Fig. 25. Turning an Engine Crank-pin in an Ordinary Lathe
Turning a Crankshaft in a Lathe.—Another example of eccentric turning is shown in [Fig. 25]. The operation is that of turning the crank-pin of an engine crankshaft, in an ordinary lathe. The main shaft is first rough-turned while the forging revolves upon its centers C and C1 and the ends are turned to fit closely the center-arms A and A1. After the sides B and B1 of the crank webs have been rough-faced, the center-arms are attached to the ends of the shaft as shown in the illustration. These arms have centers at D and D1 (located at the required crank radius) which should be aligned with the rough pin, when attaching the arms, and it is advisable to insert braces E between the arms and crank to take the thrust of the lathe centers. With the forging supported in this way, the crank-pin and inner sides of the webs are turned and faced, the work revolving about the axis of the pin. The turning tools must extend beyond the tool-holder far enough to allow the crank to clear as it swings around. Owing to this overhang, the tool should be as heavy as possible to make it rigid and it is necessary to take comparatively light cuts and proceed rather cautiously. After finishing the crank-pin and inside of the crank, the center-arms are removed and the main body of the shaft and the sides B and B1 are finished. This method of turning crankshafts is often used in general repair shops, etc., especially where new shafts do not have to be turned very often. It is slow and inefficient, however, and where crankshafts are frequently turned, special machines or attachments are used.
Fig. 26. LeBlond Lathe with Special Equipment for Crankshaft Turning
Special Crankshaft Lathe.—A lathe having special equipment for rough-turning gas engine crankshaft pins is shown in [Fig. 26]. This lathe is a heavy-duty type built by the R. K. LeBlond Machine Tool Co. It is equipped with special adjustable headstock and tailstock fixtures designed to take crankshafts having strokes up to about 6 inches. The tools are held in a three-tool turret type of toolpost and there are individual cross-stops for each tool. This lathe also has a roller steadyrest for supporting the crankshaft; automatic stops for the longitudinal feed, and a pump for supplying cutting lubricant. The headstock fixture is carried on a faceplate mounted on the spindle and so arranged as to be adjustable for cranks of different throw. When the proper adjustment for a given throw has been made, the slide is secured by four T-bolts. A graduated scale and adjusting screw permit of accurate adjustments.