In this machine, as in the wood lathe, work revolves about a fixed axis between the centers and the cutting tool moves either (1) parallel to the axis, cutting a cylindrical or spiral surface; (2) perpendicular to the axis, as in cutting the end of a cylinder; or (3) in any combination of the two directions named, cutting any variety of surfaces of revolution. The cutting of screw threads is done on the lathe.

Fig. 17.1 is a cut selected to show the principal features of a lathe of medium size. The main parts are:

AA. Bed. B. Head stock.

C. Tail stock,

D. Carriage.

Work is suspended between the centers PP, and is driven by a dog, as shown in Fig. 172; or both centers are dispensed with and flat work is secured to a chuck or a face plate screwed on the spindle-end in place of the small face-plate F. The larger face plate, under the lathe, is marked E.

Fig. 171.   Engine Lathe of Medium Size.

Fig. 171. - Engine Lathe of Medium Size.

The head stock carries a hollow steel spindle on which are keyed the gear wheel S which drives the spindle, and the small gear wheel K which drives the feed and screw-cutting gear at the left of the machine. The cone wheels 67 and their small attached gear wheel J are not attached to the spindle, but the cones drive the spindle either directly by means of a small sliding bolt which attaches them to the wheel S, or indirectly through the back gear. The back gear consists of a spindle supported on the head stock parallel to the main spindle, on one end of which is fixed the large gear wheel H and on the other end is fixed a small gear wheel not visible. The wheels of the back gear may be thrown in or out of gear with J and S by a lever L, and when they are in gear, the bolt connecting the cones with S must be dropped out of its driving notch. The cone G is driven by a belt from a corresponding cone over head. The back gear gives slower speed and greater power.

The tail stock carries the dead center which may be moved forward or backward by the hand wheel for adjusting it to hold work in the lathe. The tail stock may be clamped at any position along the bed. For supporting a bar which is to be cut tapered, the dead center may be moved perpendicular to the length of the bed.

A tool is shown clamped in the tool post T. Its cutting motions are derived from the leading screw M or from the feed rod N.

Fig. 172.

Fig. 172.

The leading screw is used in cutting threads. Any desired combination of change wheels may be placed in gear at the left to give any desired speed to the tool in relation to the speed of the lathe.

The carriage D moves along the shears or top of the bed, and the various attachments on the apron 0 serve to facilitate the quick adjustment and to regulate the various motions of the tool.

The compound slide rest, consisting of the two parts Q and R, carries the tool post and governs the motions of the tool other than in the direction along the lathe bed. The part Q travels perpendicular to the lathe bed, and the part R is mounted on a vertical pivot in Q which enables its slide to move the tool in a line making a greater or less angle with the direction of motion of the part Q.

The term back lash is frequently heard in speaking of machine tools, and particularly of lathe gearing. This is the slack motion noticed when reversing a train of gear wheels, due to the loose fitting between the teeth of wheels which mesh together.