Tc 9-524 fundamentals of machine tools

Forming a Radius Using the Compound Rest
To use the compound rest and tool to pivot and cut (Figure
7-6 1), the compound rest bolts must be loosened to allow the
compound rest to swivel. When using this method, the
compound rest and tool are swung from side to side in an arc.
The desired radius is formed by feeding the tool in or out with
the compound slide. The pivot point is the center swivel point
of the compound rest. A concave radius can be turned by
positioning the tool in front of the pivot point, while a convex
radius can be turned by placing the tool behind the pivot
point. Use the micrometer carriage stop to measure precision
depths of different radii.
7-34

---

TC 9-524
TAPER TURNING
When the diameter of a piece changes uniformly from one
end to the other, the piece is said to be tapered. Taper turning
as a machining operation is the gradual reduction in diameter
from one part of a cylindrical workpiece to another part,
Tapers can be either external or internal. If a workpiece is
tapered on the outside, it has an external taper; if it is tapered
on the inside, it has an internal taper. There are three basic
methods of turning tapers with a lathe. Depending on the
degree, length, location of the taper (internal or external), and
the number of pieces to be done, the operator will either use
the compound rest, offset the tailstock, or use the taper
attachment. With any of these methods the cutting edge of the
tool bit must be set exactly on center with the axis of the
workpiece or the work will not be truly conical and the rate of
taper will vary with each cut.
Compound Rests
The compound rest is favorable for turning or boring short,
steep tapers, but it can also be used for longer, gradual tapers
providing the length of taper does not exceed the distance the
compound rest will move upon its slide. This method can be
used with a high degree of accuracy, but is somewhat limited
due to lack of automatic feed and the length of taper being
restricted to the movement of the slide.
The compound rest base is graduated in degrees and can be
set at the required angle for taper turning or boring. With this
method, it is necessary to know the included angle of the taper
to be machined. The angle of the taper with the centerline is
one-half the included angle and will be the angle the
compound rest is set for. For example, to true up a lathe center
which has an included angle of 60°, the compound rest would
be set at 30° from parallel to the ways (Figure 7-41).
If there is no degree of angle given for a particular job, then
calculate the compound rest setting by finding the taper per
inch, and then calculating the tangent of the angle (which is
the: compound rest setting) .
For example, the compound rest setting for the workpiece
shown in Figure 7-62 would be calculated in the following
manner