asoline
engine pistons is shown in Fig. 17. The piston is held on an expanding
pin chuck which is so constructed that all of the pins are forced
outward with equal pressure and automatically conform to any
irregularities on the inside of the piston. Tool _A_ rough-turns the
outside, and just as this tool completes its cut, a center hole is
drilled and reamed in the end of the piston by combination drill and
reamer _B_. The turret is then indexed one-half a revolution and a
finishing cut is taken by tool _C_. After the cylindrical body of the
piston has been turned, tools held in a special holder _E_ attached to
the cut-off slide are used to face the ends of the piston and cut the
packing-ring grooves. While the grooves are being cut, the outer end of
the piston is supported by center _D_. The center hole in the end also
serves to support the piston while being ground to the required diameter
in a cylindrical grinding machine. The edge at the open end of the
piston may also be faced square and the inner corner beveled by a hook
tool mounted on the rear cross-slide, although this is usually done in a
separate operation. (This provides a true surface by which to hold this
end when grinding.)

[Illustration: Fig. 18. Pratt & Whitney Turret Lathe equipped with
Special Attachment for Turning Eccentric Piston Rings]

This illustration (Fig. 17) shows very clearly the stops which
automatically disengage the turret feed. A bracket _F_ is bolted to the
front of the bed and contains six stop-rods _G_ (one for each position
or side of the turret). When one of these stop-rods strikes lever _H_,
the feed is disengaged, the stop being adjusted to throw out the feed
when the tool has completed its cut. Lever _H_ is automatically aligned
with the stop-rods for different sides of the turret by a cam _J_ on the
turret base. A roller _K_ bears against this cam and, through the
connecting shaft and lever shown, causes lever _H_ to move opposite the
stop-rod for whatever turret face is in the working position. Lever _L_
is used for engaging the feed and lever _R_ for disengaging it by hand.

The indexing of the turret at the end of the backward movement of the
slide is controlled by stop _M_ against which rod _N_ strikes, thus
disengaging the lock bolt so that the turret can turn. This stop _M_ is
adjusted along the bed to a position depending upon the length of the
turret tools and the distance the turret must move back to allow the
tool