In the second operation, the piston a is reversed and held in soft jaws, which are used in place of the hardened jaws b shown in the illustration. These jaws are bored to the outside diameter of the piston, so that when closed, they hold the work true or concentric with the lathe spindle. In this operation the chucking ring by which the piston was previously held is cut off, and the end of the piston is faced true. If the crank-pin hole is to be finished, a third operation is necessary, a self-centering chuck-plate and boring and reaming tools being used. (These are not shown in the illustration.)
Turning Piston Rings in Turret Lathe.—One method of turning piston rings is shown at B in [Fig. 16]. The piston rings are cut from a cast-iron cylindrical piece which has three lugs b cast on one end and so arranged that they may be held in a three-jawed chuck. This cylindrical casting is about 10 inches long, and when the rings are to have their inside and outside surfaces concentric, the casting is held by the lugs in the regular jaws furnished with the chuck. (The arrangement used for turning and boring eccentric rings, which is that shown in the illustration, will be described later.)
The casting a, from which the rings are made, is first rough-bored with double-ended cutter n in boring-bar m, after which it is finish-bored with cutter n1 in bar m1. While taking these cuts, the bars m and m1 are supported by their extension ends which enter bushing r located in the central hole of the chuck. This furnishes a rigid support so that a heavy cut can be taken.
The outside of the casting is next rough-turned with tool k, held in the turret toolpost. This toolpost is then revolved to bring tool j into position, by which the outside is turned true to size, a broad shaving chip being taken. The toolpost is again swung around, to bring the cutting-off tool-holder l into position. This holder contains four blades set the proper distance apart to give rings of the desired width. Each blade, from right to left, is set a little back of the preceding one, so that the rings are cut off one after the other, the outer rings being supported until they are completely severed. After the first four rings are cut off, the carriage is moved ahead to a second stop, and four more rings are severed, this operation being continued until the casting has been entirely cut up into rings.
When the bore of the ring is to be eccentric with the outside, the holding arrangement shown in the illustration is used. The casting a is bolted to a sliding chuck-plate c, and the outside is rough-turned with tool k in the toolpost. Finishing tool j is then brought into action, and the outside diameter is turned accurately to size. Then the sliding chuck-plate c, carrying the work, is moved over a distance equal to the eccentricity desired, and the work is bored with cutters n and n1 as in the previous case. The turret toolpost is next revolved and the tools l are used for cutting off the rings. The reason for finishing the outside first is to secure smooth rings in cutting off, as this operation should be done when the work is running concentric with the bore, rather than with the exterior surface.
It will be evident that this method gives a far greater output of rings than is possible by finishing them in the more primitive way on engine lathes. The faces of the rings may be finished in a second operation if desired, or they may be ground, depending on the method used in the shop where the work is being done, and the accuracy required.
Fig. 17. Turning Gasoline Engine Pistons in Pratt & Whitney Turret Lathe