ld 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.

[Illustration: Fig. 26. LeBlond Lathe with Special Equipment for
Cranksh