Shoulder Turning: When a workpiece having different diameters is turned, the surface forming the step from one diameter to the other is called the shoulder, and machining this part of the workpiece is called shoulder turning.
Thread Cutting Operation
External Thread Cutting. In a thread cutting operation the first step is to remove the excess material from the workpiece to make its diameter equal to the major diameter of the screw thread, Change gears of correct size are then fitted to the end of the bed between the spindle and the lead screw. In case of all geared machines, the change gears are already fitted, only the levers are to be shifted to the required positions. The shape or form of the thread depends on the shape of the cutting tool to be used. In a metric thread, the included angle of the cutting edge should be ground exactly 60°. The top of the tool nose should be set at the centre of the workpiece. A thread tool gauge or angle gauge is usually used against the turned surface to check the cutting tool so that each face of the tool may be equally inclined to the centre line of the workpiece. The speed of the spindle is reduced by one half to one-fourth of the speed required for turning according to the type of the material being machined, and the half-nut is then engaged. The depth of cut, which usually varies from 0.05 to 0.2 mm, is applied by advancing the tool perpendicular to the axis of the work. After the tool has produced a helical groove upto the end of the work, it is quickly withdrawn by the use of the cross slide, the half nut disengaged, and the tool is brought back to the starting position to give a fresh cut. Before re-engaging the half nut, it is necessary to ensure that the tool will follow the same path it has traversed in the previous cut, otherwise the job will be spoiled. Several cuts are necessary before the full depth of thread is reached.
Where: D = Diameter, L = Length of the threaded portion
Boring: Boring is the operation of enlarging and truing a hole produced by drilling, punching, casting or forging. Boring cannot originate a hole. Boring is similar to the external turning operation and can be performed in a lathe.
- Counter Boring. Counter Boring is the operation of enlarging a hole through a certain distance from one end instead of enlarging the whole drilled surface. It is similar to a shoulder turning operation in external turning.
- Taper Boring. The principle of turning a taper hole is similar to the external taper turning operation and is accomplished by rotating the work on a chuck and feeding the tool at an angle to the axis of rotation of the work piece.