Figure 9-18.—Checking a center’s point with a center gauge.
of the pressure of the cut to bear on the work-holding
device and the spindle thrust bearings. When you must
feed the cutting tool toward the tailstock, take lighter
cuts at reduced feeds. In facing, the general practice is
to feed the tool from the center of the workpiece
outward.
PRELIMINARY PROCEDURES
Before starting a lathe machining operation, always
ensure that the machine is set up properly. If the work is
mounted between centers, check the alignment of the
dead center and the live center and make any necessary
changes. Ensure that the toolholder and cutting tool are
set at the proper height and angle. Check the
work-holding accessory to ensure that the workpiece is
held securely. Use the center rest or follower rest to
support long workpieces.
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 prevent the bearing
surfaces from fitting perfectly. This will decrease the
accuracy of your work. Make sure that there are no burrs
in the spindle hole. If you find burrs, remove them by
carefully scraping and reaming the hole with a Morse
taper reamer. Burrs will produce the same inaccuracies
as chips or dirt.
A center’s point must be finished accurately to an
angle of 60°. Figure 9-18 shows the method of checking
this angle with a center gauge. The large notch of the
center gauge is intended for this purpose. If this test
shows that the point is not perfect, you must true it in
the lathe by taking a cut over the point with the
compound rest set at 30°. You must anneal the hardened
tail center before it can be machined in this manner, or
you can grind it if a grinding attachment is available.
Figure 9-19.—Aligning lathe centers.
Figure 9-20.—Tool overhang.
CHECKING ALIGNMENT
To turn a shaft straight and true between centers, be
sure the centers are aligned in a plane parallel to the ways
of the lathe. You can check the approximate alignment
of the centers by moving the tailstockup until the centers
almost touch and observing their relative positions as
shown in figure 9-19.
To test center alignment for very accurate work, take
a light cut over at each end with a micrometer and, if
readings are found to differ, adjust the tailstock
accordingly. Repeat the procedure until alignment is
obtained.
SETTING THE TOOLHOLDER AND THE
CUTTING TOOL
The first requirement for setting the tool is to have
it rigidly mounted on the tool post holder. Be sure the
tool sets squarely in the tool post and that the setscrew
is tight. Reduce overhang as much as possible to prevent
the tool bit 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 distances represented by A and B in figure
9-12
