7106809-Lathe

MODULE

Manufacturing Processes :

III

Theory of Metal Cutting & Machine Tools

Lecture Notes: Joyjeet Ghose Senior Lecturer, Department of Production Engineering, Birla Institute of Technology, Mesra, Ranchi

The Lathe Machine

The Lathe Machine

Joyjeet Ghose, BIT, Mesra, Lecture notes on PE5005

Types of Lathes Engine lathes The basic engine lathe, which is one of the most widely used machine tools, is very versatile when used by a skilled machinist. However, it is not particularly efficient when many identical parts must be machined as rapidly as possible. •The standard engine lathe is not a high production machine, but it can be readily tooled up for many one-piece or short-run jobs. •It is also possible to modify the basic machine for many higher production applications. •The modern engine lathe provides a wide range of speeds and feeds which allow optimum settings for almost any operation. •There have been advances in headstock design to provide greater strength and rigidity. •This allows the use of high horse power motors so that heavy cuts with carbide tools are practical. •To utilize this high power without losing accuracy, new lathes incorporate heavier beds, wider hardened ways, and deepersectioned carriages. Joyjeet Ghose, BIT, Mesra, Lecture notes on PE5005

An engine lathe


Toolroom Lathe - more accurate -wider range of speeds and feed -smaller, more precise version of engine lathe

A typical toolroom engine lathe with face plate, square turrent, follower, and steady rest. Joyjeet Ghose, BIT, Mesra, Lecture notes on PE5005

TURRET LATHES -Semi-automatic -used for high production work. -In this lathe the tail stock is replaced by a hexagonal turret, on the face of which multiple tools can be fitted and fed into the work piece in proper sequence. -Machining of more than one surface can be done at the same time.


AUTOMATIC BAR MACHINE ( AUTOMATIC SCREW MACHINES) -Automatic -long pieces can be fed through headstock. -these are similar to turret lathe operations, - but with the cutting tools operated by a set cams -Used mostly in screw production that is why it is most frequently called automatic screw machines. These are sometimes called Swiss Machines - These are of two types -Single - spindle screw machines (1 tool cuts at each time) - multiple -spindle screw machines (4-6 tools cuts at the same time)


AUTOMATIC BAR MACHINE ( AUTOMATIC SCREW MACHINES)


AUTOMATIC BAR MACHINE ( AUTOMATIC SCREW MACHINES)


Schematic diagram of a six-spindle automatic bar machine


Copy lathe These types of lathe use hydraulic attachment to copy the shape of a part from a master.


Copy lathe


CNC LATHE •Computer controlled •Wide variety of process capability •Multiple axis •Indexing and contouring head •On- line and off- line programming available


Retrofitting lathe


High speed lathe


All Geared engine lathe


An engine lathe

Engine lathe Joyjeet Ghose, BIT, Mesra, Lecture notes on PE5005

A conventional engine lathe


A mini engine lathe

http://www.mini-lathe.com/Mini_lathe/Introduction/introduction.htm Joyjeet Ghose, BIT, Mesra, Lecture notes on PE5005

Component of an standard engine lathe


Block diagram of an standard engine lathe

http://www.mini-lathe.com/Mini_lathe/Introduction/introduction.htm Joyjeet Ghose, BIT, Mesra, Lecture notes on PE5005

Headstock • The headstock is the powered end and is always at the operator’s left. • This contains the speed changing gears and the revolving, driving spindle, to which any one of several types of work holders is attached. The center of the spindle is hollow so that long bars may be put through it for machining. . • A live centre, a face plate, collet or a chuck can be fitted to the spindle nose to hold and drive the work. • Headstock spindle can be driven by a stepped pulley and a belt or by transmission gears in the headstock.


Bed • •

• •

The Bed forms the base of a Lathe machine. It provides a heavy rigid frame on which all the other basic components are mounted. It must be rigid enough to resist deflection in any direction under load. The bed is made of cast iron or a steel weldment, in a box or I-beam shape, and is supported on legs, a cabinet, or a bench. The headstock and the tailstock are located at either end of the bed and the carriage rests over the Lathe bed and slides over it.


Bed ways • The ways of the lathe are the flat or V-shaped surfaces on which the carriage and the tailstock are moved left and right. • Each has its separate pair of ways, often one flat surface, for stability, and one V-way for guidance in a perfectly straight line. • These ways are hardened and scraped or ground to close tolerances. • The basic accuracy of movement of the carriage depends on the ways.


Headstock

Back Gear arrangement

Headstock belt drive


Headstock

Nose of the head stock, where various work holding devices may be fitted


Tailstock • The tailstock is located on the inner ways at the right end of the bed. • It supports the other end of the work when it is being machined between centers, and • It holds a tool for performing operations such as drilling, reaming • The tailstock is non-rotating but on hardened ways, it can be moved, to the left or right, to adjust to the length of the work. It can also be offset for cutting small angle tapers.


Carriage • The carriage can be moved left or right either by hand wheel or power feed. This provides the motion along the Z-axis. • During this travel turning cuts are made. • Carriage consists of the following parts: (1) Saddle, (2) Cross-slide, (3) Compound-slide or compound rest, (4) Tool post, and (5) Apron.


Carriage


Saddle

• •

The saddle is an H-shaped casting that fits over the bed and slides along the bed ways. It carries the cross-slide and tool post.


Cross Slide • The cross slide is mounted on the carriage and can be moved in and out (X-axis) perpendicular to the carriage motion. • This is the part that moves when facing cuts are made with power feed, or at any time a cut must be made ‘square’ with the Z-axis. • This, or the compound, is also used to set the depth of cut when turning. • The cross slide can be moved by its hand wheel or by power feed.

Cross Slide


Compound Rest: • The compound rest is fitted on the top of the cross-slide, is used to support the cutting tool. • It can be swiveled to any angle for taper turning operations and is moved manually. • It can be moved in and out by its hand wheel for facing or for setting the depth of cut. • It can also be rotated 360 degrees and fed by its hand wheel at any angle. • he compound does not have any power feed but it always moves longitudinally with the cross slide and the carriage.

Compound rest


Compound Rest:


Tool Post • The tool post is mounted on the compound rest. • This can be any of several varieties but in its simplest form is merely a slotted cylinder, which can be moved, left or right in the T-slot in the compound and clamped in place. • It can also be rotated so as to present the cutter to the work at whatever angle is best for the job.


Tool Post


Tool Post

(a) A tool post for single-point tools and (b) a quick change indexing square turret, which can hold up to four tools.


Apron • The apron attached to the front of the carriage, holds most of the control levers. These include the levers, which engage and reverse the feed lengthwise (Z-axis) or crosswise (X-axis) and the lever which engages the threading gears. • The apron is fastened to the saddle, houses the gears and mechanisms required to move the carriage and cross-slide automatically. • The apron hand wheel can be turned manually to move the carriage along the Lathe bed. This hand wheel is connected to a gear that meshes in a rack fastened to the Lathe bed. • The automatic feed lever engages a clutch that provides the automatic feed to the carriage


Feedrod • The feedrod is a long shaft that has a keyway. • The power is transmitted from the lathe spindle to the apron gears through a feedrod via a large number of gears. • The feedrod is used to move the carriage or crossslide for turning, facing and all other operations except thread cutting.


Leadscrew • The leadscrew is powered by gears from the head stock and is used for providing specific accurate mechanized movement to the carriage for cutting threads on the workpiece. • The leadscrew has a definite pitch. • A splint nut is used to engage the leadscrew with the carriage. • In some lathes, the leadscrew performs the functions of feed rod and there is no separate feed rod.


Apron mechanism • Apron mechanism is used for transferring rotary motion of the feed rod and the lead screw into feed motion of the carriage. • Both automatic longitudinal and cross-feed can be provided to the carriage by gears and clutch engagements. • The mechanism is so designed that when the half-nut is engaged with the lead screw, the automatic feed motion from the feedrod is disconnected. • There is an interlocking device when prevents simultaneous engagement of the carriage with the feed shaft and leadscrew and saves the machine from any probable damage. • This arrangement of the apron is called fool-proof mechanism.


The half nut or split nut mechanism • The half nut makes the carriage to engage or disengage with the leadscrew. • It comprises of a pair of half nuts capable of moving in or out of mesh with the lead screw. • The half nut can be engaged with the lead screw by means of a lever provided on the apron. • This mechanism is called half nut mechanism. • The half nut or split nut is used only for thread cutting.


LATHE SPECIFICATIONS The size of a lathe is specified by two or three dimensions: • Maximum swing diameter without touching the bed (C) : The largest diameter workpiece which will clear the bed of the lathe. The center is the headstock spindle center. • Maximum swing diameter without touching the cross slide (D): The largest diameter workpiece which will clear the cross slide is sometimes also specified. • Distance Between Centres (B): The longest workpiece which can be held on centers between the headstock and the tailstock. • Length of Bed (A). • The range of speeds and feeds, and the horsepower available.


Cutting tools for Lathes


Work Holding Devices In lathe work the three most common work holding methods are: • Held in a chuck • Held between centers Chucks •A chuck is one of the most important devices for holding and rotating workpieces in a lathe. •Workpiece of short length and large diameter or of irregular shape which cannot be conveniently mounted between centers are held quickly and rigidly in a chuck. •A chuck is attached to the lathe spindle by means of bolts with the back plate or screwed on the spindle nose. •There are different kinds of chucks: •Three jaw self centering chuck: •Four jaw independent chuck: •Combination chuck: It is a combination of self centering and independent chuck. •Magnetic chuck: The workpieces are held in this chuck by means of powerful electro- magnets. •Air or hydraulic operated chuck: The workpieces are held in this chuck by means of fluid pressure. •Collet chuck: Joyjeet Ghose, BIT, Mesra, Lecture notes on PE5005

Three jaw self centering chuck A three jaw chuck is used for gripping cylindrical workpieces when the operations to be performed are such that the machined surface is concentric with the work surfaces. The jaws have a series of teeth that mesh with spiral grooves on a circular plate within the chuck. This plate can be rotated by the key inserted in the square socket, resulting in simultaneous radial motion of the jaws. Since the jaws maintain an equal distance from the chuck axis, cylindrical workpieces are automatically centered when gripped.


Three jaw self centering chuck


Four jaw independent chuck • With the four jaw chuck, each jaw can be adjusted independently by rotation of the radially mounted threaded screws. • Although accurate mounting of a workpiece can be time consuming, a four-jaw chuck is often necessary for non-cylindrical workpieces.


Collet chuck: • Collets are used when smooth bar stock, or workpieces that have been machined to a given diameter, must be held more accurately than normally can be achieved in a regular three or four jaw chuck. • Collets are relatively thin tubular steel bushings that are split into three longitudinal segments over about two thirds of their length. • The smooth internal surface of the split end is shaped to fit the piece of stock that is to be held. • The external surface at the split end is a taper that fits within an internal taper of a collet sleeve placed in the spindle hole. • When the collet is pulled inward into the spindle, by means of the draw bar that engages threads on the inner end of the collet, the action of the two mating tapers squeezes the collet segments together, causing them to grip the workpiece.

A collet (a) and a collet mounting assembly (b) are shown here.


Collet chuck:


Collet chuck:


Work holding between Centers • For accurate turning operations or in cases where the long work surface is not truly cylindrical, the workpiece can be turned between centers. • Initially the workpiece has a conical center hole drilled at each end to provide location for the lathe centers. • Before supporting the workpiece between the centers (one in the headstock and one in the tailstock), a clamping device called a ‘dog’ is secured to the workpiece. • The dog is arranged so that the tip is inserted into a slot in the drive plate mounted on the main spindle, ensuring that the workpiece will rotate with the spindle.


Work holding between Centers • Lathe centers support the workpiece between the headstock and the tailstock. • The center used in the headstock spindle is called the ‘live’ center. It rotates with the headstock spindle. • The ‘dead’ center is located in the tailstock spindle. This center usually does not rotate and must be hardened and lubricated to withstand the wear of the revolving work. • The workpiece must have perfectly drilled and countersunk holes to receive the centers. • The center must have a 60-degree point.


Work holding between Centers

For accurate machining, cylindrical parts can be turned between centers.

Hardened “dead” centers are mounted in the tailstock; they do not rotate with the workpiece and must be lubricated. Joyjeet Ghose, BIT, Mesra, Lecture notes on PE5005

Hardened “live” centers are mounted in the tailstock; they rotate with the workpiece and do not need lubricatio

Lathe accessories Carriers or lathe dogs and Catch plates or Drive plates

• Carriers or lathe dogs and catch plates are used to hold workpiece when it is held between centers. • Carriers or lathe dogs are attached to the end of the workpiece by setscrews; catch plates are either screwed or bolted to the nose of head stock spindle. • A projecting pin from the carriers fits into the slots provided in the catch plate

A catch plate with live centre


Lathe accessories Face plate • A face plate consists of a circular disc bored out and thread to fit the nose of the spindle. • This has radial, plain and T slots for holding work by bolts and clamps. • Face plates are used for holding workpieces which cannot be held conveniently held between centers or chucks.

A face plate Joyjeet Ghose, BIT, Mesra, Lecture notes on PE5005

Lathe accessories Angle plates • This is a cast iron plate having two faces machined to make them absolutely at right angles to each other. • Holes and slots are provided on both faces so that it may be clamped on the face plate and can hold the workpiece on the other face by clamps and bolts. • Angle plates are used in conjunction with a face plate when the holding surface of the workpiece should be kept horizontal, as for example, in machining a flange of a pipe elbow. • When eccentric jobs are bolted on the face plate, a balance weight or counter weight must be added.


Lathe accessories Mandrels • A mandrel is a device for holding and rotating hollow workpiece that has been previous drilled or bored. • The work revolves with the mandrel which is mounted between two centers. • It is generally made of high carbon steel. • The ends are slightly smaller in diameter and flattened to provide effective gripping surface of the lathe dog screws.


Lathe accessories Steady rest • A steady rest consists of cast iron base, which may be made to slide on the lathe bed ways and clamped at any desired position where support is necessary. • This is so designed that the upper position is hinged at one end which facilitates setting and removal of the workpiece without disturbing the position of the steady rest. • There are three jaws on the steady rest, two on the lower base and one on the upper frame, the jaws may be adjusted radially by rotating individual screws to accommodate work of different diameters. • The main function of the steady rest is to provide support to a long slender work. • For a very long work more than one steady rest may be used. • However the carriage cannot be fed to the full length of the work when steady rest is used.


Lathe accessories Steady rest • The steady rest supports long, small diameter stock that otherwise could not be turned. The steady rest can also replace the tailstock to allow for cutting tool access at the outboard end of your workpiece. • To mount the steady rest: 3. Secure to bedway from below with the locking plate. 4. A single hex bolt, along with a nut and washer, is used to hold the steady rest in place. See Figure. 5. The bearing surfaces on the steady rest should receive periodic lubrication while in use to prevent premature wear.


Lathe accessories Steady rest • To adjust the Steady Rest: 1. Loosen the lock nuts. 2. Open the sliding fingers by turning the knurled screws until they fit around the workpiece. Secure the steady rest in position. 3. Tighten the knurled screws so that the fingers are snug, but not tight against the workpiece. Tighten the setscrews and the lock nuts. 4. Lubricate the brass points with machine oil.


Lathe accessories Follower rest: • • • • •

A follower rest consists of a “C” like casting having two adjustable jaws which support the work. The rest is bolted to the back end of the carriage and moves with it. Before setting the follower rest, the end of the workpiece is machined slightly wider than the jaws to provide the true bearing surface. The tool is slightly in advance position than the jaws, and the tool is fed longitudinally be the carriage, the jaws always follow the tool giving continuous support to the workpiece. The follower rest prevents the job from springing away when the cut is made and is used in finish turning operation.


Lathe accessories Follower rest: • The follow rest is normally used with small diameter stock to prevent the workpiece from “springing” under pressure from the turning tool. To install the follow rest: 1. The follow rest is secured to the saddle with two cap screws. See Figure . 2. The bearing surfaces on the follow rest are similar to those on the steady rest, and should be lubricated to prevent premature wear.


Cutting Conditions After deciding on the machine tool and cutting tool, the following main cutting conditions have to be considered: • Cutting speed • Depth of cut • Feed rate Feed, speed, and depth of cut have a direct effect on productivity, tool life, and machine requirements. Therefore these elements must be carefully chosen for each operation. Whether the objective is rough cutting or finishing will have a great influence on the cutting conditions selected.


Cutting Conditions (Changing cutting speed)

In belt driven lathes the cutting speed may be changed using different pulley combinations Joyjeet Ghose, BIT, Mesra, Lecture notes on PE5005

Cutting Conditions (Changing feed)

In some lathes feed can be changed automatically using the levers in different positions as given in the chart Joyjeet Ghose, BIT, Mesra, Lecture notes on PE5005

Cutting Conditions (Changing feed)


Roughing Cuts •When roughing, the goal is usually maximum stock removal in minimum time with minor consideration given to tool life and surface finish. •The first is to use a heavy feed because this makes the most efficient use of power and, with less tool contact, tends to create less chatter. •There are some exceptions where a deeper cut is more advantageous than a heavy feed, especially where longer tool life is needed. •Increasing the depth of cut will increase tool life over an increase in feed rate. But, as long as it is practical and chip formation is satisfactory, it is better to choose a heavy feed rate. • Even more important, tool life is greatly reduced at high cutting speeds unless coated carbide or other modern tool materials are used, and these also have practical speed limits. •Tool life is decreased most at high speeds, although some decrease in tool life occurs when feed or depth of cut is increased. This stands to reason, because more material will be removed in less time. • It becomes choice then, between longer tool life and increased stock removal. Since productivity generally outweighs tool costs, the most practical cutting conditions are usually those, which first, are most productive, and second, will achieve reasonable tool life.


Finishing Cuts: •When taking finishing cuts, feed rate and depth of cut are of minor concern. •The feed rate cannot exceed that which is necessary to achieve the required surface finish and the depth of cut will be light. • However, the rule about speed will still apply. The speeds will generally be higher for finish cuts, but they must still be within the operating speed of the tool material. •Tool life is of greater concern for finish cuts. It is often better to strive for greater tool life at the expense of material removed per minute. •If tool wear can be minimized, especially on a long cut, greater accuracy can be achieved, and matching cuts which result from tool changes, can be avoided. •One way to minimize tool wear during finishing cuts is to use the maximum feed rate that will still produce the required surface finish. The less time the tool spends on the cut, the less tool wear can occur. •Another way to minimize tool wear during a long finishing cut is to reduce the speed slightly. •Coolant, spray mist, or air flow, will also extend tool life because it reduces the heat of the tool. Joyjeet Ghose, BIT, Mesra, Lecture notes on PE5005

Lathe operations Turning • Turning is a metal cutting process used for the generation of cylindrical surfaces. • Normally the workpiece is rotated on a spindle and the tool is fed into it radially, axially, or both ways simultaneously, to give the required surface. • The term ‘turning’, in the general sense, refers to the generation of any cylindrical surface with a single point tool. • Turning is the most commonly used operation in Lathe. By turning operation excess material from the work piece is removed to produce a cylindrical or cone shaped surface. • Two of the common types of turning are: Straight turning and taper turning.


Straight turning •In this operation the work is held in the spindle and is rotated whole the tool is fed past the work piece in a direction parallel to the axis of rotation. •The surface generated is a cylindrical surface.


Design Considerations for Turning Operations • Parts should be designed so that can be fixtured and clamped in the work holding devices • Dimensional accuracy and surface finish specified should be as wide as possible • Avoid sharp corners, tapers, and major dimensional variations in the part • Use near-net-shape forming • Cutting tools should be able to travel across workpiece without obstruction • Standard cutting tools, inserts, and toolholders should be used • Materials should be selected for their machineability


Guidelines for Turning Operations • • • •

Minimize tool overhang Support workpiece rigidly Use machine tools with high stiffness and high damping capacity When tools begin to vibrate and chatter, modify one or more of the process parameters, such as tool geometry, cutting speed, feed rate, depth of cut, or use of cutting fluid

Chip Collection Systems • • • • •

Drop them on a conveyor belt Dragging the chips from a setting tank Using augers with feed screws Magnetic conveyors Vacuum methods


High-Speed Machining, Ultraprecision Machining, and Hard Turning • High-Speed Machining – High speed: 600 - 1,800 m/min – Very high speed: 1,800 - 1,800 m/min – Ultrahigh speed: > 18,000 – Important factors • Power and stiffness of the tools • Stiffness of tool holder • Spindle design • Inertia of the machine-tool components • Fast feed drives • Level of automation • Selection of appropriate cutting tool • •

Ultraprecision Machining – uses a single-crystal diamond, also known as diamond turning Hard turning – When hardness increases, machinability decreases – Uses polycrystalline cubic boron nitride, cermit, or ceramic cutting tools – Competes successfully with the grinding process Joyjeet Ghose, BIT, Mesra, Lecture notes on PE5005

Taper turning •A taper may be defined as a uniform increase or decrease in diameter of a work piece measured along its length. •In a Lathe taper turning is an operation to produce a conical surface by gradual reduction in diameter from a cylindrical job. •Taper turning can be done by the following ways; • By a form tool. • By setting over the tailstock. • By swiveling the compound rest. • By taper turning attachment. • By compound feed.


Taper Geometry

D

d B A

C

α

α

Where,

l

D = Large diameter of taper in mm. d = small diameter of taper in mm. l = length of taper part in mm 2α = full taper angle α = angle of taper angle or half taper angle.

The amount of taper in a workpiece is specified by ratio of the difference in diameters of the taper to its length. This is termed as conicity and designated by letter K.

D−d K= l

From figure D−d AB = 2 BC = l D−d ⇒ tan α = 2l K ⇒ tan α = 2 ⇒ K = 2 tan α


Taper turning methods Taper turning by a form tool Taper turning by a form tool uses a tool which is a broad nose tool having straight cutting edge. The tool is set on the work piece at half taper angle, and is fed straight into the work to generate a tapered angle. This method is limited to turn limited length taper only. This is due to the reason that the metal is removed by entire cutting edge, and any increase in length of the taper will necessitate the use of a wider cutting edge. This will require excessive cutting pressure, which may distort the work due to vibration and spoil the work due to vibration and spoil the work surface.

Work piece

Tool

feed


Taper turning methods Taper turning by setting over the tailstock The principle of turning taper by this method is to shift the axis of rotation of the workpiece, at an angle to the lathe axis, and feeding the tool parallel to the lathe axis. The angle at which axis of rotation of the workpiece is shifted is equal to half angle of taper. The amount of setover is limited. This method is suitable for turning small taper on long jobs. The main disadvantage of this method is that the live and dead centres are not equally stressed and the wear is not uniform. Moreover, the lathe dog being set at an angle, the angular velocity is not constant. From geometry : BC = setover

BC = AB sin α setover = L sin α If the angle α , the anle of taper, is very small, for all practical purposes

D B d A

S

α α

C

l

sin α = tan α ⇒ setover = L tan α D−d ⇒ setover = L 2l entire length of the work X conicity ⇒ setover = 2 if the taper is turned on the entire length of the workpiece, then l = L ⇒ setover =


D−d 2

Taper turning methods Taper turning by swiveling the compound rest This method employs the principle of taper turning by rotating the workpiece on the lathe axis and feeding the tool at an angle to the axis of rotation of the workpiece. The tool is mounted on the compound rest, is attached to a circular base, graduated in degrees, which may be swiveled and clamped at any desired angle. Once the compound rest is set at the desired half taper angle, rotation of the compound slide will cause the tool to be fed at an angle and generate the corresponding taper. This method is limited to turn a short but steep taper owing to limited movement of the cross slide. The movement of the tool in this method is controlled by hand, thus this gives low production rate and poor surface capacity.


Taper turning methods Taper turning by taper turning attachment •

•

•

•

The principle of taper turning by taper turning attachment is to guide the tool in a straight path set at an angle to the axis of rotation of the workpiece, while the work is being held by a chuck or between centres aligned to the lathe axis. A taper turning attachment consists of a frame or bracket which is attached to the rear end of the lathe bed and supports a guide bar pivoted at the centre. The bar having graduations in degrees may be swiveled on either side of the zero graduation and is set at any desired angle with the lathe axis. When taper turning attachment is used, the cross slide is first made free from the lead screw by removing the binder screw.


Taper turning methods Taper turning by taper turning attachment • The rear end of the cross slide is tightened with the guide block by means of bolt. When longitudinal feed is engaged, the tool mounted on the cross slide will follow the angular path, as the guide block will slide on the guide bar set at an angle to the lathe axis. • Taper turning by this method does not disturb the alignment of the live and dead centre. • By this process both steep and small taper can be made over any length of the workpiece.


Taper turning methods

Taper turning attachment Joyjeet Ghose, BIT, Mesra, Lecture notes on PE5005

Other related lathe operations





Related turning operations: (a) chamfering, (b) parting, (c) threading, (d) boring, (e) drilling, (f) knurling. Joyjeet Ghose, BIT, Mesra, Lecture notes on PE5005

Other related lathe operations Facing: Facing is an operation for generating flat surface at the ends of a work piece. In this operation the feed given is in a direction perpendicular to the axis of rotation. • • • • • • •

First, clamp the part securely in a lathe chuck. Then, install a facing tool Bring the tool approximately into position, but slightly off of the part. Always turn the spindle by hand before turning it on. This ensures that no parts interfere with the rotation of the spindle. Move the tool outside the part and adjust the saddle to take the desired depth of cut. Then, feed the tool across the face with the cross slide. After facing, there is a very sharp edge on the part. Break the edge with a file.


Other related lathe operations • Chamfering: It is a operation of beveling the extreme end of a work piece. This done to remove unwanted metal projections at the ends and to protect end of the work piece from being damaged and to have a better look. • Knurling: Knurling is process of embossing a diamond shaped pattern on the surface of the work piece. The purpose of knurling is to provide an effective gripping surface on a work piece to prevent it from slipping when operated by hand. Knurling is done with a special tool called knurling tool. This tool consists of a set of hardened steel rollers in a holder with teeth cut on their surface in definite pattern.


Other related lathe operations • Grooving or Recessing Operations: Grooving or recessing operations is the operation of reducing the diameter of a workpiece over a very narrow surface. Grooving or recessing operations, sometimes also called necking operations, are often done on workpiece shoulders to ensure the correct fit for mating parts. • Drilling/reaming/ Boring: These are operations to accurately make holes on a workpiece. These operations uses the tailstock of the lathe. The tool is held on the tailstock and is fed toward the rotating work piece.


Other related lathe operations Parting: In. this operation a flat nose tool is used to cut the work piece, with feed in the direction perpendicular to the axis of rotation. A parting tool is deeper and narrower than a turning tool. It is designed for making narrow grooves and for cutting off parts. When a parting tool is installed, ensure that it hangs over the tool holder enough that the holder will clear the workpiece (but no more than that). Ensure that the parting tool is perpendicular to the axis of rotation and that the tip is the same height as the center of the part. A good way to do this is to hold the tool against the face of the part. Set the height of the tool, lay it flat against the face of the part, then lock the tool in place. When the cut is deep, the side of the part can rub against sides of the groove, so it's especially important to apply cutting fluid. In this clip, a part is cut off from a piece of stock.


Thread Nomenclature


Types of thread


Cutting Screw Threads

Fig : (a) Cutting screw threads on a lathe with a single-point cutting tool. (b) Cutting screw threads with a single-point tool in several passes, normally utilized for large threads. The small arrows in the figures show the direction of feed, and the broken lines show the position of the cutting tool as time progresses. (c) A typical carbide insert and toolholder for cutting screw threads. (d) Cutting internal screw threads with a carbide insert. Joyjeet Ghose, BIT, Mesra, Lecture notes on PE5005

Thread cutting operation


Thread cutting operation •

In thread cutting operation the first step is to remove the excess material from the workpiece to make its diameter equal to the major diameter of the thread to be cut.

•

The shape or form of the thread depends on the shape of the cutting tool to be used. The tool point must be ground so that it has the same angle as the thread to be cut. In a metric thread the included angle of the cutting edge should be ground exactly 600.Typical angles are 60° for Vee threads, and 29° for ACME threads. A thread gauge can be used to measure thread angles. (also called Centre Gauge or Fish Tail Gauge).

•

The top of the nose of the tool should be set at the same height as the centre of the workpiece.

•

The correct gear ratio is required between the machine spindle and the lead screw. This can be determined in the following manner:


Thread cutting operation Thread cutting calculations: To calculate the gears required for cutting a thread of certain pitch can calculated from the following formula: The gear of the spindle shaft is the driver and the gear on the leadscrew is the driven gear.

Driver teeth Speed of the leadscrew Pitch of the screw to be cut = = Driven teeth Speed of the spindle Pitch of the lead screw Note: Often engine lathes are equipped with a set of gears ranging from 20 to 120 teeth in steps of 5 teeth, and one gear with 127 teeth. To cut metric thread on English leadscrews: The cutting of metric thread on a lathe with an English leadscrew may be carried out by introducing a translating gear of 127 teeth. If the leadscrew has n threads per inch to cut p mm pitch then,

Driver teeth = Driven teeth

Pitch of the screw to be cut (p) 5 pn =  1 127  127 Pitch of the lead screw  ×  5  n

The factor 127/5 from the fact that 25.4 mm is equal to 1 inch. So one translating gear, with 127 teeth is necessary. Joyjeet Ghose, BIT, Mesra, Lecture notes on PE5005

Thread cutting operation 5. Change gears of correct sizes are then fitted between the spindle and the leadscrew.

When the Change gears are not fitted and when the Change gears are fitted (in this case a compound drive is used) Joyjeet Ghose, BIT, Mesra, Lecture notes on PE5005

Thread cutting operation To change gears in a all geared lathe: 1. Loosen the nut below the middle gear and rotate the bracket so the middle gear moves away from gear F. 2. Loosen the cap screw at the center of the middle gear and slide it away from gear G. 3. Gear F can be removed by loosening the cap screw in its middle. Gear G has a setscrew in its rim. Loosen this screw and pull the gear off of the shaft. 4. Replace these two gears with the gears which will produce the desired pitch and secure with screws provided.


Thread cutting operation • •

The speed of the spindle should be at a lower value and the half nut is engaged. In thread cutting there are two methods of feeding the tool into the workpiece. In the first method the tool is feed perpendicularly into the workpiece. In the second method the tool is feed at half the angle of thread by swiveling the compound rest. The second method has distinct advantages over the first as it permits to have a top rake, cuts with single cutting edge, allows chips to flow easily, and reduces the strain on the tool. So the later method is used for roughing cuts and the first method is used for finishing cuts.

•

After the tool has produced a helical groove upto the end of the work, the tool is withdrawn by the use of cross slide. Thread catching: The complete depth of cut of the thread cannot be attained in a single pass. Several cuts have to be taken till the required depth of cut is obtained. For this, the tool has to be withdrawn from the thread groove after completing each cut and then brought back to the starting position. Therefore we should have a suitable method so that the tool follows the previously cut thread groove, otherwise the threads will be spoiled. The process of engaging the thread with the same groove is called thread catching or thread chasing. The following methods can be used for thread catching:

•


Thread cutting operation •

•

When the length of the threaded part is short, after each cut, the carriage is brought back to its starting position by reversing the direction of rotation of lead screw. Therefore in this case the half nut is not disengaged from the leadscrew so the relative position is maintained. When threading long jobs, the above mentioned method is not suitable, as it requires lot of time. So after each cut the machine is stopped, the carriage is disengaged from the leadscrew, by disengaging the half nut. It is then brought back to the starting position by rotating the hand wheel in suitable direction. If the leadscrew pitch is an exact multiple of the pitch to be cut than the half nut can engaged anywhere and the tool will follow the previously cut groove. But if not, a reference dial present on the right hand side of the apron called thread chasing dial has to be used. A fixed zero mark is provided on the saddle surface adjacent to the periphery of the dial. When the first cut is to be taken, the half nut is engaged when zero mark and in subsequent cuts the half nut should be engaged when the zero mark coincides with the same mark on the dial.


Machining time calculation for turning operation πDN m/min 1000 Where D = Diameter of the work in mm (Average of initial and final) N = Speed of the work in rpm. If f be the feed in mm/rev. and l be the length of the job in mm , l o be the length of overetravel of the tool in mm (usually taken as 2 to 3 mm), then l + lo Machining time = min f ×N Cutting Speed V =

This is the time required for one pass. A job is completed in several passes. Let , Pr & P f = No . of roughing and finishing passes respectively. d r & d f = depth of cut for roughing and finishing in mm. A = Total maching allowance A f = Finish machining allowance. Then, Pr =

A - Af dr

and P f =

A

Af

Df

Di

Af df l (mm)

For facing operation the diameter used for calculating is the average of the blank diameter and the lowest diameter (zero in case of complete facing). Joyjeet Ghose, BIT, Mesra, Lecture notes on PE5005

CAPSTAN AND TURRET LATHE


CAPSTAN AND TURRET LATHE The standard engine lathe is versatile, but it is not a high production machine. When production requirements are high, more automated turning machines must be used. The turret lathe represents the first step from the engine lathe toward high production turning machines. The turret lathe is similar to the engine lathe except that tool-holding turrets replace the tailstock and the tool post-compound assembly. The ‘skill of the worker’ is built into these machines, making it possible for inexperienced operators to reproduce identical parts. In contrast, engine lathe requires a skilled operator and requires more time to produce parts that are dimensionally the same. The principal characteristic of turret lathes is that the tools for consecutive operations are set up for use in the proper sequence. Although skill is required to set and adjust the tools properly, once they are correct, less skill is required to operate the turret lathe.


Advantages of Turret Lathes The difference between the engine and turret lathes is that the turret lathe is adapted to quantity production work, whereas the engine lathe is used primarily for miscellaneous jobbing, toolroom, or single-operation work. The features of a turret lathe that make it a quantity production machine are: •Tools may be set up in the turret in the proper sequence for the operation. •Each station is provided with a feed stop or feed trip so that each cut of a tool is the same as its previous cut. •Multiple cuts can be taken from the same station at the same time, such as two or more turning and/or boring cuts. •Combined cuts can be made; tools on the cross slide can be used at the same time that tools on the turret are cutting. •Rigidity in holding work and tools is built into the machine to permit multiple and combined cuts. •Turret lathes can also have attachments for taper turning, thread chasing and duplicating, and can be made. Joyjeet Ghose, BIT, Mesra, Lecture notes on PE5005

Horizontal Turret Lathes Horizontal turret lathes are made in two general designs and are known as the ram and saddle-types. The ram-type turret lathe is shown in Figure has the turret mounted on a slide or ram which moves back and forth on a saddle clamped to the lathe bed. The saddle-type turret lathe shown in Figure has the turret mounted directly on a saddle which moves back and forth with the turret.

Ram-type horizontal turret lathe has the turret mounted on a slide or ram. Joyjeet Ghose, BIT, Mesra, Lecture notes on PE5005

Horizontal Turret Lathes

Saddle-type turret lathe has the turret mounted directly on the saddle. Joyjeet Ghose, BIT, Mesra, Lecture notes on PE5005

Differences between a Ram type or Capstan and Saddle type or a Turret lathe •

The turret of a capstan lathe is mounted on a short slide or ram which slides on the saddle. The saddle is clamped on bedways after adjusting the length of the workpiece. Thus in a capstan lathe, the travel of the turret is dependent upon the length of the travel of the ram. This limits the maximum length of the work to be machined in one setting.

•

The turret of a turret lathe is mounted on a saddle which slides directly on the bed. This feature enables the turret to be moved on the entire length of the bed and can machine longer work.

:


Differences between a Ram type or Capstan and Saddle type or a Turret lathe :

• In the case of turret lathe, the turret is mounted on the saddle which slides directly on the lathe bedways. This type of construction provides utmost rigidity to the tool support as the entire cutting load is taken up by the lathe bed directly. In the case of a capstan lathe as the ram feeds into the work, the overhanging of the ram from the stationary saddle presents a non-rigid construction which is subjected to bending, deflection or vibration under heavy cutting load..


Differences between a Ram type or Capstan and Saddle type or a Turret lathe • :


On a capstan lathe the hexagonal turret can be moved back and forth much more rapidly without having to move the entire saddle unit. Thus capstan lathes are particularly handy for small articles which require light and fast cuts. While operating the machine by hand, there is less fatigue to the operator, due to lightness of the ram, whereas in the case of turret lathe hand feeding is a laborious process due to the movement of the entire saddle unit.

Differences between a Ram type or Capstan and Saddle type or a Turret lathe • :

• •

Some turret lathes are equipped with crosswise movement of the hexagonal turret. The crosswise movement may be effected by hand or power. This feature enables turning of large diameters, facing, contour turning and many other operation on the lathe. Heavier turret lathes are equipped with power chucks like air operated chucks for holding large workpieces quickly. In the case of a capstan lathe, the cross slide is mounted on a carriage which rests on the bedways between head stock and the ram. The carriages rests on both the front and rear ways on the top of the bed. Some turret lathe are equips with side hung type carriage. The carriage of this type does not require support from the rear bedways but slides on the top and bottom guideways provided at the front of the lathe. This construction enables larger diameter of work to be swung above the lathe bedways. There is no rear tool post on this type of machine as the carriage does not extend upto rear bedways.


Turret indexing mechanism

• The turret 1 is mounted on the spindle 5, which rests on bearing on the turret saddle. • The index plate 2, the bevel gear 3 and the indexing ratchet 4 are keyed to the spindle 5.



• The plunger 14 fitted within the housing and mounted on the saddle locks the index plate by spring pressure 15 and prevents any rotary movement of the turret as the tool feeds into the work. • A pin 13 fitted on the plunger 14 projects out of the housing. • An actuating cam 10 and indexing pawl 7 are attached to the lathe bed 9 at the desired position. Joyjeet Ghose, BIT, Mesra, Lecture notes on PE5005


• Both the cam and the pawl are spring loaded. • As the turret reaches the backward position , the actuating cam 10 lifts the plunger 14 out of the groove in the index plate due to the riding of the pin 13 on the beveled surface of the cam 10 and thus unlocks the index plate 2. • The spring loaded pawl 7 which by this time engages with a groove on the ratchet plate 4 causes the turret to rotate as the turret head moves backward. Joyjeet Ghose, BIT, Mesra, Lecture notes on PE5005


• When the index plate or the turret rotates through one sixth of revolution, the pin 13 and plunger 14 drops out of cam 10 and the plunger locks the index plate at the next groove. • The turret is thus index by one sixth of revolution and again locked into the new position automatically. • The turret holding the next tool is now fed forward and the pawl is released from the ratchet plate by the spring pressure. Joyjeet Ghose, BIT, Mesra, Lecture notes on PE5005


• The synchronized movement of the stop rods with the indexing of the turret can also be understood from the figure above. • The bevel pinion 6 meshes with bevel gear 3 mounted on the turret spindle. • The extension of the pinion shaft carries a plate holding six adjustable stops rods 8. • As the turret rotates through one sixth of revolution the bevel gear 3 caused the plate to rotate. Joyjeet Ghose, BIT, Mesra, Lecture notes on PE5005


• The ratio of the teeth between the pinion and gear are so chosen that when the tool mounted on the face of the turret is indexed to bring it to the cutting position, the particular stop rod for controlling the longitudinal travel of the tool is aligned with stop 12.



• The setting of the stop rods 8 for limiting the feed of each operation may be adjusted by unscrewing the lock nuts and rotating the stop rods on the plate. • Thus six stop rods may be adjusted for controlling the longitudinal travel of the tools mounted on the six faces of the turret.


Different types of tool holders used in turret lathes










Tool Layout •

•

• • •

• • •

In order to perform any work on turret lathes, proper planning for systematic operations to be carried out in advance before setting the work on lathe. The following procedures should be adopted to plan and execute a work. For effective planning and control, for each turret lathe upto-date capacity chart is an essential requirement. This chart is supplied by the manufacturers contains every working details of the machine such as the maximum and minimum diameter of the work that can be mounted, maximum length of stroke of the turret and saddle, maximum length of the cross slide movement, tools available etc. For tooling layout, a drawing of the finished part is required. Proper selection of tools and tool holder is to be made. Then the finished drawing is to be superimposed on the capacity chart and the tools to be used are drawn out in proper sequence. The length of travel of each tool is now calculated from the chart and position of stop decided. Proper spindle speed, feed and depth of cut is then decided. The work and the tools are then set on the machine according to the planned chart. A typical example of such chart is given below. Joyjeet Ghose, BIT, Mesra, Lecture notes on PE5005

Tool Layout


Production of a hexagonal bolt The planning for Production of a hexagonal bolt is given below: • The capacity chart is made available. • The drawing of the finished hexagonal bolt is taken into consideration.

10 mm 37 mm

16 mm

Hexagonal Bolt

• •

The tools and equipments such as bar stop, roller steady turning tool holder, roller steady bar ending tool holder, self opening die head, chamfering tool, parting tool are collected. The drawing of the work and tools are superimposed on the capacity chart to decide the length of travel of the tool and the position of stops.


Production of a hexagonal bolt

REAR SQUARE TURRET

T2

T3

T6

T1

WORK

HEX TURRET

T5 FRONT SQUARE TURRET

T1 = Bar stop, T2 = Roller steady box turning tool, T3 = Bar ending tool, T4 = Self-opening die head, T5 = Chamfering tool and T6 = Parting tool

•Proper speeds and feeds for each operation are next calculated.


T4

Production of a hexagonal bolt •Setting and machining operation are performed in the following order: Setting of bar stops: The bar stop is placed on the first turret face. The bar stop is set at a distance of 70 mm from the collet face. An extra length of 10 mm than the bolt length is allowed, 4mm for parting off and 6 mm clearance of the collet face so that the parting off tool may penetrate deep into work without interference.

Setting of the roller steady box turning tool: The roller steady box turning tool is set on the next turret face for turning a diameter of 16 mm. The stop for turning tool is set 20 mm from the collet face.


Production of a hexagonal bolt Setting of bar ending tool: The bar ending tool is set on the next turret face and is brought into operation after turning the bar. The stop is adjusted in position accordingly.

Setting of self opening die head: The self opening die head is mounted on the next face of the turret and dies are fitted into it to cut a thread of 16 mm diameter. The stop is adjusted in position keeping in view the pulling out length of the die for self releasing. Joyjeet Ghose, BIT, Mesra, Lecture notes on PE5005

Production of a hexagonal bolt Setting of chamfering tool: The chamfering tool is mounted on the four station turret on the cross slide and the extreme longitudinal position of the saddle is adjusted by a stop. The cross feed movement of the cross slide is also adjusted by a stop.

Setting of parting off tool: The parting off tool is set on the rear tool post on the cross slide and longitudinal position of the parting tool is adjusted by the stop set at a distance of 6 mm from the turret face.


7106809-Lathe

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