than the bore of the cylinder. A piece is then cut out with a saw or parting tool and this enables the two ends to be pressed together so that the ring can be sprung into the cylinder. When in place the rings spring back, and their pressure against the cylinder walls causes the piston to be steam tight. The latest method of making these rings is to cast a cylinder of the form shown in Fig. 36 with lugs at the bottom, which can be bolted to the horizontal table of a turning and boring mill. This machine is provided with a turret head which has four tools (Fig. 37) for four operations. Tool No. 1 faces the top of the casting, and is then followed by No. 2 in which two cutting tools are employed to rough-bore the inside and rough-turn the outside at one operation. No. 3 is a similar tool for the finishing cuts of the boring and turning. A depth of 5 or 6 inches of the casting is now ready to be cut off into rings. This is done by means of a special holder No. 4 containing six parting tools, so arranged that the top tool stands out beyond the next one below it and so on. Each tool enters the casting a little before the tool below it and thus each ring is cut off before the one below it, for it is of course impossible to cut off all the rings simultaneously.
The piston rods are either screwed into the pistons or have plain coned ends which are secured to the piston by a large nut screwed on the front side. The rods are turned nearly to size in a lathe, and finished to the exact diameter in a grinding machine. In this machine an
