g two micrometers or gages, one set
for the radial dimension _x_ and the other for the chordal distance _y_,
the work may be done in a comparatively short time.

[Illustration: Fig. 20. Testing Concentricity of Button with Dial Gage]

After the buttons have been tightened, all measurements should be
carefully checked; the work is then mounted on the faceplate of the
lathe, and one of the buttons, say _b_, is set true by the use of a test
indicator as shown in Fig. 19. When the end of this indicator (which is
one of a number of types on the market) is brought into contact with the
revolving button, the vibration of the pointer _I_ shows how much the
button runs out of true. When the pointer remains practically
stationary, thus showing that the button runs true, the latter should be
removed. The hole is then drilled nearly to the required size, after
which it is bored to the finish diameter. In a similar manner the other
buttons are indicated and the holes bored, one at a time. It is evident
that if each button is correctly located and set perfectly true in the
lathe, the various holes will be located at the required
center-to-center dimensions within very close limits.

[Illustration: Fig. 21. Drilling a Bushing Hole]

Fig. 20 shows how one of the buttons attached to a plate in which three
holes are to be bored is set true or concentric. The particular
indicator illustrated is of the dial type, any error in the location of
the button being shown by a hand over a dial having graduations
representing thousandths of an inch. Fig. 21 shows how the hole is
drilled after the button is removed. It will be noted that the drill is
held in a chuck, the taper shank of which fits into the tailstock
spindle, this being the method of holding small drills. After drilling,
the hole is bored as shown in Fig. 22. The boring tool should have a
keen edge to avoid springing, and if the work when clamped in position,
throws the faceplate out of balance, it is advisable to restore the
balance, before boring, by the use of a counter-weight, because the
lathe can be rotated quite rapidly when boring such a small hole.

[Illustration: Fig. 22. Boring a Bushing Hole]

When doing precision work of this kind, the degree of accuracy will
depend upon the instruments used, the judgment and skill of the workman
and the care exercised. A good general rule to follow when locating
bushings or buttons is to use the method which is the most direct and