Turning

Turning is a metal removal process in which job is given rotational motion while the cutting tool is given linear (feed and depth of cut) motion. Different types of turning operations are mentioned as follows:

Straight Turning: It is the operation of producing a cylindrical surface of a job by removing excess material. In this operation, the job rotates and tool is fed longitudinally by giving the desired depth of cut (Figure 19.11).

Figure 19.11 Straight Turning on Lathe

Face Turning or Facing: Face turning is also known as facing operation. It is the operation of making the ends of a job to produce a square surface with axis of operation or to make a desired length of the job. In this operation job rotates and the tool advances in perpendicular direction to the axis of the job rotation (Figure 19.12).

Figure 19.12 Face Turning on Lathe

Shoulder Turning: If a job is turned with different diameters, the steps for one diameter to the other so formed, the surface is known as shoulder turning. There are several types of shoulder turning such as square, radius, bevelled, etc., as shown in Figure 19.13. It is also known as step turning.

Figure 19.13 Shoulder Turning on Lathe

Eccentric Turning: When a job having more than one axis of rotation, each axis may be parallel with each other but never coincides; turning of different cylindrical surfaces of the job is known as eccentric turning. In Figure 19.14, the job is first turned through centres C₁ − C₁ and then through centres C₂ − C₂.

Figure 19.14 Eccentric Turning on Lathe

Taper Turning: Taper turning is an operation in which taper cylindrical surface, i.e., cone type surface is produced as shown in Figure 19.15.

Figure 19.15 Taper Turning on Lathe

Taper on a cylindrical surface of a job can be produced by the following methods:

• Taper turning by swivelling compound rest: Job rotates on lathe axis and tool moves on angular path. It can be applied from any angle 0° − 90° for short length of taper up to 150 mm (approximate) . It is used for shorter length and steeper angle. Here, D_l and D_s are larger and shorter diameters, l is length of the job, and α is angle of taper.
• Taper turning by off-setting the tailstock: Job rotates at an angle to the lathe axis and the tool travels longitudinally to the lathe axis. Any angle 0° − 8°, long job of smaller diameter can be turned by this method. It is also used for internal taper turning.
• Taper turning attachment: Job rotates on lathe axis and tool moves in guided angular path. Long jobs of steeper angle of taper (0° − 12°) can be done by this attachment. Guide rail is set as per angle of taper. It is applied for mass production.
• Taper tuning by a form tool: Job rotates on lathe axis and tool moves crosswise direction, perpendicular to the lathe axis. Very small length of taper and any angle 0° − 90°. Tool itself designed as per requirements. It is used for mass production for Chamfering on bolts, nuts, bushes, etc.
• Taper turning by combination fed: Job rotates on lathe axis and tool travels on resultant path, for any length and any angle. Taper angle is to be determined by trial and error method. It is applied by hand feeds by combined feeding of tool (axial and perpendicular) for taper turning.

Parting-off (Grooving)

It is the operation of cutting-off/grooving a bar after it has been machined to the required shape and size. In this operation, the job is held on a chuck, rotates to the turning speed and the parting-off tool is fed into the job very slowly until the tool reaches to the centre of the job. The parting-off operation is shown in Figure 19.16.

Figure 19.16 Parting-off Operation on Lathe

Knurling

Knurling is the process of embossing, producing a roughened surface on a smooth surface of a cylindrical job to provide effective gripping, for example, thimble and ratchet of micrometer and plug gauge handle. Knurling tools (single, two, or three sets of rollers) are held rigidly on tool post, pressed against the rotating (one third speed of the turning) surface of a job, leaving exact facsimile of the tool on the surface of the job as shown in Figure 19.17.

Figure 19.17 Knurling Operation on Lathe

Thread Cutting

For thread cutting on the lathe, there is definite relationship between the speeds of the job and tool. The relationship is obtained by gear ratio selection which depends on the pitch of the job, pitch of the lead screw, number of start of thread on the job. Every machine is supplied with a spur gear box (a set of 23 gears) having teeth from 20 to 120 with an interval of 5 and a special gear or transfer gear is of 127 teeth for cutting metric thread. Two 20 teeth spurs are available. Lead screw has single start thread. The simple process of thread cutting on lathe is shown in Figure 19.18.

Figure 19.18 Thread Cutting on Lathe

Steps for Thread Cutting on Lathe

1. Hold the job on machine and turn up to major diameter of the thread.
2. Choose suitable thread cutting tool.
3. Select slower speed of the lathe spindle.
4. Calculate the change gear ratio based on the following formula: 
5. Fix the calculated change gear ratio to the head stock spindle, intermediate shaft, and lead screw shaft.
6. Choose suitable depth of cut. Three or four cuts are necessary to complete the thread.
7. Arrange job and tool proper position and give desired depth of cut.
8. Engage half nut with respect to chasing dial according to odd/even threads.
9. Allow the movement of the tool up to the portions of the job necessary for thread cutting then lifting the tool from the job.
10. Disengage the half nut, move the carriage to the right side up to the position from where second cut will start. Allowing the second depth of cut again engages the half nut with respect to chasing dial.

Drilling

The operation of producing a circular hole by removing metal by rotating the cutting edges of a drill is known as drilling. But on lathe drill is static and only feed motion is given through the movement of tail stock and rotating motion is given to the job. Metal is removed by shearing and extrusion. Drilled hole will be slightly oversized than the drill used due to the non-alignment of the drill and vibration of the spindle. For producing accurate hole, the drill bit should be chosen slightly undersize and subsequent reaming or boring operation is essential after drilling. Drilling on lathe is very easy. Drill bit is held in tail stock in place of dead centre and moved in forward direction applying pressure at the end of the rotating job. Drill moves up to the length of the hole required as shown in Figure 19.19.

Figure 19.19 Drilling on Lathe

Tapping

Tapping is an operation for producing internal thread. A hole of minor diameter is produced in the job by holding the drill tool in tail stock and applying pressure on the rotating job in chuck. After drilling the hole, tap is hold in tail stock and inserted in drilled hole of the rotating job as shown in Figure 19.20.

Figure 19.20 Tapping on Lathe

Reaming

The operation of finishing and sizing a previous drilled hole using a multi-edges straight cutting tool named as reamer is known as reaming. Very small amount of material (0.4 mm) removal is possible by this operation. Reaming operation on lathe is very similar to drilling on lathe as shown in Figure 19.21.

Figure 19.21 Reaming on Lathe

Boring

The operation of enlarging and finishing a previous drilled hole throughout its length by means of an adjustable single edge cutting tool (named as boring tool) is known as boring. Boring on lathe is also very similar to drilling but this process is used to enlarge the drilled hole as shown in Figure 19.22.

Figure 19.22 Boring on Lathe

Spinning

Spinning is a process to produce a circular homogeneous pot or household utensil. In this operation, the sheet metal job is held between a former attached with headstock spindle and the tail stock centre and rotates at high speed with the former. The long round nose forming tool fixed rigidly on special tool post presses the job on the periphery of the former as shown in Figure 19.23. Thus, the job is deformed exactly in the shape of former and the operation is known as spinning. It is chipless machining process.

Figure 19.23 Spinning on Lathe