Bed is mounted on the legs of the lathe which are bolted to the floor. It forms the base of the machine. It is made of cast iron and its top surface is machined accurately and precisely. Headstock of the lathe is located at the extreme left of the bed and the tailstock at the right extreme. Carriage is positioned in between the headstock and tailstock and slides on the bed guideways.
The top of the bed has flat or ‘V’ shaped guideways. The tailstock and the carriage slides on these guideways. Inverted ‘V’ shaped guideways are useful in better guide and accurate alignment of saddle and tailstock. The metal burrs resulting from turning operation automatically fall through. Flat bed guideways can be found in older machine tools. It is useful in heavy machines handling large workpieces. But then the accuracy is not high.
Headstock is mounted permanently on the inner guideways at the left hand side of the leg bed. The headstock houses a hollow spindle and the mechanism for driving the spindle at multiple speeds. The headstock will have any of the following arrangements for driving and altering the spindle speeds
- Stepped cone pulley drive
- Back gear drive
- All gear drive
The spindle rotates on two large bearings housed on the headstock casting. A hole extends through the spindle so that a long bar stock may be passed through the hole. The front end of the spindle is threaded on which chucks, faceplate, driving plate and catch plate are screwed. The front end of the hole is tapered to receive live center which supports the work. On the other side of the spindle, a gear known as a spindle gear is fitted. Through this gear, tumbler gears and a main gear train, the power is transmitted to the gear on the leadscrew.
Tailstock is located on the inner guideways at the right side of the bed opposite to the headstock. The body of the tailstock is bored and houses the tailstock spindle or ram. The spindle moves front and back inside the hole. The spindle has a taper hole to receive the dead centre or shanks of tools like drill or reamer. If the tailstock handwheel is rotated in the clockwise direction, the spindle advances. The spindle will be withdrawn inside the hole, if the handwheel is rotated in anti-clockwise direction.
To remove the dead centre or any other tool from the spindle, the handwheel is rotated in anticlockwise direction further. The movement of the spindle inside the hole may be locked by operating the spindle clamp located on top of the tailstock.In order to hold workpieces of different lengths, the tailstock can be locked at any desired position on the lathe bed. Tailstock clamping bolts and clamping pates are used for this purpose.
Tailstock is designed to function as two units-the base and the body. The base of the tailstock is clamped to the bed. The body is placed on the base and can be made to slide sidewards-perpendicular to the bed guideways upto a certain distance.
The uses of tailstock
- It supports the other end of the long workpiece when it is machined between
- It is useful in holding tools like drills, reamers and taps when performing drilling, reaming and
- The dead centre is off set by a small distance from the axis of the lathe to turn tapers by set over
- It is useful in setting the cutting tool at correct height aligning the cutting edge with lathe
Carriage is located between the headstock and tailstock on the lathe bed guideways. It can be moved along the bed either towards or away from the headstock. It has several parts to support, move and control the cutting tool. The parts of the carriage are :
- compound rest
- compound slide
- tool post
It is an “H” shaped casting. It connects the pair of bed guideways like a bridge. It fits over the bed and slides along the bed between headstock and tailstock. The saddle or the entire carriage can be moved by providing hand feed or automatic feed.
Cross-slide is situated on the saddle and slides on the dovetail guideways at right angles to the bed guideways. It carries compound rest, compound slide and tool post. Cross slide handwheel is rotated to move it at right angles to the lathe axis. It can also be power driven. The cross slide hand wheel is graduated on its rim to enable to give known amount of feed as accurate as 0.05mm.
Compound rest is a part which connects cross slide and compound slide. It is mounted on the cross-slide by tongue and groove joint. It has a circular base on which angular graduations are marked. The compound rest can be swiveled to the required angle while turning tapers. A top slide known as compound slide is attached to the compound rest by dove tail joint. The tool post is situated on the compound slide.
This is located on top of the compound slide. It is used to hold the tools rigidly. Tools are selected according to the type of operation and mounted on the tool post and adjusted to a convenient working position. There are different types of tool posts and they are:
– Single screw tool post
– Four bolt tool post
– Four way tool post
– Open side tool post
Single screw tool post
The tool is held by a screw in this toolpost. It consists of a round bar with a slotted hole in the centre for fixing the tool by means of a setscrew. A concave ring and a convex rocker are used to set the height of the tool point at the right position. The tool fits on the flat top surface of the rocker. The tool post is not rigid enough for heavy works as only one clamping screw is used to clamp the tool.
Four way tool post
This type of tool post can accommodate four tools at a time on the four open sides of the post. The tools are held in position by separate screws and a locking bolt is located at the centre. T h e required tool may be set for machining by swiveling the tool post. Machining can be completed in a shorter time because the required tools are pre-set.
There are several mechanisms to make the carriage and crose-slide move automatically and to change the direction of their movement. Some important mechanisms are dealt with as follows.
Tumbler gear arrangement
Tumbler gears are located in the headstock just below the spindle gear. For the purpose of moving the carriage towards or away from the headstock, this mechanism along with feed rod or leadscrew is used.
Tumbler gears are two small pinions mounted on a bracket. This bracket is pivoted about the axis of the stud gear. The position of the bracket can be arranged in three different stages namely i) neutral ii) forward & iii) reverse. Hence, the direction of rotation of the leadscrew and the feed rod is reversed.
When the bracket is held in neutral position, both the tumbler gears A and B stand disengaged from the spindle gear and main gear train. And so carriage will not get any movement.
When the bracket is arranged in the forward position as shown in the diagram, only one of the tumbler gears(B) comes between the spindle gear and the main gear train. In that position, the leadscrew or the feed rod rotates in the direction of the headstock spindle rotation. The carriage moves towards the headstock providing the cutting tool with longitudinal feed.
When the bracket is arranged in the reverse position, both the tumbler gears come in contact between the spindle gear and the main gear train. The carriage moves towards the tailstock to give the tool the longitudinal feed in the opposite direction.
Apron is attached to the carriage and hangs over the front side of the lathe bed. It is useful in providing power and hand feed to both carriage and cross-slide. It is also used to provide power feed to the carriage during thread cutting through two half nuts.
Power is transmitted from the spindle to the leadscrew and feed rod through the spindle gear and tumbler gear arrangement. A worm is mounted on the feed rod by a sliding key. The worm meshes with a worm gear on whose axis another gear G1 is attached. Gear G1 is attached to a small gear G2 by a bracket as shown in the diagram. Gear G4 is positioned to be in mesh with the rack gear always. Another gear G3 is mounted on the same axis of gear G4. The carriage handwheel meant for longitudinal feed is attached to the gear G5 on the same axis. The gears G3 and G5 are always in mesh. The gear G6 is attached to the cross slide screw.
- Hand feed and power feed to the carriage
- Hand feed and power feed to the cross slide
Hand feed to the carriage
Feed selection lever is kept in neutral position and the carriage handwheel is rotated. The gear G4 attached to the rack gets rotation through the gears G5 and G3. The carriage moves longitudinally.
Power feed to the carriage
When feed selection lever is kept in up position (U), the gear G2 will mesh with gear G3. Gear G4 gets rotation through gear G3 and the carriage gets automatic (power) feed.
Hand feed to the cross slide
Feed selection lever is kept in neutral position. The cross slide will move on rotation of the cross slide handwheel.
Power feed to the cross slide
When the feed selection lever is kept in down position (D), gear G2 will be in contact with gear G6. The rotation of G6 will make the cross slide screw also to rotate and the cross-slide moves automatically.
Power feed to the carriage for thread cutting
When the two half nuts in the apron are made as one unit, leadscrew makes the carriage to move automatically and cut threads of required pitch value.
The feed selection lever can be kept in neutral, up and down positions to obtain the following movements.
The leadscrew is a long threaded shaft used as master screw. It is brought into operation during thread cutting to move the carriage to a calculated distance. Mostly leadscrews are Acme threaded.
The leadscrew is held by two bearings on the face of the bed. A gear is attached to the lead screw and it is called as gear on leadscrew. A half nut lever is provided in the apron to engage half nuts with the leadscrew.
Leadscrew is used to move the carriage towards and away from the headstock during thread cutting. The direction of carriage movement depends upon the direction of rotation of the leadscrew.When the leadscrew is kept stationary, the half nuts are engaged with the leadscrew to keep the carriage locked at the required position.
Feed rod is placed parallel to the leadscrew on the front side of the bed. It is a long shaft which has a keyway along its length. The power is transmitted from the spindle to the feed rod through tumbler gears and a gear train. It is useful in providing feed movement to the carriage except for thread cutting and to move cross-slide. A worm mounted on the feed rod enables the power feed movements.
The spindle is located in the headstock and it receives the driving power from the motor. The spindle speed should be changed to suit different machining conditions like type of material to be cut, the diameter and the length of the work, type of operation, the type of cutting tool material used, the type of finish desired and the capacity of the machine. In order to change the spindle speeds, any one of the following methods are employed.
- Step cone pulley drive
- Back geared drive
- All geared drive
Step cone pulley drive
It is simple in construction. The belt is arranged on the four different steps of the cone pulley to obtain four different speeds.
A step cone pulley is attached with the spindle contained within the headstock casting. The cone pulley has four steps (A, B, C & D). Another cone pulley having four steps (E, F, G and H) is placed parallel to the spindle cone pulley. Both the cone pulleys are connected by a flat belt. The belt can be arranged between the steps A & H, B & G, C & F and D & E. The cone pulley at the bottom is connected to the electric motor by a ‘V’belt. So the cone pulley at the bottom rotates at a particular speed.
The belt is arranged on any of the four steps to obtain different spindle speeds. The spindle speed is increased if the belt is placed on the smaller step of the driven pulley. The spindle speed will be maximum when the belt is arranged between A & H and the speed will be minimum when the belt is arranged between D & E.
Back gear mechanism
Back gear mechanism is housed within the headstock of the lathe. A step cone pulley having steps ABCD and a small pinion ‘P’ are mounted on the spindle and rotates freely on it. The gear ‘S’ is keyed to the headstock spindle. So, the spindle will rotate only when the gear ‘S’ rotates.
The step conepulley ABCD and the gear ‘S’ can be kept seperately or made as one unit with the help of a pin ‘T’. When the pin is disengaged, the conepulley along with the gear P will rotate freely on the spindle and the spindle will not rotate. There is another shaft parallel to the spindle axis having back gears Q and R mounted on it. These back gears can be made to mesh with gears P and S or kept disengaged from them. The spindle can get drive either from the cone pulley or through back gears.
Drive from step conepulley
When the spindle gets drive from the conepulley, the backgears Q and R are disengaged from the gears P and S. The pin ‘T’ is engaged with conepulley. The belt can be arranged on the steps A,B,C or D to get four different direct speeds for the spindle.
Drive through back gears
Back gears Q and R are engaged with gears P and S. The pin ‘T’ is disengaged from the conepulley to make the conepulley and the spindle separate units. When the conepulley gets drive through the belt, the power is transmitted through the gears P,Q and R to the gears. Because of number of teeth on these gears, the spindle rotates at slower By arranging the belt on the different steps of the conepulley, four different spindle speeds are obtained.
Uses of back gear arrangement
- The spindle gets four direct speeds through the conepulley and four slower speeds through the back
- Slower speeds obtained by this arrangement are useful when turning on larger workpieces and cutting coarse
All geared headstock
Modern lathes are equipped with all geared headstocks to obtain different spindle speeds quickly. Casting of the all geared headstock has three shafts (1,2& 3) mounted within it. The intermediate shaft(2) has got three gears D, E and F as a single unit and rotate at the same speeds. The splined shaft (1) which is above the intermediate shaft has got three gears A, B and C mounted on it by keys. These three gears can be made to slide on the shaft with the help of a lever. This movement enables the gear A to have contact with the gear D or the gear B with gear E or the gear C with the gear F.
Likewise the spindle shaft(3) which is also splined has three gears G, H and I. With the help of a lever, these three gears can be made to slide on the shaft. This sliding move- ment enables the gear G to have contact with gear D or the gear H with the gear E or the gear I with the gear F. By altering the positions of the six gears namely A, B, C, G, H and I the following arrangements can be made within the headstock. Nine different spindle speeds are obtained.
The gear combinations are