This section is from the book "Machine Shop Work", by Frederick W. Turner, Oscar E. Perrigo, Howard P. Fairfield. Also available from Amazon: Machine shop work.
The round nose is used solely for turning concave surfaces, being held as high on the work as proper cutting will allow, as shown in Fig. 93.
To make the hand lathe more rapid and more certain in operation, it is frequently provided with a tool holder called the slide rest, as shown in Fig. 94. This holds the tool rigidly and guides it mechanically, so that the work is done more rapidly than with the hand tools. Slide rest tools are miniatures of those used on larger lathes, hence a description will not be given at this point.
Fig. 92. Position of Tool for Turning Steel.
This type of machine tool is well illustrated by Fig. 95, which shows the common cone belt-driven screw-cutting engine lathe of ordinary dimensions. It is commonly sold in sizes from 10-inch swing to 30-inch swing. Larger sizes are usually built to order. When an engine lathe is used for turning, the tool is rigidly held in a "tool post" clamped to the cross-slide, and is not directly hand controlled. The modern engine lathe is designed usually so that by combining a direct belt-driven cone and suitable back gears a range of at least eight rotative spindle speeds are obtained. The engine lathe illustrated in Fig. 95 has a strong cast-iron bed A carried on four well braced legs that may be bolted to the floor, though the weight of the machine may be sufficient to hold it in position. On the left-hand end of the bed there is fastened the headstock B, which carries the main running gear of the machine.
Fig. 93. Hand-Turning a Fillet.
At each end of the headstock there is a bearing for the spindle. Running loosely on the spindle and between the bearings is the cone pulley C to which the pinion D is attached.
The back gearing is designed to reduce the speed of the spindle without changing the belt speed. The mechanism of the back gearing is clearly shown in Fig. 96. The large gear E alone shows in Fig. 95. It is driven by the pinion D which is attached to the cone. Referring interchangeably to Figs. 95 and 96, a pinion on the same sleeve as the gear E drives the gear at the right of the cone c, which gear is keyed to the work spindle. When the back gearing is not in use, it is thrown out of mesh with the gears on the pulley and spindle by means of a shaft having eccentric bearings upon which it turns; at the same time the cone pulley locks fast to the gear at its right; the work spindle then turns with the cone pulley. With the back gearing in use the spindle runs more slowly, with the belt on the same step of the cone, than it does when driven direct.
Fig. 94. Hand Lathe Slide Rest.