Machinery Repairman

Figure 6-43.—Tool overhang.
inch times the diameter of the workpiece equals 5°.
The point of a high-speed steel cutting tool being used
to cut aluminum should be set slightly more than 5°
above center, while the points of tools used to cut
copper, brass, and other soft metals should be set
exactly on center. The point of cast alloy (stellite and
so on), carbide, and ceramic cutting tools should be
placed exactly on center regardless of the material
being cut. The tool point should be placed on center
for threading, turning tapers, parting (cutting-off), or
boring.
You can adjust the height of the tool in the
toolholder illustrated in figure 6-43 by moving the
half-moon wedge beneath the toolholder in or out as
required. The quick-change toolholder has an
adjusting screw to stop the tool at the correct height.
Some square turret toolholders require a shim beneath
the tool to adjust the height.
There are several methods you can use to set a
tool on center. You can place a dead center in the
tailstock and align the point of the tool with the point
of the center. The tailstock spindle on many lathes
has a line on the side that represents the center. You
can also place a 6-inch rule against the workpiece in a
vertical position and move the cross-slide in until the
tool lightly touches the rule and holds it in place.
Look at the rule from the side to determine if the
height of the tool is correct. The rule will be straight
up and down when the tool is exactly on center and
will be at an angle when the tool is either high or low.
6-25
METHODS OF HOLDING THE WORK
You cannot perform accurate work if the work is
improperly mounted. The following are requirements
for proper mounting:
The work center line must be accurately
centered along the axis of the lathe spindle.
The work must be held rigidly while being
turned.
The work must not be sprung out of shape by
the holding device.
The work must be adequately supported
against any sagging caused by its own weight
and against springing caused by the action of
the cutting tool.
There are four general methods of holding work in
the lathe: (1) between centers, (2) on a mandrel, (3) in a
chuck, and (4) on a faceplate. Work may also be
clamped to the carriage for boring and milling; the
boring bar or milling cutter is held and driven by the
headstock spindle.
Holding Work Between Centers
To machine a workpiece between centers, drill
center holes in each end to receive the lathe centers.
Secure a lathe dog to the workpiece and then mount the
work between the live and dead centers of the lathe.
CENTERING THE WORK.—To center drill
round stock such as drill-rod or cold-rolled steel,
secure the work to the head spindle in a universal
chuck or a draw-in collet chuck. If the work is too
long and too large to be passed through the spindle,
use a center rest to support one end. It is good shop
practice to first take a light finishing cut across the
face of the end of the stock to be center drilled. This
will provide a smooth and even surface and will help
prevent the center drill from “wandering” or breaking.
The centering tool is held in a drill chuck in the
tailstock spindle and fed to the work by the tailstock
handwhecl (fig. 6-44).
Figure 6-44.—Drilling center hole.