t is set to the same angle
given on the drawing, but this is not always the case. If the draftsman
had given the included angle of 90 degrees, as shown at _B_, which would
be another way of expressing it, the setting of the compound rest would,
of course, be the same as before, or to 45 degrees, but the number of
degrees marked on the drawing does not correspond with the angle to
which the rest must be set. As another illustration, suppose the valve
were to be turned to an angle of 30 degrees with the axis as shown at
_C_. In this case the compound rest would not be set to 30 degrees but
to 60 degrees, because in order to turn the work to an angle of 30
degrees, the rest must be 60 degrees from its zero position, as shown.
From this it will be seen that the number of degrees marked on the
drawing does not necessarily correspond to the angle to which the rest
must be set, as the graduations on the rest show the number of degrees
that it is moved from its zero position, which corresponds to the line
_a--b_. The angle to which the compound rest should be set can be found,
when the drawing is marked as at _A_ or _C_, by subtracting the angle
given from 90 degrees. When the included angle is given, as at _B_,
subtract one-half the included angle from 90 degrees to obtain the
required setting. Of course, when using a compound rest, the lathe
centers are set in line as for straight turning, as otherwise the angle
will be incorrect.

Rules for Figuring Tapers

+---------------------+---------------------+--------------------------+
| Given | To Find | Rule |
+---------------------+---------------------+--------------------------+
|The taper per foot. |The taper per inch. |Divide the taper per foot |
| | | by 12. |
| | | |
|The taper per inch. |The taper per foot. |Multiply the taper per |
| | | inch by 12. |
| | | |
|End diameters and |The taper per foot. |Subtract small diameter |
| length of taper in | | from large; divide by |
| inches. | | length of taper, and |
| | | multiply quotient by 12.|
| |