In this article we will discuss about:- 1. Meaning of Lathe 2. Working Principle of Lathe 3. Size of a Lathe 4. Specification of a Lathe 5. Classification of Lathe 6. Accessories Supplied with Lathe 7. Preparing Lathe for Operation 8. Safety Guidelines for Working on Lathe 9. Precautions on Lathe Operation 10. Check-Out Procedures for Operation on Lathe 12. Safety in Using Lathe and Other Details.
Contents:
- Meaning of Lathe
- Working Principle of Lathe
- Size of a Lathe
- Specification of a Lathe
- Classification of Lathe
- Accessories Supplied with Lathe
- Preparing Lathe for Operation
- Safety Guidelines for Working on Lathe
- Precautions on Lathe Operation
- Check-Out Procedures for Operation on Lathe
- Safety in Using Lathe
- Use of Angle Plates
- Parting Operation
- Finish Turning
- Grooving or Necking Operation and Under-Cutting
1. Meaning
of Lathe:
A lathe is probably the oldest machine tool, stemming from the early tree lathe, which was turned by a rope passed around the work a few times and attached to a springly branch overhead. The work was supported by two dowels struck in adjacent trees. The operator’s foot supplied the motion, which was intermittent and fluctuating.
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The tool was held in the operator’s hand. Later a strip of wood called a “lath” was used to support the rope and hence named as Lathe. From this crude beginning and over period of more than two centuries, the modern engine lathe has evolved. Until about 1770, lathes were useless for metal cutting because they lacked power and a holding device strong enough and accurate enough to guide the tool.
For its development to the form in which we know it now, we owe much to Henry Mauldsley, who developed the sliding carriage and in 1800 built a screw cutting lathe. Now-a-days, it has become a general purpose machine tool, employed in production and repair work, because it permits a large variety of operations to be performed on it.
2. Working Principle of Lathe
:
Lathe removes undesired material from a rotating workpiece in the form of chips with the help of a tool which is traversed across the work and can be fed deep in work. The tool material should be harder than the workpiece and the latter held securely and rigidly on the machine. The tool may be given linear motion in any direction.
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A lathe is used principally to produce cylindrical surfaces and plane surface, at right angles to the axis of rotation. It can also produce tapers and bellows etc. Operation of turning is done on parts as small as those used by watches to huge parts weighing several tons.
A lathe basically consists of a bed to provide support, a headstock, a cross slide to traverse the tool, a tool post mounted on the cross slide. The spindle is driven by a motor through a gear box to obtain a range of speeds.
The carriage moves over the bed guideways parallel to the workpiece and the cross slide provides the transverse motion. A feed shaft and lead screw are also provided to power the carriage and for cutting the threads respectively. Fig. 12.1 A shows an engine lathe.
Operations which can be performed on lathe:
(i) Turning,
(ii) Facing,
(iii) Taper turning,
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(iv) Eccentric turning,
(v) Boring,
(vi) Drilling,
(vii) Reaming,
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(viii) Threading,
(ix) Knurling,
(x) Scroll cutting.
In addition to it, with the help of special attachments, operations like key-way cutting, cam and gear cutting, shaping, milling, fluting and grinding can also be performed on this machine.
3. Size of a Lathe
:
Size of a lathe is specified in any one of the following ways:
(i) The height of the centres measured over the lathe bed, or
(ii) Swing or maximum diameter that can be rotated over the bed ways, or
(iii) Swing or diameter over carriage. This is the largest diameter of work that will revolve over the lathe saddle, or
(iv) Maximum job length in mm that may be held between the centres (head stock and tail stock centres), or
(v) Bed length in metres which may include the head stock length also, or
(vi) Diameter of the hole through lathe spindle for turning bar material.
Sometimes the shape of bedways and horsepower of the driving motor is also given in specification. The size of the lathes (Capstan) using collect chuck is often given by the maximum diameter of the bar that may pass through the hollow spindle or the chuck.
4. Specification of a Lathe
:
A lathe is generally designated by:
(a) Swing, i.e., the largest work diameter that can be swung over the lathe bed.
(b) Distance between head stock and tail stock centre.
Some manufacturers designate the lathes by the swing and length of the bed.
Bar automatic lathes are specified by the maximum diameter of the bar which can be accommodated.
In order to specify a lathe completely, the following specifications should be included:
1. (a) Height of centres (b) Type of bed, i.e. straight, semi-gap, or gap type (c) Centre distance.
2. (a) Swing over bed. (b) Swing over cross slide, (c) Swing in gap. (d) Gap in front of face plate, (e) Width of bed.
3. (a) Spindle Speeds Range, (b) Spindle Nose (Type), (c) Spindle bore, (d) Taper in nose.
4. (a) Metric thread pitches, (b) Lead screw pitch, (c) Longitudinal feeds, (d) Cross feeds.
5. (a) Cross slide travel, (b) Top slide travel, (c) Tool section.
6. (a) Tailstock sleeve travel. (b) Taper in sleeve bore.
7. Motor horsepower and RPM.
8. Shipping dimensions—length x width x height x weight.
5. Classification of Lathe
:
It is difficult to make a suitable classification of lathe as there are so many variables in the size, design, method of drive, arrangement of gears, different precision classes and purpose.
In general, the following classification covers most of the lathes used today:
(i) Speed Lathe:
It is so named because of the very high speed of the headstock spindle. It is the simplest form of lathe and consists of a simple head-stock, a tail-stock, and tool-post. It has no gear box, lead screw and carriage. Tools are hand operated. Cone-pulley is the only source provided for the speed variation of the spindle. Such a machine finds its intensive application in wood turning, metal spinning and polishing operations.
(ii) Engine Lathe or Centre Lathe:
It is the most important machine tool in the lathe family and by far most widely used. Its name is derived from the fact that early machine tools were driven by separate engines or from a central engine with overhead belts and shafts. The stepped cone- pulley or geared head are often used for varying the speed of lathe spindle.
A tailstock is provided to facilitate holding the work between centres and permit use of tools like drills, taps, etc. The cutting tools are controlled either by hand or by power and can be fed both in cross and longitudinal directions with reference to the lathe axis with the help of a carriage feed rod and lead screw.
A wide range of attachments can be fitted on it to increase its utility. These are available in sizes to handle upto 1 m diameter jobs and 1 to 4 m long.
(iii) Turret Lathe:
It is a production machine used to perform a large number of operations simultaneously. In it, several tools are set on a revolving turret to facilitate doing large number of operations on a job with minimum wastage of time. An index able square tool post is provided on the cross- slide for mounting the turning and parting-off tools.
The turret usually accommodates six tools for different operations like drilling, countersinking, reaming, tapping etc. which can be successively brought into working positions by indexing the turret. Some special tool holders to perform simultaneous multi-tool operations are also available. They are widely used for repetitive batch production.
(iv) Capstan Lathe:
These are similar to turret lathe and incorporate capstan slide which moves on an auxiliary slide and can be clamped in any position. It is best suited for fast production of small parts because of its light weight and short stroke of capstan slide.
(v) Tool Room Lathe:
It is the modern engine lathe which is equipped with all necessary accessories for accurate tool room work. It is a geared head driven machine with considerable range in spindle speeds and feeds. These are best suited for production of small tools, dies, gauges, etc.
(vi) Bench Lathe:
In is a small lathe which can be mounted on the work bench for doing small precision and light jobs.
(vii) Gap Bed Lathe:
In these lathes, a gap is provided on the bed near the headstock with a view to handle jobs having flanges or some other protruding parts. Very often a removable portion is provided in the bed so that when not required, it can be inserted-
(viii) Hollow Spindle Lathes:
These lathes are provided with spindles having large through bores in order to facilitate turning the ends of long tubular workpiece. The long jobs are supported on a steady or some other outboard support.
(ix) Vertical Turret Lathes:
These have vertical orientation and are used for turning large components which can be conveniently mounted on the machine table. The turret head moves in two axes to enable turning, boring and facing.
Apart from the above types, there are special purpose lathes such as Crank Shaft, Car-wheel, Multi-cut and Duplicating lathes, etc.
6. Accessories
Supplied with Lathe: (Fig. 12.14)
Accessories generally supplied with a lathe are as below:
I. Standard Accessories:
(i) Driving plate,
(ii) Reduction sleeve,
(iii) Dead centres,
(iv) Set of service tools,
(v) Operator’s manual.
II. Special Accessories:
(i) Face plate (with jaws),
(ii) Face plate (without jaws),
(iii) Live centre,
(iv) Self-claming chuck,
(v) Collet chuck,
(vi) Steady rest,
(vii) Follow rest,
(viii) Roller steady rest,
(ix) Spring loaded head stock centre,
(x) Sping loaded centre with face driver,
(xi) 3-jaws self-centering chuck,
(xii) 4-jaws independent chuck,
(xiii) Rear tool holder,
(xiv) Square turret tool post.
(xv) Drill holder,
(xvi) Longitudinal stop,
(xvii) Longitudinal bar stop,
(xviii) Machine lamp,
(xix) Splash guard,
(xx) Coolant equipment,
(xxi) Splash guard,
(xxii) Taper turning attachment,
(xxiii) Hydro-copying attachment.
7. Preparing Lathe for Operation
:
Before starting the work on lathe, it must be inspected for safe and proper operation as follows. The various parts should be cleaned and lubricated using lubricant grade specified by the manufacturer. It should be ensured that lathe is not locked or engaged in back gear. It should be ensured that there is no binding by moving the carriage along the ways. The cross slide movement should not have any play, which can be adjusted by the jibs.
The desired work holding attachment is then mounted after leaning the spindle nose and applying a drop of lub oil on threaded nose spindle before attaching the chuck or face-plate. Drive mechanism should be adjusted for the desired speed and feed. Tailstock should be properly aligned, if to be used.
The tool holder is mounted in the tool post. To avoid tool chatter, the excessive overhang of compound rest should be avoided. The work is mounted then, ensuring adequate clearance between work and the various machine parts.
8. Safety Guidelines for Working on Lathe
:
i. Before operating the machine, one should fully understand its operation controls, and how to stop it.
ii. All safety guards should be in position.
iii. Limit switches should not be meddled with or removed.
iv. Since loose garments are likely to be caught in the revolving job, these should be avoided.
v. Long and unruly hair are not desirable for safety.
vi. Safety goggles are preferred to avoid damage to eyes by flying chips.
vii. Avoid wearing a ring, bracelet, or a watch.
viii. Avoid taking measurements with caliper, micrometre gauge when the job is in motion.
ix. For cleaning, oiling and setting, the machine should be stopped.
x. Good housekeeping, clean and tidy work area free from oil and chips is essential to avoid accidents.
xi. One should not play with swarf.
xii. One should not run in workshop.
xiii. Machine should not be left running but operator should be alert when doing a job.
xiv. Before starting lathe it should be ensured that the chuck or face plate is mounted securely.
xv. No attempt should be made to mount or remove the chuck from the spindle using machine power.
xvi. It is preferable to keep a wooden cradle below chuck on bed guideways when mounting/removing it.
xvii. Chuck key should be removed after tightening the job in the chuck.
xviii. Before starting the lathe spindle by power, lathe spindle should be revolved by one revolution by hand to make it sure that no fouling is there.
xix. Safe distance from revolving chuck should be maintained.
xx. Revolving chuck should never be stopped by hand.
xxi. One should not rest/kneel on the revolving chuck even when stationary.
xxii. Tool should be properly ground, fixed at correct height and properly secured, and work should also be firmly secured.
xxiii. Tools and instruments should not be placed over lathe bed.
xxiv. It should be ensured that carriage is free to move, tailstock properly secured for turning between centres.
xxv. Chips should not be allowed to wind around a revolving job and cleared as often as possible.
xxvi. No attempt should be made to clean the revolving job with cotton waste.
9. Precautions on Lathe Operation
:
i. Sliding parts of the lathe should be cleaned and lubricated periodically.
ii. Speed selector control should not be changed when the spindle is rotating.
iii. Before moving the carriage, the carriage clamping screw should be unlocked.
iv. If chuck is moving forward, reverse switch should not be operated.
v. Chuck should be secured properly.
vi. Chips should be cleared using a brush.
vii. The thread grooves of the lead screw should be cleaned occasionally using a piece of cotton cord.
viii. Excess tightening of tailstock centre or heavy roughing cuts may cause excess heat and scour the centre or burn the tip. To avoid this, the centre should be lubricated frequently.
ix. The jibs of the cross slide and the top slide should be checked and adjusted periodically.
x. Correct size of lathe centres should be used.
xi. On hearing unusual noise, machine should be stopped.
Care of Lathes:
Guiding surfaces of lathes must be protected from damage at all times since the accuracy of the lathe depends on them. Spare tools should never be kept on the machine bed. Lubrication procedure should be followed strictly. The swarf should be removed at the earliest opportunity.
The alignment of various parts (spindle, tailstock and guideways) should be checked from time to time. The machine should never be loaded beyond its capacity. Guards and other safety devices should never be removed. Electric wiring should be renewed as and when necessary.
10. Check-Out Procedures for Operation on Lathe
:
Before operation on lathe, it must be inspected for safe and proper operating condition.
The various check-out procedures are:
i. To clean and lubricate the lathe using the recommended type and grades of lubricants. In fact cleaning is essential to maintain the accuracy built into the lathe. The accumulated chips should be removed using 50 mm paint brush. No attempt should be made to remove chips by hand or by air hose. All painted surfaces should be wiped with a soft cloth.
The tailstock should be taken to the extreme right on the ways and the dirty oil, chips and dirt removed from the machine ways using a soft cloth. A light coating of machine oil on the machined surfaces is recommended to prevent rusting. The lead screw can be cleaned by wrapping a piece of heavy cord around it and rotating the screw at slow speed and permitting the cord held in both hands to feed along the threads.
ii. All guards should be in their position.
iii. Check the carriage for binding by moving it along the ways. If binding is observed, it should be corrected.
iv. Ensure that the spindle is not locked or engaged in back gear. This is done by loosening belt tension and turning the spindle by hand.
v. Check for play in the cross slide movement. If same is noticed, the gibs should be adjusted according to the instruction of the manufacturer.
vi. Mount the desired work-holding attachment after cleaning the spindle nose with a soft brush. The tapered section of the spindle must be carefully cleaned before attempting to mount the chuck. The threaded nose spindle must have a drop of lub oil applied before attaching the chuck or face plate.
vii. Adjust the drive mechanism for the desired speed and feed.
viii. Adjust tailstock position to accommodate the length of work. The tailstock should be locked on the ways by tightening the clamp bolt nut. The fine adjustment is done by rotating the hand wheel and it should then be locked in position by tightening the binding lever.
ix. Mount the work and check for adequate clearance between work and various machine parts.
x. Clamp the cutter bit into the appropriate tool holder and mount it in the tool post. It should be ensured that compound rest overhang is not much, which otherwise would result into tool chatter and a poorly machined surface.
xi. It should also be ensured that tool height is at centre of workpiece.
xii. It should be ensured that no tools are placed on the lathe ways or carriage before finally starting the machine and powering the spindle.
11. Safety in Using Lathe
:
Operator should follow various precautions to ensure safe working conditions. Sleeves should be rolled up, and rings, wrist watch, jewellery and neck tie removed. Tools should never be placed over lathe ways or carriage. Lathe should be cleaned at regular intervals using a paint brush to remove the accumulated chips.
All painted surfaces should be wiped with a soft cloth. The tailstock should be moved to the extreme right on the ways and remaining dirty oil, chips and dirt from the machined surfaces removed by using a soft cloth. Air hose should never be used for removing chips as flying chips are injurious.
If machine is not to be used for some time, then a light coating of machine oil should be applied on the machined surfaces to prevent rust. Lead screw should be cleaned occasionally.
Use of apron and safety goggles is must. All works should be clamped solidly. Correct size tool and work-holding devices should be used. Work must be checked frequently when being machined between centres, as the work may expand due to heating up and may thus damage the tailstock centre.
All guards should be in place before starting the lathe. It should be ensured by turning the faceplate or chuck by hand that work will not bind or strike any part of the lathe. Chips should never be removed by hand. Small diameter work should not be allowed to project too far from the chuck without support from the tailstock.
The cutting tool should never be allowed to run into the chuck or dog. Ample clearance must be made for tool when it is moved left to the farthest point to be machined.
If the work is to be repositioned or removed from the lathe, the cutting tool should be moved clear of the work to prevent accidental injuries. Machine should never be tried to be stopped by running in reverse direction as chuck could spin off and cause serious injury.
Before engaging automatic feed, one should be aware of the direction of travel and speed of carriage. Key should always be removed from the chuck. Tools must be placed on a tool board or the lathe tray. If odd noise or vibration develops, lathe should be stopped immediately and should not be operated till the fault is cleared.
Sharp edges and burrs should be removed before removing the work from lathe. Chips can be removed with a brush or short stick to ensure that these do not stick in recesses.
12. Use of Angle Plates
:
Fig. 12.50 shows the use of angle plates in conjunction with face plates for machining workpiece like elbow pipes. Angle plate is a cast iron plate having two faces machined to make them absolutely at right angles to each other.
Holes and slots provided in angle plate enable it to be bolted to face plate and also clamp the workpiece. In order to overcome effect of eccentricity and reduce vibrations, counter-weights have to be added suitably at other end of the face plate.
13. Parting Operation
:
In parting operation the material is cut off after machining. It is important that the cutting tool is ground with the correct front, side and end clearances on the tool. A concave rake should be ground on the top of the cutter to reduce the width of the chip. (Refer Fig. 12.51 (a)).
This helps in preventing the seizure/binding of the tool in the groove. The cutting edge of the tool must be kept sharp to permit easy penetration into the work, otherwise the tool may slip and even break due to sudden digging by building up of pressure.
Work should be held close to the chuck and set exactly at 90° to the work surface (Refer Fig. 12.51 (b)). The cutting edge should be set on centre when cutting off stock 25 mm or less in diameter and 1.5 mm above centre for each 25 mm large diameter. The tool must be lowered as the work diameter is reduced.
Spindle speed is selected about one-third of that used for turning operation. It is also necessary to tighten the compound and cross slide to prevent play. Ample feed should be employed to provide a continuous chip. If slow feed is used, hogging will result, i.e., the tool will not cut continuously but will ride on the surface for a revolution or two, and then bite in suddenly. Enough coolant should be used.
14. Finish Turning
:
During rough machining maximum metal is removed and very little oversize dimension is left for finish machining. A right-cut finishing tool as shown in Fig. 12.52 is used. It is mounted and centred. It should be checked and ensured that adequate clearance exists between compound rest and the revolving lathe dog.
When reversing the work to permit machining its entire length, the section under the lathe dog set screw should be protected by inserting a piece of soft aluminium or copper sheet.
15. Grooving or Necking Operation and Under-Cutting
:
Some workpiece require turning of groove or neck in order to terminate a thread or to provide adequate clearance for mating pieces. Fig. 12.53 shows the three types of grooves that could be provided. The round groove is preferable compared to square which tends to weaken the shaft.
For cutting groove, the tool is set on centre and fed in until it just touches the work surface. The micrometer dial is set on the cross-feed screw to zero and the tool fed in to the desired depth. Square grooves are machined using a parting tool.
Under-Cutting:
It is similar to grooving operation but is performed inside a hole. Square nose parting tool is used for this purpose. It is done at the end of an internal thread or a counter bore to provide clearance for the tool or any mating part under cutting operation.
Shoulder Turning:
In some cases, it is required to turn the piece to several different diameters. The surface forming the step from one diameter to the other is called the shoulder. Four different kinds of shoulders are shown in Fig. 12.55. The points to which different diameters are to be cut are located and lines scribed with a hermaphrodite caliper.
A right-cut tool is used to make the square and angular type shoulder. A round nose tool is ground to the proper radius using a fillet and is used to machine filleted shoulder. While machining on angular shoulder, the cut is made from the smaller diameter to the larger diameter.
It is the operation of bevelling the extreme end of a workpiece. Chamfer is provided for better look, to enable nut to pass freely on threaded workpiece, to remove burrs and protect the end of the workpiece from being damaged. Fig. 12.56 shows the operation of chamfering which is usually performed after knurling, thread cutting, etc.
It is the process of embossing a diamond shaped regular pattern on the surface of a workpiece using a special knurling tool. This tool consists of a set of hardened steel rollers in a holder with the teeth cut on their surface in a definite pattern.
The tool is held rigidly on the tool post and the rollers are pressed against the revolving workpiece to squeeze the metal against the multiple cutting edges. The knurling tool may have one or two rollers.
Usually three sets of two rollers (rough, medium and fine pitches) are mounted on a revolving holder. Any one set may be brought into operation by revolving the unit. Knurling is done at the slowest speed and plenty of oil is used.
The purpose of knurling is to provide an effective gripping surface on a workpiece to prevent it from slipping when operated by hand.
To ensure the specified accuracy of machining is a key requirement placed on the manufacturing process. Machining accuracy can be interpreted as the degree to which the machined part meets the accuracy specifications established by its engineering drawing.
The accuracy of the part encompasses its dimensions, form, relative position of its surfaces, and surface finish. The accuracy of form means the conformity of a surface to a true geometrical form in an axial and a cross section.
When planning a manufacturing process, account must be taken of factors that give rise to machining errors. In lathe work, these factors are mainly the lack of accuracy and stiffness of the machine tool, cutting tool, and tool holder; errors in locating the work and its deformations caused by clamping, cutting forces, and heating (Fig. 12.57); measuring errors; and some other factors.
In the course of machining, the machining complex (the machine-fixture-tool-workpiece structural loop) is deformed by the cutting forces. For instance, the live centre can deflect an amount h1 (Fig. 12.58), and the dead centre, h2, relative to the axis OO of the unloaded machine. The workpiece will deflect an amount h3 and the tool carriage, h4. Actually, these deflections may appear in combination or singly, resulting in workpiece cylindricity errors (Fig. 12.59).
In order to ensure the accuracy of the finished part, the sequence of operations is taken from the following considerations:
a. Machining is started with roughing cuts, where the maximum amount of stock is removed. This helps expose the workpiece defects and relieve the internal stresses that cause deformations. All roughing operations involve considerable cutting forces that affect machining accuracy; therefore, roughing must precede finishing cuts.
b. Workpiece surfaces where defects are impermissible should be machined at the initial stages of the manufacturing process.
c. Work surfaces whose machining lowers the stiffness of the work to the least degree should be machined first of all.
d. Finishing operations must be performed at the final stages of the process in order to minimize the chance of damaging the surfaces already machined.
e. Workpiece surfaces with strictly tolerance relative position should be machined in a single setup.
Single pieces are machined to size by removing the stock in consecutive cuts. The workpiece is measured before each cut to find the machining allowance and the necessary depth of cut. This is repeated until the size of the part is within the specified tolerance.
When a job lot is to be processed, only the initial work part is machined as described above to determine the necessary relative position of the tool and the part, and then all the other parts of the lot are machined with this setting.
The requirements placed on the operator skill, on the accuracy of the machine, cutting tool, fixture, workpiece measurement, etc., grow stricter with the accuracy of the finished part, that is, higher accuracy of machining takes more time and labour.
That is why distinctions are made between the economically feasible and the maximum possible accuracy of machining. The economically feasible accuracy is relative; it determines the choice of a machining method that provides for specified accuracy achieved with minimum labour inputs at the shortest working time. Fig. 12.60 illustrates the cost of machining as a function of machining accuracy.
The maximum possible accuracy can be achieved by a highly skilled machinist under working conditions which ensure machining with the specified accuracy. Comparison between the feasible and the maximum possible accuracy can serve as a measure of how well a given production process has been designed.