ters by twice the center distance; find the angle corresponding to
the quotient, in a table of sines, and double the angle.

_Example:_ If the disk diameters are 1 and 1.5 inch, respectively, and
the center distance is 5 inches, find the included angle [alpha].

1.5 - 1
------- = 0.05.
2 x 5

The angle whose sine is 0.05 equals 2 degrees 52 minutes; then, 2 deg.
52 min. x 2 = 5 deg. 44 min. = angle [alpha].

[Illustration: Fig. 17. Setting Center Mark in Line with Axis of Lathe
Spindle by use of Test Indicator]

[Illustration: Fig. 18. Jig-plate with Buttons attached, ready for
Boring]

=Use of the Center Indicator.--=The center test indicator is used for
setting a center-punch mark, the position of which corresponds with the
center or axis of the hole to be bored, in alignment with the axis of
the lathe spindle. To illustrate, if two holes are to be bored, say 5
inches apart, small punch marks having that center-to-center distance
would be laid out as accurately as possible. One of these marks would
then be set central with the lathe spindle by using a center test
indicator as shown in Fig. 17. This indicator has a pointer _A_ the end
of which is conical and enters the punch mark. The pointer is held by
shank _B_ which is fastened in the toolpost. The joint _C_ by means of
which the pointer is held to the shank is universal; that is, it allows
the pointer to move in any direction. Now when the part being tested is
rotated by running the lathe, if the center-punch mark is not in line
with the axes of the lathe spindle, obviously the outer end of pointer
_A_ will vibrate, and as joint _C_ is quite close to the inner end, a
very slight error in the location of the center-punch mark will cause a
perceptible movement of the outer end, as indicated by the dotted lines.
When the work has been adjusted until the pointer remains practically
stationary, the punch mark is central, and the hole is bored. The other
center-punch mark is then set in the same way for boring the second
hole. The accuracy of this method depends, of course, upon the location
of the center-punch marks. A still more accurate way of setting parts
for boring holes to a given center-to-center distance is described in
the following:

=Locating Work by the Button Method.=--Among the different methods
employed by machinists and toolmakers for accurately locating work such
as jigs, etc., on the faceplate of a lathe, the one most commonly used
is