[Illustration: Fig. 29. Spherical Turning Attachment for Engine Lathe]
Sketch _B_ illustrates how a concave surface can be turned. The cross-slide is adjusted until swivel pin is in line with the lathe centers, and the carriage is moved along the bed until the horizontal distance between center _b_ of the swivel, and the face of the work, equals the desired radius of the concave surface. The turning is then done by swinging the compound rest as indicated by the dotted lines. The slide can be turned more evenly by using the tailstock center to force it around. A projecting bar is clamped across the end of the slide at _d_, to act as a lever, and a centered bar is placed between this lever and the tailstock center; then by screwing out the tailstock spindle, the slide is turned about pivot _b_. The alignment between the swivel pin and the lathe centers can be tested by taking a trial cut; if the swivel pin is too far forward, the tool will not touch the turned surface if moved past center _c_, and if the pin is too far back, the tool will cut in on the rear side.
=Spherical Turning Attachments.=--When spherical turning must be done repeatedly, special attachments are sometimes used. Fig. 29 shows an attachment applied to a lathe for turning the spherical ends of ball-and-socket joints. The height or radius of the cutting tool and, consequently, the diameter of the turned ball, is regulated by adjusting screw _A_. The tool is swung around in an arc, by turning handle _B_ which revolves a worm meshing with an enclosed worm-wheel. As will be seen, the work is held in a special chuck, owing to its irregular shape.
[Illustration: Fig. 30. Attachment for Turning Spherical End of Gasoline Engine Piston]
Another spherical turning attachment is shown in Fig. 30. This is used for machining the ends of gasoline engine pistons. The cross-slide has bolted to it a bar _A_ carrying a roller which is pressed against a forming plate _B_ by a heavy spring _C_. The forming plate _B_, which is attached to a cross-piece fastened to the ways of the lathe bed, is curved to correspond with the radius required on the piston end, and when the tool is fed laterally by moving the cross-slide, it follows the curve of plate _B_. The piston is held in a special hollow chuck which locates it in a central position and holds it rigidly.