Various operations are performed in a lathe machine other than plain turning. These are:-
- Straight turning
- Step turning
- Eccentric turning
- Taper turning
- Thread cutting
Facing is the operation of machining the ends of a piece of work to produce flat surface square with the axis. The operation involves feeding the tool perpendicular to the axis of rotation of the work.
Turning in a lathe is to remove excess material from the workpiece to produce a cylindrical surface of required shape and size.
The work is turned straight when it is made to rotate about the lathe axis and the tool is fed parallel to the lathe axis. The straight turning produces a cylindrical surface by removing excess metal from the workpieces.
Step turning is the process of turning different surfaces having different diameters. The work is held between centres and the tool is moved parallel to the axis of the lathe. It is also called shoulder turning.
Chamfering is the operation of bevelling the extreme end of the workpiece. The form tool used for taper turning may be used for this purpose. Chamfering is an essential operation after thread cutting so that the nut may pass freely on the threaded workpiece.
Grooving is the process of cutting a narrow goove on the cylindrical surface of the workpiece. It is often done at end of a thread or adjacent to a shoulder to leave a small margin. The groove may be square, radial or bevelled in shape.
Forming is a process of turning a convex, concave or any irregular shape. For turning a small length formed surface, a forming tool having cutting edges conforming to the shape required is fed straight into the work.
Knurling is the process of embossing a diamond shaped pattern on the surface of the workpiece. The knurling tool holder has one or two hardened steel rollers with edges of required pattern. The tool holder is pressed against the rotating work. The rollers emboss the required pattern. The tool holder is fed automatically to the required length.
Knurls are available in coarse, medium and fine pitches. The patterns may be straight, inclined or diamond shaped.
The purpose of knurling is
- to provide an effective gripping surface
- to provide better appearance to the work
- to slightly increase the diameter of the work
Undercutting is done
- at the end of a hole
- near the shoulder of stepped cylindrical surfaces
- at the end of the threaded portion in bolts
It is a process of enlarging the diameter if done internally and reducing the diameter if done externally over a short length. It is useful mainly to make fits perfect. Boring tools and parting tools are used for this operation.
If a cylindrical workpiece has two separate axes of rotating, one being out of centre to the other, the workpiece is termed as eccentric and turning of different surfaces of the workpiece is known as eccentric turning. The distance between the axes is known as offset. Eccentric turning may also be done on some special machines. If the offset distance is more, the work is held by means of special centres. If the offset between the centres is small, two sets of centres are marked on the faces of the work. The work is held and rotated between each set of centres to machine the eccentric surfaces.
A taper may be defined as a uniform increase or decrease in diameter of a piece of work measured along its length.
Taper turning methods
- Form tool method
- Compound rest method
- Tailstock setover method
- Taper turning attachment method
- Combined feed method
Form tool method
A broad nose tool is ground to the required length and angle. It is set on the work by providing feed to the cross-slide. When the tool is fed into the work at right angles to the lathe axis, a tapered surface is generated.
This method is limited to turn short lengths of taper only. The length of the taper is shorter than the length of the cutting edge. Less feed is given as the entire cutting edge will be in contact with the work.
Compound rest method
The compound rest of the lathe is attached to a circular base graduated in degrees, which may be swiveled and clamped at any desired angle. The angle of taper is calculated using the formula
The compound rest is swiveled to the angle calculated as above and clamped. Feed is given to the compund slide to generate the required taper.
Tailstock setover method
Turning taper by the setover method is done by shifting the axis of rotation of the workpiece at an angle to the lathe axis and feeding the tool parallel to the lathe axis. The construction of tailstock is designed to have two parts namely the base and the body. The base is fitted on the bed guideways and the body having the dead centre can be moved at cross to shift the lathe axis.
The amount of setover – s, can be calculated as follows
The dead centre is suitably shifted from its original position to the calculated distance. The work is held between centres and longitudinal feed is given by the carriage to generate the taper.
The advantage of this method is that the taper can be turned to the entire length of the work. Taper threads can also be cut by this method.
The amount of setover being limited, this method is suitable for turning small tapers (approx. upto 8°). Internal tapers cannot be done by this method.
Taper attachment method
The taper attachment consists of a bracket which is attached to the rear end of the lathe bed. It supports a guide bar pivoted at the centre. The bar having graduation in degrees may be swiveled on either side of the zero graduation and set at the desired angle to the lathe axis. A guide block is mounted on the guide bar and slides on it. The cross slide is made free from its screw by removing the binder screw. The rear end of the cross slide is tightened with the guide block by means of a bolt. When the 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 of the lathe axis. The depth of cut is provided by the compound slide which is set parallel to the cross-slide.
The advantage of this method is that long tapers can be machined. As power feed can be employed, the work is completed at a shorter time. The disadvantage of this method is that internal tapers cannot be machined.
Combined feed method
Feed is given to the tool by the carriage and the cross-slide at the same time to move the tool at resultant direction to turn tapers.
Thread cutting is one of the most important operations performed in a lathe. The process of thread cutting is to produce a helical groove on a cylindrical surface by feeding the tool longitudinally.
- The job is revolved between centres or by a The longitudinal feed should be equal to the pitch of the thread to be cut per revolution of the work piece.
- The carriage should be moved longitudinally obtaining feed through the leadscrew of the
- A definite ratio between the longitudinal feed and rotation of the headstock spindle should be found Suitable gears with required number of teeth should be mounted on the spindle and the leadscrew.
- A proper thread cutting tool is selected according to the shape of the It is mounted on the toolpost with its cutting edge at the lathe axis and perpendicular to the axis of the work.
- The position of the tumbler gears are adjusted according to the type of the thread (right hand or left hand).
- Suitable spindle speed is selected and it is obtained through back
- Half nut lever is engaged at the right point as indicated by the thread chasing
- Depth of cut is set suitably to allow the tool to make a light cut on the
- When the cut is made for the required length, the half nut lever is The carriage is brought back to its original position and the above procedure is repeated until the required depth of the thread is achieved.
- After the process of thread cutting is over, the thread is checked by suitable gauges.