Machinery Repairman

Figure 6-39.—Lathe alignment test bar.
The Test Bar Method
The first step in the test bar method is to make the
test bar. A test bar may be any metal bar, 3 inches in
diameter and approximately 18 to 24 inches long.
(See fig. 6-39 for a sample). Next take a light cut on
each end of the test bar WITHOUT CHANGING
THE TOOL SETTING. Measure the diameter of
each end. A difference in diameter indicates a
misalignment. Adjust the machine leveling screws
and repeat the procedure until a cut on both ends of
the test bar results in the same diameter.
PREPARING THE CENTERS
The first step in preparing the centers is to see that
they are accurately mounted in the headstock and
tailstock spindles. The centers and the tapered holes
in which they are fitted must be perfectly clean.
Chips and dirt left on the contact surfaces will impair
accuracy by preventing a perfect fit of the bearing
surfaces. Be sure that there are no burrs in the spindle
hole. If you find burrs, remove them by carefully
scraping or reaming the surface with a Morse taper
reamer. Burrs will produce the same inaccuracies as
chips and dirt.
Center points must be accurately finished to an
included angle of 60°. Figure 6-40 shows the method
of checking the angle with a center gauge. The large
notch of the center gauge is intended for this
particular purpose. If the test shows that the point is
not perfect, true the point in the lathe by taking a cut
over the point with the compound rest set at 30°. To
true a hardened tail center, either anneal it and then
machine it or grind it if a grinding attachment is
available.
Aligning and Testing
Figure 6-40.—Checking the center point with a center gauge.
6-23
To turn a shaft straight and true between centers,
be sure the centers are in the same plane parallel to the
ways of the lathe. You can align the centers by
releasing the tailstock from the ways and then moving
the tailstock laterally with two adjusting screws. At
the rear of the tailstock are two zero lines, and the
centers are approximately aligned when these lines
coincide. To check the approximate alignment,
move the tailstock up until the centers almost touch
and observe their relative positions as shown in
28.105