........................................................................................................... Joining of Metals by the grooving technique......................................................................................... Carry Out soft Soldering ....................................................................................................................... WEEK 10
Assembling of Oxy-acetylene Plant …...……………………………………..…… … Selection of various welding Regulators .............................................................................................. Perform Gas welding by various Welding Technique ........................................................................... WEEK 11 Regulate Current And Determine Polarity for Metal Arc Welding l ..................................................... Determine Polarity and select Current .................................................................................................. Perform Various Arc Welding Joints by Down and up Hand Operation .............................................. WEEK 12 Threading .............................................................................................................................................. Tapping ................................................................................................................................................. Practical use of taps ............................................................................................................................... Dieing .................................................................................................................................................... Care of dies ............................................................................................................................................ Practical application of die ....................................................................................................................
CHAPTER 6 : THE USE OF MACHINING PROCESSES WEEK 13 ............................................................................................................ Controling Distursion in Welding Operation : ...................................................................................... Controling Distursion by Stop Back method ........................................................................................
Controling Distursion by Skipping ....................................................................................... Apply Pre and Post Heating Technique ................................................................................................ WEEK 14 WEEK 15
Basic Turning Operations ..................................................................................................... Practical work .......................................................................................................................
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WEEK 1: SAFETY PRECAUTION 1.1
OBSERVATION OF WORKSHOP SAFETY
The need for safety Almost everyone working in a factory/school workshop has at some stage in his or her career suffered an injury requiring some kind of treatment or first aid. It may have been a cut finger or something more serious. He cause may have been carelessness by the victim or a colleague, defective safety equipment, not using he safety equipment provided, the true cause was most likely a failure to think ahead. You must learn to work safely. Your workplace will have its own safety rule so obey them at all times. Ask if you don’t understand any instruction and do report anything which seems dangerous, damaged or faulty.
Hazards Accidents result most commonly for the following reason: Careless or hurried movement about the workshop causing collisions or knocking over of tools or materials. Careless movement is particular dangerous near machinery. Loose clothing or long hair becoming caught in revolving parts f machines, particularly the drilling spindle. Failing to secure work properly before commencing machining. Lack of care in handling hot metal
1.2
GENERAL RULES FOR SAFETY
All injuries, no matter how slight, should be reported immediately. Breakages and all damage to equipment should be reported. A file should never be used without a handle. The correct size and dye of smith’s tongs should be used when foxing short work. Hot work being transferred from forge to anvils should be held downwards close to the ground, to minimize danger of burns resulting from accidental collisions with other students. When preparing the pickle bath for cleaning copper and gilding metal, the acid must be added to the water never vice versa.
Behavior It is very important to study all the safety instructions and regulations for the work in hand, but the craftsman should be aware of safe pratice all the time. He must study the work to be done, the environment in which it is to be done, the tools to be used and the best method to be used. He must realties what the dangers will be I he adopts a wrong and thoughtless approach. He must always have a thoughtful and positive approach to safety for his own sake and the sake of his workmate
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Fig .1 The Correct Way of dressing in the Workshop.
1.3
PERSONAL SAFETY
To avoid injuries to yourself and your workmates on the shop floor, the following points must form part of the general code of behavior: 1. Do not act foolishly on the shop floor. 2. Do not operate machines that you have not used before without learning about them. 3. Always tidy in the workshop 4. Always protect your eyes 5. Do not wear loose clothing 6. Do not lift heavy loads 7. Always protect your feet 8. Do not use hand tools that are not in good condition 9. Make sure all moving machinery is fenced 10. Do not run in workshops 11. Do not throw tools or materials about the shop floor 12. Remember to place warning notice on faulty machines 13. Always keep away from suspended loads 14. Always cooperate with your colleagues 15. Always protect your hands. 6
Fig. 1 The types of Safety goggles in the Workshop.
g. 1.2. Safety footwear and dangers in use of hand tools.
Safety rules for particular tools, machine tools and processes 7
Files:- File must not be used without handles. The tang can cause injury to the wrist. Files with chipped tips are also dangerous.
Spanners:- Spanners of the correct sizes must be used for nuts to avoid injury to the user when the spanner slips. Over size spanner also round off the edges of the nuts. Do not extend spanner with pieces of tube because this strains the spanner and the fastening.
Socket Wrenches:- Allen key of the correct size must be used for socket screws (allen screws). Worn socket wrenches must never be used as they can slip and injure the user.
Hammer:- Hammers Must be securely fixed to the handles before use. Badly fixed head can fly off while in use and injure co workers.
Chisels:- Chisel with mushroomed heads must not be used as the sharp edges can cause injury to the user for the protection.
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WEEK 2 : WORKSHOP SAFETY AND PPE Guards Moving machine parts must be safeguarded to protect operators from serious injury. Belts, gears, shafts, pulleys, fly wheels, chains, and other moving parts must be guarded if there is a chance they could contact an employee. Fig ( 1.4 ) As mentioned before, the hazards associated with moving machinery can be deadly. There are three types of barrier guards that protect people from moving machinery. They consist of the following:
Fixed guards
Interlocked guards
Adjustable guards
Safety guards must never be removed when a tool is being used. For example, portable circular saws must be equipped with guards. An upper guard must cover the entire blade of the saw. A retractable lower guard must cover the teeth of the saw, except when it makes contact with the work material. The lower guard must automatically return to the covering position when the tool is withdrawn from the work. Housekeeping Good housekeeping shall be maintained in all shops, yards, buildings, and mobile equipment. Supervisors are responsible for good housekeeping in or around the work they are supervising. As a minimum, the following requirements shall be adhered to: 1.
Material shall not be placed where anyone might stumble over it, where it might fall on someone, or on or against any support unless the support can withstand the additional weight.
2.
Aisles and passageways shall be kept clear of tripping hazards. 9
3.
Nails shall be removed from loose lumber or the points turned down.
4.
Ice shall be removed from all walkways and work areas where it may create a hazard or interfere with work to be done. If ice cannot be removed readily, sand or other approved materials shall be applied.
5.
Trash and other waste materials shall be kept in approved receptacles. Trash shall not be allowed to accumulate and shall be removed and disposed of as soon as practicable, at least once per shift (or more often if needed).
6.
Disconnect switches, distribution panels, or alarm supply boxes shall not be blocked by any obstruction which may prevent ready access.
7.
Machinery and equipment shall be kept clean of excess grease and oil and (operating conditions permitting) free of excessive dust. Pressure gauges and visual displays shall be kept clean, visible, and serviceable at all times. Drip pans and wheeled or stationary containers shall be cleaned and emptied at the end of each shift.
Tutorial 1
10
(1) Look at figure (1.5) and write the correct number beside each statement: ( ( ( ( ( ( ( ( ( ( ( ( ( (
) Loose tools carried while mounting a ladder. ) Badly arranged tools. ) Climbing up on unstable supports. ) Carrying things that limit the vision ahead. ) Throughing tools. ) Lift wood pieces with nails. ) Uncleaned workshop floor. ) Pointing compressed air on others. ) Broken bottles on the ground. ) Lift pipes in the middle of the workshop. ) Standing bellow lifted materials. ) Carrying long bars. ) Carrying heavy loads. ) Uncovered pits.
(2) Which of the four sketches bellow shows the correct way to lift a heavy load?
(3) What special precautions should be taken with regard to the storage of flammable substances?
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WEEK 4: MEASUREMENT 4.1ELEMENT OF MEASUREMENT Measurement of components is a vital part of both manufacturing and inspection processes. A large percentage of such measurement is concerned with linear dimension such as lengths, widths, thicknesses, diameters, etc and this involves the use of a wide variety of measuring instruments. The basic principle of linear measurement is that of comparison with a standard of length. The engineer’s rule is a simple example of such a comparative measuring standard, the dimension being read by direct observation. The engraving of the division on the rule necessitates the use of another standard of comparison, and in turn such standards are indirectly related to the use of an absolute standard of linear measurement.
4.2MICROMETER The operation of a micrometer depends upon the principle that the distance moved by a nut along a screw is proportional to the number of revolutions made by the nut. Therefore, by controlling the number of revolutions and fractions of a revolution made by the nut, the distance it moves along the screw can be predicted accurately. This principle forms the basis of a number of measuring devices. The principle applies whether the nut is fixed and the screw rotates or the screw is fixed and the nut rotates. In application of the principle the following devices are needed. 1. A precision screw 2. A means of counting one revolution of the screw. 3. A means of measuring parts of a revolution. Examination of any micrometer measuring instrument will show how these devices are incorporated. The screw thread is rotted by the thimble which indicates the partial revolution. The whole revolutions are counted on the barrel of the instrument.
Fig. 4.1 Micrometer scale reading This article is about the measuring device. For the unit of length, see micrometre.
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Fig. 4.2 Outside, inside, and depth micrometers A micrometer pronounced sometimes known as a micrometer screw gauge, is a device used widely in mechanical engineering and machining for precisely measuring, along with other metrological instruments such as dial calipers and vernier calipers. Micrometers are often, but not always, in the form of calipers. Colloquially the word micrometer is often shortened to mic.
2.3
TYPES
Basic types The image shows three common types of micrometers; the names are based on their application: • • • • •
Outside micrometer (aka micrometer caliper) Inside micrometer Depth micrometer Bore micrometer Tube micrometer
An outside micrometer is typically used to measure wires, spheres, shafts and blocks. An inside micrometer is commonly used to measure the diameter of holes, and a depth micrometer typically measures depths of slots and steps. The bore micrometer is typically a three anvil head on a micrometer base used to accurately measure inside diameters. Tube micrometers are used to measure the thickness of tubes.
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Caliper
Fig. 4.3
Vernier Caliper
A caliper (British spelling also caliper) is a device used to measure the distance between two symmetrically opposing sides. A caliper can be as simple as a compass with inward or outward-facing points. The tips of the caliper are adjusted to fit across the points to be measured, the caliper is then removed and the distance read by measuring between the tips with a measuring tool, such as a ruler. They are used in many fields such as metalworking, mechanical engineering, gunsmithing, handloading, woodworking and woodturning.
Fig. 4.4 Vernier caliper Parts of a vernier caliper: 1. 2. 3. 4. 5. 6. 7. 8.
Outside jaws: used to measure external lengths Inside jaws: used to measure internal lengths Depth probe: used to measure depths Main scale (cm) Main scale (inch) Vernier (cm) Vernier (inch) Retainer: used to block movable part to allow the easy transferring a measurement
A variation to the more traditional caliper is the inclusion of a vernier scale; this makes it possible to directly obtain a more precise measurement. Vernier calipers can measure internal dimensions (using the uppermost jaws in the picture at right), external dimensions using the pictured lower jaws, and depending on the manufacturer, depth measurements by the use of a probe that is attached to the movable head and slides along the centre of the body. This probe is slender and can get into deep grooves that may prove difficult for other measuring tools. The vernier scales will often include both metric and Imperial measurements on the upper and lower part of the scale. 21
Vernier calipers commonly used in industry provide a precision to a hundredth of a millimetre (10 micrometres), or one thousandth of an inch. A more accurate instrument used for the same purpose is the micrometer.
Fig. 4.5 Vernier caliper reading
To read a vernier caliper: • • • •
Read the centimeter mark on the fixed scale to the left of the 0-mark on the vernier scale. (2 cm on caliper seen above) Find the millimeter mark on the fixed scale that is just to the left of the 0-mark on the vernier scale. (2.5 cm) Look along the ten marks on the vernier scale and the millimeter marks on the adjacent fixed scale, until you find the two that most nearly line up. (0.08 cm) To get the correct reading, simply add this found digit to your previous reading. (2.58 cm)
Example:
Reading: 2.64 The engineer’s rule used for making direct measurements depends upon the uvula alignment of a mark or surface on the work to be measured with the nearest division on its scale. This may appear to be a relatively simple exercise, but in practice errors can very easily. It is important when making measurements with an engineer’s rule, to have the eye directly opposite and at 900 to the mark on the work. If the mark is looked at sideway, there will be an error known as parallax.
Fig. 4.6(a) Steel rule 22
Fig. 4.6 (b) Using steel rule for measurement The steel rule should be: 1. Made from hardened and tempered spring steel. 2. Engine divided, that is the graduations should be precision engraved into the metal. 3. Ground on the edges so that it can be used for scribing straight lien and as a straight edge when testing a surface for flatness. 4. Satin chrome finished so as t reduce glare, make it easier to read and prevent corrosion.
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WEEK 5: DRILL AND DRILLING OPERTATION 5.1
DRILL BIT
Fig 5.1. Type of Dill bit Some drill bits: Spade, lip and spur (brad point), masonry bit, twist drill
Drill bits are cutting tools used to create cylindrical holes. Bits are held in a tool called a drill, which rotates them and provides axial force to create the hole. Specialized bits are also available for non-cylindrical-shaped holes. This article describes the types of drill bits in terms of the design of the cutter. The other end of the drill bit, the shank, is described in the drill bit shank article. Drill bits come in standard sizes, described in the drill bit sizes article. A comprehensive drill and tap size chart lists metric and imperial sized drills alongside the required screw tap sizes. The term drill can refer to a drilling machine, or can refer to a drill bit for use in a drilling machine. In this article, for clarity, drill bit or bit is used throughout to refer to a bit for use in a drilling machine, and drill refers always to a drilling machine.
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25
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Twist drill The twist drill bit is the type produced in largest quantity today. It drills holes in metal, plastic, and wood. The twist drill bit was invented by Steven A. Morse[1] of East Bridgewater, Massachusetts in 1861. He received U.S. Patent 38,119 for his invention on April 7, 1863. The original method of manufacture was to cut two grooves in opposite sides of a round bar, then to twist the bar to produce the helical flutes. This gave the tool its name. Nowadays, the drill bit is usually made by rotating the bar while moving it past a grinding wheel to cut the flutes in the same manner as cutting helical gears. Tools recognizable as twist drill bits are currently produced in diameters covering a range from 0.05 mm (0.002") to 100 mm (4"). Lengths up to about 1000 mm (39") are available for use in powered hand tools. 27
The geometry and sharpening of the cutting edges is crucial to the performance of the bit. Users often throw away small bits that become blunt, and replace them with new bits, because they are inexpensive and sharpening them well is difficult. For larger bits, special grinding jigs are available. A special tool grinder is available for sharpening or reshaping cutting surfaces on twist drills to optimize the drill for a particular material. Manufacturers can produce special versions of the twist drill bit, varying the geometry and the materials used, to suit particular machinery and particular materials to be cut. Twist drill bits are available in the widest choice of tooling materials. However, even for industrial users, most holes are still drilled with a conventional bit of high speed steel. The most common twist drill (the one sold in general hardware stores) has a point angle of 118 degrees. This is a suitable angle for a wide array of tasks, and will not cause the uninitiated operator undue stress by wandering or digging in. A more aggressive (sharper) angle, such as 90 degrees, is suited for very soft plastics and other materials. The bit will generally be self-starting and cut very quickly. A shallower angle, such as 150 degrees, is suited for drilling steels and other tougher materials. This style bit requires a starter hole, but will not bind or suffer premature wear when a proper feed rate is used. Drills with no point angle are used in situations where a blind, flat-bottomed hole is required. These drills are very sensitive to changes in lip angle, and even a slight change can result in an inappropriately fast cutting drill bit that will suffer premature wear.
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WEEK 8: THE USE OF FITTING TOOLS (THREADING) Threading A thread is a spiral or helical ridge found on nuts (Internal Thread) and bolts (External threads).
Fig (8.1) Pitch: is the distance between corresponding points (crests)
Dieing (External Thread): Dies are used for cutting external threads on round bar or tubes. Dies are made of Hardened High Carbon Steel or High Speed Steel.
Fig (8.2) Split die is held in place in the stock. The split permits a small amount of adjustment in the size of the die by adjusting the screws in the stock. Since split dies cut their thread complete in one cut, the die thread are tapered and back off for one third of their length.
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Fig (8.3)
Tapping (Internal thread): Taps are used to cut the internal screw threads. Taps are made of hardened High Carbon Steel or High Speed Steel. The ends of the shank are square to fit a wrench. Usually taps are provided in set of three -- taper, second and plug tap.
Fig
(8.4)
Fig (8.5) Taper Tap The tap is tapered off for a length of 8 to 10 threads and is the first tap to be used in a hole to start the thread form.
Second Tap The tap is tapered off for a length of 4 to 5 threads to facilitate picking up the threads cut by the taper tap.
Plug Tap This is fully threaded throughout its length and is called a 'bottoming' tap. This tap used to cut the bottom of a blind hole.
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Fig (8.6)
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Tutorial 5 Type of cut
use
Sketch
1) Complete the following table: Drawing
Operation Name
Tool Use
40
2) Complete the following table: 3) Complete the following table: Tool name
use
drawing
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WEEK 9: METAL JOINING OPERATION
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SIMPLE OPERATIONS DEVELOPMENT Consider the template shown in figure below
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JOINING OF METAL BY THE GROOVING TECHNIQUES
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46
METAL JOINING USING SOLDERING PROCESS
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WEEK 10: METAL JOINING USING OXY-ACETYLENE WELDING The general term 'fabricated product' is used to describe a wide range of engineering products made from metal sheet, plate, angle or channel sections, or other types of rolled sections which are joined together by means of a welding, brazing, or riveting process. Welding is the most common method of fabricating and repairing metal products, therefore many different welding techniques have been developed to meet the requirements of modern industry. Two basic methods of fusion welding only are mentioned in this chapter.
Oxy-Acetylene Welding The basic principle of the oxy-acetylene process is quite simple. When oxygen and acetylene gas are mixed in the correct proportions and ignited, a flame which reaches a temperature of over 3000°C is obtained, this being sufficient to melt all commercial metals. The edges of plates to be joined together are brought to the fusing temperature by heat generated with the aid of this oxygen/acetylene flame, while a filler metal in the form of a welding rod of suitable composition is fed into the joint. The molten edges of the plates and the filler metal flow together and upon cooling form one complete piece, the joint being as strong as the actual parent metal. Two systems of oxy-acetylene welding are in general use: (i) Low Pressure; (ii) High Pressure.
Low-Pressure System In this system acetylene is supplied at low pressure from a special generator by the action of water on calcium carbide. This low-pressure acetylene is purified, dried, and stored in a gasstorage holder, then as required is fed through the main supply line, into a hydraulic backpressure valve, and then into the blow-pipe. The function of the back-pressure valve is to prevent oxygen from passing back into the acetylene-supply line and creating an explosive mixture.
High-Pressure System In the high-pressure system both the oxygen and the acetylene are supplied from seamlesssteel cylinders, and the manufacturers claim the following advantages over the low-pressure system:(i) Greater safety. (ii) Ease of adjustment. (iii) Higher working efficiency, accounted for by the intimate mixture of gases in the blowpipe, and the slightly higher flame temperature due to the high purity of the gas. (iv) Plant entirely portable, can be easily taken to any desired site. (v) Simplicity of operation. (vi) Accurate regulation of both oxygen and acetylene cylinders.
Standard Equipment A high-pressure plant comprises the following items of equipment which are arranged as shown in Fig. I. (i) Supply of oxygen in a steel cylinder painted black, into which is fitted an oxygen-pressure regulator. (ii) Supply of compressed acetylene in a steel cylinder painted maroon, into which is fitted an acetylene-pressure regulator. Note Acetylene gas is unstable when compressed to high pressures and is therefore contained in the cylinder dissolved in acetone. The cylinder is packed with a porous spongy material such as kapoc, asbestos, prepared charcoal, or other suitable material saturated with acetone, and the acetone absorbs large quantities of acetylene under pressure. (iii) A high-pressure blow-pipe as shown in Fig. 3(b), equipped with a range of interchangeable nozzles suitable for welding the various types and thicknesses of-metal. (iv) Two lengths of special non-porous rubber hose, one blue leading from the oxygen regulator to blow-pipe inlet marked '0', the other red leading from the acetylene regulator to 48
the blow-pipe inlet marked 'A'. Standard connections are fitted to the hose at both regulator and blow-pipe ends. (v) Keys to suit the cylinder-valve spindles. (vi) A supply of welding rods and fluxes; these are selected according to the type of metal being welded. (vii) A pair of welding goggles fitted with lenses as recommended in B.S. 679: also protective clothing including a leather apron and a pair of gloves. (viii) A welding table equipped with suitable firebricks, and a wire brush for cleaning the metal prior to welding. (ix) A spark lighter for igniting the blow-pipe. (x) A trolley for transporting cylinders and equipment.
Regulators The reducing valve or pressure regulator used in both oxygen and acetylene cylinders perform two functions: (i) reduce the high cylinder pressure to the much lower working pressure used in the blow-pipe; (ii) maintain a steady working pressure in spite of variations in cylinder pressure.
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WEEK 11: METAL ARC WELDING (PRINCIPLES) 11.1 Introduction The arc is produced by a low-voltage, high-amperage electric current jumping an air gap between the electrode and the joint to be welded. The heat of the electric arc is concentrated on the edges of two pieces of metal to be joined. This causes the metal edges to melt. While these edges are still molten additional molten metal, transferred across the arc from a suitable electrode, is added. This molten mass of metal cools and solidifies into one solid piece. The greatest bulk of electrodes used with manual arc welding are coated electrodes. A coated electrode consists mainly of a core wire of closely controlled composition having a concentric covering of flux and/or other material, which will melt uniformly with the core wire forming a partly vapourised and partly molten screen around the arc stream. This shield protects the arc from contamination by atmospheric gases. The liquid slag produced performs three important functions. 1. Protects the solidifying weld metal from any further contamination from the atmosphere. 2. Prevents rapid cooling of the weld metal. 3. Controls the contour of the completed weld. The function of an electrode is more than simply to carry the current to the arc. The core wire melts in the arc and tiny globules of molten metal shoot across the arc into the molten pool (arc crater in parent metal) during welding. These tiny globules are explosively forced through the arc stream. They are not transferred across the arc by the force of gravity, otherwise it would not be possible to use the manual arc-welding process for overhead welding. The arc stream and other basic features of manual gas shielded metal-arc welding are illustrated in Fig. 13.12. The coating on electrodes has several functions some of which are listed. 1. To facilitate striking the arc and to enable it to burn stably. 2. Serves as an insulator for the core wire. 3. It provides a flux for the molten pool, which picks up impurities and forms a protective slag which is easily removed. 4. It stabilises and directs the arc and the globules of molten core metal as shown in Fig. 13.12. 5. It provides a protective non-oxidising or reducing gas shield (smoke-like gas) around the arc to keep oxygen and nitrogen in the air away from the molten metal. 6. It increases the rate of melting (i.e. metal deposition) and so speeds up the welding operation. 50
7. It enables the use of alternating current. 8. Additions to the coating can be made (during manufacture) which will replace any alloying constituents of the core wire or the parent metal which are likely to be lost during the welding process. 9. It gives good penetration. 10. It increases or decreases the fluidity of the slag for special purposes. It can, for example, reduce the fluidity of electrodes used for overhead welding.
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Fig 2
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WEEK 12: THE USE OF A.C. AND D.C WELDING MACHINE D.C. GENERATORS With a d.c. welding machine the electric current is produced by means of a generator which is driven by a petrol or diesel engine, or alternatively is driven by an a.c. or d.c. electric motor. The engine-driven generator-set can operate quite independently of any electrical supply, and for this reason is used on site work where no mains supply of electricity is available. The motor-driven type of generator-set is chiefly used for the type of welding work performed inside a workshop, and is therefore often permanently mounted on the floor, but the portable type of set is' also available. The electric motor provides a good, constant speeddrive for the generator, ant: is not affected by the load imposed upon it.
A.C. Transformers The a.c. welding machine employs a transformer instead of a generator to provide the required welding current. See Fig. 8(c). The a.c. Transformer as its name implies is an instrument which transforms or steps down the voltage of the normal mains electrical supply to a voltage suitable for welding, i.e. between 80 and 100 volts. Unlike the d.c. Generator the a.c. Transformer has no moving parts and for this reason is usually referred to as a static plant. The advantages claimed for the a.c. welding plant are:1. Low initial cost. 2. No moving parts, therefore negligible maintenance. 3. Higher electrical efficiency. 4. Easy to transport. The disadvantages are few but important:1. Coated electrodes must always be used. 2. Voltage higher than in d.c. system, therefore risk of shock greater. 3. Welding of non-ferrous materials more difficult than with d.c. system.
Electric Welding Equipment 1. A d.c. Generator or an a.c. Transformer. 2. A properly designed electrode-holder (Fig. 9) possessing the following features:(a) Light in weight to reduce fatigue. (b) Well insulated. (c) Well balanced. 53
(d) Must not overheat. (e) Locate and eject the electrode easily. 3. Two lengths of flexible cable to carry the current to and from the One cable runs from the welding plant to the electrode holder; the other from the plant to the work bench known as the ground cable. diameter of these cables will be governed by the voltage and the distance it has to be carried from the machine. 4. A wide selection of different-gauge electrodes of various materials. 5. A head shield or face screen fitted with special coloured lenses as recommended in B.S. No. 679. See Figs. 11 (a) and 11(b). Note An electric arc produces a brilliant light and gives off ultra-violet and infra-red rays which are very dangerous to the eyes and skin, therefore never attempt to look at the arc with the naked eye. The helmet type of head shield fits over the head, and leaves both The face screen provides adequate protection, but needs holding by hand. The coloured lenses are classified according to the amperes to ~ 6. A leather apron and a pair of gloves. 7. A welding booth designed to protect all other personnel from tiE glare and radiation. 8. A chipping hammer for the removal of slag from the weld. 9. A wire brush to clean the weld, and to remove spatter. 10. A steel bench insulated from the booth. 11. A wooden duck board to safeguard the welder from damp floors.
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STRIKING THE ARC The first step towards successful electric-arc welding is learning to and to maintain the arc, and run a straight bead of metal, techniques require a good deal of practice. First set the control unit to the correct current specified for the size of electrode being used. (The value recommended by the makers is only a mate; the final setting is made as the welding operation proceeds. I electrode is then brought into contact with the plate by one of two me 1. The tapping-motion method as shown in Fig. 10(a) in which the electrode is brought straight down on the plate and instantly withdrawn a of 3-5 mm, this distance being equal to the core diameter of the electrode.
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2. The scratching method as shown in Fig. 10(b) in which the el tilted at an angle and is then given a slight circular movement similar to that of striking a match. As in the previous method the el promptly raised a distance equal to its diameter, otherwise it will stick to the plate. If the electrode does stick the holder should be given a sharp twist; failing this the electrode should be released from holder, or the electric current should be switched off. Welding currents may vary from 20-600 amperes; and for striking a arc on open circuit 55-60 volts is required, whilst an a.c. set requires volts. Once the arc has been struck the arc voltage will drop to 20-25 Before striking an arc the operator should have his head shield O£ screen in position and observe the arc through the glass filters of the grade as specified in B.S. 679. After mastering the art of quickly and easily striking the arc the electrode is held at an angle of 60-70° to the plate, while it is moved evenly and across the plate starting from the left edge towards the right, forming a continuous bead which must be even, free from holes, and penetrating well into the parent metal. The heat generated from the arc forms a pool molten metal in the plate and the electrode begins to melt, transf. metal from the electrode to the plate. This transfer of metal also takes against gravity when welding in the overhead position.
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WEEK 13: CONTROLLING DISTORTION IN WELDING OPERATIONS 13.1
WELDING DEFECTS
Distortion We don't have to tell you about the effect of heat on a welded structure. Distortion can be corrected by proper jigging, preheating or post-weld slow cooling or stress-relief heat treatment, and sometimes by peening the weld metal and heat-affected zone if your manufacturing specification allows it. Sequencing your welding also may balance stresses and eliminate the problem. Reducing amperage to lower heat input and using smaller electrodes are done for the same reason. Pulsed rent GMAW is another way to lower heat input. If the weldment is out-of-shape, out-of-alignment, or not dimensionally correct, it’s simply a useless piece of metal.
Bad weld profiles Unacceptable weld profiles have a considerable effect on the performance of your weld under load. If one pass of a multipass weld has a bad profile it can cause incomplete fusion or slag inclusions, even though the next weld passes will partially remelt the first pass. A bad weld profile on one pass can cause subsequent cover passes to be out of size, out of contour, or create many other weld defects that will be covered up by follow-up passes, making the problems impossible to see without special test procedures. The problem is most severe when the bad pass is the first, or root pass of the weld. That is why very experienced welders re often used to make the root passes on pipelines and less experienced welders follow them up making the added passes needed to fill the joint.
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13.2
controlling distortion in welding
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WEEK 14: WOOD WORKING TOOLS AND OPERATIONS 14.1 Use of spokeshave Tools
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14.2 Use of Wooden Planes Tools
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14.3
Use of Wood working hammer and saws
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WEEK 15: METAL MACHINING PROCESS (TURNING)
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15.1
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15.3: The assembly part of the spindle nose of lathe machine
15.4: Different types of tool holding device of the lathe machine
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