Thread cutting is one of the most important operations performed in a lathe. The principle of thread cutting is to produce a helical groove on a cylindrical or conical surface by feeding the tool longitudinally when the job is revolved between centres or by a chuck. The longitudinal feed should be equal to the pitch of the thread to be cut per revolution of the workpiece. The lead screw of the lathe, through which the saddle receives its traversing motion, has a definite pitch. A definite ratio between the longitudinal feed and rotation of the headstock spindle should therefore be found so that the relative speeds of rotation of the work and the lead screw will result in cutting of a screw of the desired pitch. This is affected by Change gears arranged between the spindle and the lead screw or by the Change Gear Mechanism or feed box used in a modern lathe where it provides a wider range of feed and the speed ratio can be easily and quickly changed.
THREAD CUTTING OPERATION
(a) 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 After the tool has produced a helical groove upto the end of the work, it is quickly withdrawn by the use of the cross slide, the half nut disengaged, and the tool is brought back to the starting position to give a fresh cut.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.
(b) Internal Thread Cutting. The principle of cutting internal threads is similar to that of an external thread, the only difference being in the tool used. The tool is similar to a boring tool with cutting edges ground to the shape conforming to the type of thread to be cut. The hole is first bored to the root diameter of the thread. The tool is fixed on the tool post or on the boring bar after setting it at right angles to the lathe axis, using a thread gauge/angle gauge. The depth of cut is given by the compound slide and the thread is finished in the usual manner.