ricultural machinery, etc., are not turned at all but are ground from
the rough.

=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.

[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