Fig. 28. (A) Spherical Turning with Compound Rest.
(B) Concave Turning
Spherical Turning.—Occasionally it may be necessary to turn a spherical surface in the lathe. Sketch A, [Fig. 28], shows how a small ball-shaped end can be turned on a piece held in a chuck. The lathe carriage is adjusted so that the pin around which the compound rest swivels is directly under the center a. The bolts which hold the swivel are slightly loosened to allow the top slide to be turned, as indicated by the dotted lines; this causes the tool point to move in an arc about center a, and a spherical surface is turned. Light cuts must be taken as otherwise it would be difficult to turn the slide around by hand.
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.
Fig. 29. Spherical Turning Attachment for Engine Lathe
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.