Piston Turning in Pratt and Whitney Turret Lathe.—A turret lathe equipped with tools for turning, facing and grooving automobile gasoline 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.)
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 tools to clear as they swing around.
Fig. 18. Pratt & Whitney Turret Lathe equipped with Special
Attachment for Turning Eccentric Piston Rings
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.