s to clear as they swing around.

[Illustration: Fig. 19. Tool Equipment for Machining Worm Gear
Blanks--Davis Turret Lathe]

=Attachment for Turning Piston Rings.=--Fig. 18 shows a special
attachment applied to a Pratt & Whitney turret lathe for turning
eccentric, gas-engine piston rings. The boring of the ring casting,
turning the outside and cutting off the rings, is done simultaneously.
The interior of the casting is turned concentric with the lathe spindle
by a heavy boring-bar, the end of which is rigidly supported by a
bushing in the spindle. The slide which carries the outside turning tool
is mounted on a heavy casting which straddles the turret. The outside of
the ring casting is turned eccentric to the bore as a result of an
in-and-out movement imparted to the tool by a cam on shaft _A_ which is
rotated from the lathe spindle through the gearing shown. For each
revolution of the work, the tool recedes from the center and advances
toward it an amount sufficient to give the required eccentricity. When
the turning and boring tools have fed forward about 2 inches, then the
cutting-off tools which are held in holder _B_ come into action. The end
of each cutting-off tool, from right to left, is set a little farther
away from the work than the preceding tool, so that the end rings are
always severed first as the tools are fed in by the cross-slide. A
number of the completed rings may be seen in the pan of the machine.

[Illustration: Fig. 20. Turning Bevel Gear Blanks in Davis Turret
Lathe--First Operation]

=Turning Worm-gear Blanks in Turret Lathe.=--This is a second operation,
the hub of worm-gear blank _G_ (Fig. 19) having previously been bored,
reamed, and faced on the rear side. The casting is mounted upon a
close-fitting arbor attached to a plate bolted to the faceplate of the
lathe, and is driven by two pins which engage holes on the rear side.
The rim is first rough-turned by a tool _A_ which operates on top, and
the side is rough-faced by a toothed or serrated cutter _B_. A similar
tool-holder having a tool _C_ and a smooth cutter _D_ is then used to
turn the rim to the required diameter and finish the side. The end of
the hub is faced by cutters mounted in the end of bars _E_ and _F_, one
being the roughing cutter and the other the finishing cutter. The work
arbor projects beyond the hub, as will be seen, and forms a pilot that
steadies these cutter bars. The curved rim of the gear is turned to the
req