Machinery Repairman

Figure 6-41.—Aligning lathe centers.
figure 6-41. To produce very accurate work,
especially if it is long, use the following procedure to
determine and correct errors in alignment not
otherwise detected.
Mount the work to be turned, or a piece of stock
of similar length, on the centers. With a turning tool
in the toolpost, take a small cut to a depth of a few
thousandths of an inch at the headstock end of the
work. Then, remove the work from the centers to
allow the carriage to be run back to the tailstock
without withdrawing the tool. Do not touch the tool
setting. Replace the work in the centers, and with the
tool set at the previous depth take another cut coming
in from the tailstock end. Compare the diameters of
these cuts with a micrometer. If the diameters are
exactly the same, the centers are in perfect alignment.
If they are different, adjust the tailstock in the
direction required by using the setover adjusting
screws. Repeat the test and adjustment until a cut at
each end produces equal diameters.
Another method you can use to check for positive
alignment of lathe centers is to take a light cut over
the work held between the centers. Then, measure the
work at each end with a micrometer. If the readings
differ, adjust the tailstock to remove the difference.
Repeat the procedure until the centers are aligned.
Truing Centers
To machine or true a lathe center, remove the
faceplate from the spindle. Then, insert the live
center into the spindle and set the compound rest at an
angle of 30° with the axis of the spindle, as shown in
figure 6-42. If you are using a three- or four-jaw
chuck, secure the material that you are using to
manufacture your center from in the chuck and
proceed from this point. Place a round-nose tool in
the toolpost and set the cutting edge of the tool at the
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exact center point of the lathe center. Machine a light
cut on the center point and test the point with a center
gauge. All lathe centers, regardless of their size, are
finished to an included angle of 60°.
If you must regrind a dead center for the tailstock,
it is best to do it using a cylindrical grinder. The
cylindrical grinder will be covered in chapter 10.
SETTING THE TOOLHOLDER AND
CUTTING TOOL
The first requirement for setting the tool is to have
it rigidly mounted on the toolpost holder. Be sure the
tool sits squarely in the toolpost and that the setscrew
is tight. Reduce overhang as much as possible to
prevent the tool from springing during cutting. If the
tool has too much spring, the point of the tool will
catch in the work, causing chatter and damaging both
the tool and the work. The relative distances of A and
B in figure 6-43 show the correct overhang for the
tool bit and the holder. When a quick-change
toolholder is used, tool overhang should not exceed
twice the width of the cutting tool, or of the shank,
when you use a carbide insert cutting tool.
The point of the tool must be correctly positioned
on the work. When you are using a high-speed
cutting tool to straight turn steel, cast iron, and other
relatively hard metals, set the point about 5° above
center. A rule measurement of approximately 3/64
Figure 6-42.—Machining a lathe center.