[Illustration: Fig. 23. Example of Work illustrating Accumulation of Errors]
=Eccentric Turning.=--When one cylindrical surface must be turned eccentric to another, as when turning the eccentric of a steam engine, an arbor having two sets of centers is commonly used, as shown in Fig. 24. The distance _x_ between the centers must equal one-half the total "throw" or stroke of the eccentric. The hub of the eccentric is turned upon the centers _a--a_, and the tongued eccentric surface, upon the offset centers, as indicated by the illustration. Sometimes eccentrics are turned while held upon special fixtures attached to the faceplate.
[Illustration: Fig. 24. Special Arbor for Turning Eccentrics]
When making an eccentric arbor, the offset center in each end should be laid out upon radial lines which can be drawn across the arbor ends by means of a surface gage. Each center is then drilled and reamed to the same radius _x_ as near as possible. The uniformity of the distance _x_ at each end is then tested by placing the mandrel upon the offset centers and rotating it, by hand, with a dial indicator in contact at first one end and then the other. The amount of offset can also be tested either by measuring from the point of a tool held in the toolpost, or by setting the tool to just graze the mandrel at extreme inner and outer positions, and noting the movement of the cross-slide by referring to the dial gage of the cross-feed screw.
[Illustration: 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 _C_{ 1}_ and the ends are turned to fit closely the center-arms _A_ and _A_{ 1}_. After the sides _B_ and _B_{ 1}_ 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 _D_{ 1}_ (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 _B_{ 1}_ 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.