Metal-cutting Machines

A lathe is known to be essentially a machine tool for producing and finishing surfaces of workpieces. The machine is designed to hold and revolve work around an axis of rotation so that it may be subjected to the action of a cutting tool moving in a horizontal plane through the axis of the work. When the cutting tool moves in a longitudinal direction or parallel to the axis, the operation is known as “turning”, when it moves in a transverse direction, it is known as “facing”. In addition to turning and boring, which the machine is primarily designed for, many other operations, such as drilling, threading, and by employing special adapters grinding and milling, may be performed on a lathe.

Lathes used in shop practice are known to be of different designs and sizes. These lathes fall various types, either according to their characteristic constructional features, or according to the work for which they are designed. The size of a lathe is determined by the diameter and length of work that may be swung between centers. Lathes of comparatively small size, which may be mounted on a bench, are termed bench lathes, and are intended for small work of considerable accuracy; lathes provided with tools held in a revolvable turret are called “turret lathes” lathes in which workpieces to be treated are held in a chuck are known as “chucking lathes”; lathes in which most of operations are performed automatically are named “automatic lathes”.

Besides there are also many special-purpose lathes such as crankshaft lathes and wheel lathes for turning crankshafts or engine driving wheels respectively; screw-cutting lathe used for metal-turning operations is fitted with a power-actuated carriage and cross-slide for clamping and holding the cutting tool. In engine lathes the cutting tools are generally guided by the machine tool itself, in other words, they are operated mechanically, while in some lathes the cutting tools are guided by hand. The engine lathe consists essentially of the following basic parts: the bed, the headstock, the tailstock, the feet mechanism, and the carriage.

The bed is rigid casting with two longitudinal walls firmly connected by cross ribs integral with the casting. The bed serves as a base to support and align the rest of the machine. The upper surface of the bed is provided with parallel V-type and flat ways or guides for accurate aligning of the sliding parts of the lathe – the carriage and the tailstock. The headstock is located and firmly bolted to the left - hand side of the bed and carries a pair of bearing in which the spindle rotates. Many modern lathes have a motor built into the headstock with the spindle serving as the motor shaft. The spindle being one of the most important parts of a lathe, is a steel hollow shaft with a taper bore for the insertion of the live or running centre on which the piece to be turned is placed. The other end of the work is supported by the non-rotating dead or cup centre. The nose of the spindle is accurately threaded for chucks to be screwed on it. The chucks, in turn, hold and revolve workpieces together with the spindle. The head-stock also incorporates the change gearbox driven by a set of speed change levers. The change gearbox serves for running the lathe at different speeds require in turning and boring workpieces of various diameters.

The tailstock located at the right-hand side of the bed, is a casting carrying a non-rotating sleeve, which together with the nut can be advanced screw operated by the hand wheel. The tailstock may be moved anywhere along the lathe bed and can be clamped in place at any point. On changing the position, the tailstock slide along the two inner bed ways one of which named flat way is of rectangular cross-section and the other one is of V-section. The tailstock sleeve mounts a hollow spindle with a standard tare bore for holding the lathe centers or tapered tool shanks. The dead centre fits in a Morse taped hole in the sleeve, thereby bringing the end of the tailstock screw against the rear of the centre and forcing it out. The tailstock spindle has a large area bearing in both the front and rear of the tailstock. To facilitate measurement of the spindle travel the tailstock spindle is graduated.

The feed mechanism for both longitudinal and cross feeds of the engine lather is simple and easy to operate. It comprises a cone of gears, an intermediate shaft and a set of sliding shaft and carries a tumbler gear which is dropped into engagement with a gear on the cone corresponding to the thread or feed selected on the index plate above.

Movement of the carriage and the cross-slide can be reversed either by reversing the feed mechanism with the reverse handle or by shifting the single lever located on the carriage apron.