known as the button method. This scheme is so named because
cylindrical bushings or buttons are attached to the work in positions
corresponding to the holes to be bored, after which they are used in
locating the work. These buttons, which are ordinarily about 1/2 inch in
diameter, are ground and lapped to the same size and the ends squared.
The diameter should, preferably, be such that the radius can be
determined easily, and the hole through the center should be about 1/8
inch larger than the retaining screw, so that the button can be shifted.

As an illustration of the practical application of the button method, we
shall consider, briefly, the way the holes would be accurately machined
in the jig-plate in Fig. 18. First the centers of the seven holes should
be laid off approximately correct by the usual methods, after which
small holes should be drilled and tapped for the clamping screws _S_.
After the buttons _B_ are clamped lightly in place, they are all set in
correct relation with each other and with the jig-plate. The proper
location of the buttons is very important as their positions largely
determine the accuracy of the work. A definite method of procedure that
would be applicable in all cases cannot, of course, be given, as the
nature of the work as well as the tools available make it necessary to
employ different methods.

[Illustration: Fig. 19. Setting a Button True Preparatory to Boring, by
use of Test Indicator]

In this particular case, the three buttons _a_, _b_ and _c_ should be
set first, beginning with the one in the center. As this central hole
must be 2.30 and 2.65 inches from the finished sides _A_ and _A_{1}_,
respectively, the work is first placed on an accurate surface-plate as
shown; by resting it first on one of these sides and then on the other,
and measuring with a vernier height gage, the central button can be
accurately set. The buttons _a_ and _c_ are also set to the correct
height from side _A_{1}_ by using the height gage, and in proper
relation to the central button by using a micrometer or a vernier
caliper and measuring the over-all dimension _x_. When measuring in this
way, the diameter of one button would be deducted to obtain the correct
center-to-center distance. After buttons _a_, _b_ and _c_ are set
equidistant from side A_{1} and in proper relation to each other, the
remaining buttons should be set radially from the central button _b_ and
the right distance apart. By havin