Description of various parts of lathe has been given below:
Part # 1. Bed:
It is supported on broad box-section columns and is made of cast iron. Its upper surface is either scraped or ground and the guiding and sliding surfaces are provided. The bed consists of two heavy metal slides running lengthwise, with ways or V’s formed upon them. It is rigidly supported by cross girths. The outer guide ways provide bearing and sliding surfaces for the carriage, and the inner ways for the tailstock.
Three major units mounted on bed are:
i. The head-stock,
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ii.The tailstock and
iii. The carriage.
The scraped or ground guiding and sliding surfaces on the lathe bed ensure the accuracy of alignment of these three units. The headstock is permanently fixed to the bed, the tailstock is adjustable for position to accommodate workpiece of different lengths. The carriage can be traversed to and between the headstock and the tailstock either manually or by power.
Lathe bed is made of high grade special cast iron having high vibration damping qualities. Lathe bed is secured rigidly over cabinet leg and end leg and all other parts are fitted on it. (Refer Fig. 12.2).
Top surface of bed is machined accurately. The important considerations in design of lathe bed are its rigidity, alignment and accuracy. In its use, every care should be taken to avoid formation of scratches, nicks and dents by falling tools/spanners and it should be lubricated regularly to avoid rusting.
The lathe bed being the main guiding member for accurate machining work, it should be sufficiently rigid to prevent deflection under cutting forces; should be massive with sufficient depth and width to absorb vibrations; should be designed to resist the twisting stresses set up due to resultant of two forces; should be seasoned naturally to relieve the stresses set up during casting.
Fig. 12.3 (a) shows a section through a typical lathe bed. It may be noted that while top surfaces take the weight of the head-stock, carriage and tailstock, narrow surfaces like ‘G’ act as guiding surfaces for movement of carriage and tailstock.
Fig. 12.3 (b) shows some alternative arrangements of guides and supports. It is important to note that guiding surfaces should not be too far apart to avoid cross-wind or jamming effect.
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End of Bed Gearing:
The motion of head-stock spindle is also transmitted to the feed gearbox through the gearing at the end of the bed. The drive is then transmitted through the feed gear box; selectively to the lead screw or the feed shaft, depending on whether the machine is being used for screw cutting or plain turning.
The position of end of bed gearing ensures a constant relationship between the traverse speed of the carriage and the spindle speed, irrespective of how the latter may be varied. Provision is also made for reversing the direction on the traverse of the carriage at the end of the bed gearing. The traverse rate of carriage on the bed is controlled by means of the gearing at the end of the bed in conjunction with the feed gearbox.
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Most of the latest types of lathes have the reversing mechanism built into the headstock and actuated by means of a separate lever. In certain lathes, the lead screw and feed shaft may be run, constantly in one direction, and the reversal of the saddle traverse is obtained by means of reverse gearing in the apron.
The changes in the feed rate normally required for turning or threading can be made in the feed gearbox, but when special threads have to be cut, it may be necessary to alter the ratio of the gearing at the end of the bed.
Part # 2. Head Stock:
It supports the main spindle in the bearings and aligns it properly. It also houses necessary transmission mechanism with speed changing levers to obtain different speeds. Cone pulley or gears or combination of both could be used to change speed of spindle.
Accessories mounted on head stock spindle:
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1. Three jaw chuck
2. Four jaw chuck
3. Lathe centre and lathe dog
4. Collet chuck
5. Face plate
6. Magnetic chuck.
The complete head stock consists of the headstock casting which is located on the ways of the bed at the left side of the operator, the hollow spindle in which the live centre is rigidly held by a taper, and the necessary gears and mechanisms for obtaining the various spindly speeds.
The centre line of the headstock is parallel to the guide ways, in both horizontal and vertical planes. All the modern lathes employ all-geared headstock. However, where greater simplicity and low cost are the criteria, cone-drive headstock can be used. A geared headstock may be driven either direct from a line shaft or from an independent motor, the drive being transmitted to the constant speed main drive pulley.
Headstock also incorporates the self- contained clutch and brake mechanism by which the pulley may be coupled to the shafting in the headstock, as required. Usually arrangements are provided so that when the pulley is running free the spindle is braked automatically.
Sliding gearing is generally employed for obtaining the various speed changes, the gears being mounted on multi spindle shafts and traversed axially thereon by external control levers through selector mechanism.
A separate speed change gearbox is placed below headstock to reduce the speed in order to have different feed rates for threading and automatic lateral movements of carriage. The feed shaft is used for most turning operations and lead screw is used for cutting threads etc.
Part # 3. Main Spindle:
It is a hollow cylindrical shaft and long slender jobs can pass through it. The spindle end facing the tailstock is called the spindle nose. The spindle nose has a morse taper hole (self-locking taper) and threads on outside. The morse taper is used to accommodate lathe centre or collet chuck and threaded portion for chuck or face plate. The design of the lathe spindle and its bearings forms important feature, as the thrust of the cutting tool tends to deflect the spindle.
Anti-friction bearings are used in the headstock, and the spindle, which is made of high-tensile steel suitably hardened and tempered, is supported in roller bearings. The front spindle bearings take both the axial and radial loads on the spindle and the rear bearing is so designed that the spindle may float axially from the front bearings to allow the expansion and contraction. The front bearing is made adjustable by a flanged plate and is preloaded in assembly to avoid any possibility of slackness during the cut, with consequent vibration.
The spindle nose is designed for rapid mounting and removal of chucks and fixtures on it, and also for positioning them accurately and securely. For this purpose, screwed type spindle nose with two locating cylindrical surfaces in front and rear, and threads in between is used. The overhang of the spindle nose is kept to minimum to guard against bending.
The spindle is made hollow to allow long bars to pass through. On the front side, it has a taper socket to mount a live centre which rotates with the spindle. The various face plates and chucks are secured to the flange of the spindle nose by bolts or studs, and positioned by taper spigot.
Part # 4. Tail Stock:
It is movable casting located opposite the headstock on the ways of the bed.
It is used for two purposes:
(i) To support the other end of the work when being machined, and
(ii) To hold a tool for performing operations like drilling, reaming, tapping, etc.
It contains the dead centres the adjusting screw and the hand wheel.
The body of the tailstock is adjustable on the base which is mounted on the guideways of the bed and can be moved to and fro. The object of making the body adjustable on the base is to provide means for lining up the centre, carried in the moving spindle, with the headstock centre, or for off-setting this centre to permit tapers to be turned.
Axial adjustment of the dead centre in the movable spindle in the tailstock body is provided for by means of a hand-wheel, which is attached to a screw engaging the nuts in the rear of the movable spindle. It can be located by any position in the body by means of a lever. The spindle is bored or ground to a taper gauge to take centre which may be of the fixed or revolving type.
Fig. 12.6 shows sketch of tailstock. Tailstock spindle 4 is displaced in body 1 by turning screw 5 with the hand wheel 7. Spindle is fixed in position by operating lever 3. The spindle carries a taper-shank centre 2. The tailstock is moved along the machine’s slide ways by hand or with the aid of the saddle.
The tailstock can be locked with lever 6, which is connected to the rod 8 and lever 9. Clamping pressure can be adjusted with nut 11 and screw 12. Greater force in clamping can be exerted with nut 13 and screw 14 which holds lever 10 against the bed.
Part # 5. Carriage (Fig. 12.7):
It is located between the headstock and the tailstock. It is fitted on the bed and slides along the bed guide ways and can be locked on the bed at any desired position by tightening the carriage lock screw. It can be moved manually with a hand wheel or with power feed.
It consists of saddle and apron and slides over the ways between the headstock and tailstock. It has the form of letter H and is bridged across the lathe bed to carry the cross slide, compound rest, and tool rest, and is fitted to the outside ways and gibed to the bed. It also carries the compound rest.
The saddle carrying the cross slide and tool post can be locked in any position when carrying out surfacing operation. For turning, the carriage is driven by a feed shaft, which rotates a pinion mounted in a casting at the front of the carriage (apron).
This union engages with a rack along the front of the bed, so that it pulls itself and the carriage along the bed. For screw cutting, the carriage movement is obtained by engaging a split nut over the lead screw which then rotates in relation to the spindle rotation.
It provides three movements to the tool:
(i) Longitudinal feed—through carriage movement.
(ii) Cross feed—through cross slide movement.
(iii) Angular feed—through top slide movement.
It consists of following 5 main parts:
(a) Apron
(b) Saddle
(c) Compound rest consisting of a swivel and top slide
(d) Cross slide
(e) Tool post
(a) The Apron (Fig. 12.8):
It is fastened to the saddle and hangs over the front of the bed. It contains the gears and clutches for transmitting motion from the feed rod to the carriage, and the split nut which engages with the lead screw during cutting threads. It converts the rotary motion of the feed shaft or the lead screw to a translatory motion of the carriage longitudinally on the bed or of the cross slide transversely on the carriage.
The lead screw is coupled to the carriage by means of a split nut fixed in the apron, and the feed-shaft generally drives the carriage through worm gearing. Two types of aprons are extensively used, one type incorporating drop worm mechanism and the other friction or dog clutches.
The apron is fitted to the front position of the saddle facing the operator. It consists of a hand wheel for saddle movement, pinion to engage with the rack for saddle movement, a lever to engage the automatic feed for the saddle, automatic feed clutch, split nut (half nut), and lead screw. It houses control of carriage and cross slide.
It contains controls to transmit motion from the feed rod or lead screw to the carriage and the cross slide. It houses gears, levers, hand wheels and clutches to operate the carriage by hand or by automatic power feed. A lever is provided to engage the split nut for cutting the thread.
(b) Saddle:
It is made up of a H shaped casting. Generally it has a V guide and a flat guide on one side for mounting it on the lathe bed guide ways. It also aids the saddle to slide along the bed guideways by operating a handwheel. The other side of saddle is provided with a male dove tail to accommodate the cross slide with a jib.
(c) Compound Rest:
It supports the tool post and cutting tool in its various positions. It may be swivelled on the cross- slide to any angle in the horizontal plane; its base being graduated suitably. A compound rest is necessary in turning angles and boring short tapers and in turning angles and forms on forming tools.
(d) Cross slide:
It is provided with a female dovetail on one side and assembled on the top of the saddle with its male dovetail. A tapered gib is provided between the saddle and cross slide dovetails to permit required fit for movement of cross slide on saddle.
Top surface of cross slide is provided with T slots to enable fixing of rear tool post or coolant attachment. Front side is graduated in degrees to facilitate swivelling of the compound rest to the desired angle.
Compound rest consists of swivel and top slide and is mounted on the cross slide.
Swivel is directly assembled on the cross slide and can be swivelled on either side to give the desired angle to the compound rest. It is provided with a male dovetail on the top surface.
Top slide is provided with a female dovetail and is assembled on the swivel with a tapered jib for adjustments. Top slide can be made to slide on the swivel by a precision screw rod and is moved manually.
With the help of the top slide, the tool post can get horizontal, perpendicular, or angular movements to one axis of the bed guide ways depending upon the position of the swivel.
(e) Tool Post:
It is used to hold various cutting tool holders. The holders rest on a wedge which is shaped on the bottom to fit into a concave-shaped ring (segmental type), which permits the height of the cutting edge to be adjusted by tilting the tool. (Refer Fig. 12.12)
It is fixed on the top slide. It gets its movement by the movement of the saddle, cross slide and top slide.
Three types of tool posts are commonly used:
1. Ring and rocker tool post.
2. Quick change tool post.
3. Square head tool post.
Ring and Rocker tool post consists of a circular tool post with a slot for accommodating the tool or tool holder, a ring, a rocker and a tool clamping screw. One side of the ring is flat and other side is spherical on which the rocker sits. The tool is clamped on the rocker. The height of the tool tip can be adjusted with the help of the ring and rocker arrangement.
Four way tool post has provision for holding four separate tool holders, which may be swivelled to a variety of positions.
The square-turret type tool holder is illustrated in Fig. 12.13. Tapered arbor 3 with a threaded end is secured in the locating bore of top slide 5. Square turret 6 is fitted over the taper.
On turning handle 4 clockwise, its boss 2 goes down on the thread of arbor 3 and exerts pressure through spacer 1 and a thrust bearing on the turret, making it sit tight on the taper. When being tightened, the turret is held against rotation by a ball, which is jammed between the surfaces formed by a slot on the arbor base and a hole in the turret.
To change the position of the tool, handle 4 is turned counter clockwise. Boss 2 goes up the thread of arbor 3, releasing turret 6. At the same time boss 2 turns turret 6 by means of friction elements which engage the annular recess on the underside of the boss. The friction elements are fastened to the turret by pins 7.
The spring-loaded ball, which is forced into its hole, does not prevent the turret from turning.
If handle 4 takes an inconvenient position when tightened, the necessary adjustment can be made by changing the thickness of spacer 1.
Part # 6. Overload Safety Devices:
Important parts of lathe like gears, feed screws, lead screws may get overloaded due to one reason or other. Shear pins and slip clutches are used to prevent damage by overloading. A shear pin is designed to be of a particular diameter and shape and can withstand a particular torque. When the designed torque is exceeded, the shear pin shears and the machine motion stops.
A slip clutch also protects the feed rod and connecting mechanisms. It is designed to release the feed rod when a specific force is exceeded. It also permits the feed rod to be automatically re-engaged when the force is reduced.