Tc 9-524 fundamentals of machine tools

THREAD CUTTING TOOL BITS
Cutting V-threads with a 60 degrees thread angle is the
most common thread cutting operation done on a lathe. V-
threads. with the 60 degree angle. are used for metric thread
cutting and for American (National) threads and Unified
threads. To properly cut V-shaped threads. the single point
tool bit must be ground for the exact shape of the thread
form. to include the root of the thread (Figure 7-75).
For metric and American (National) thread forms. a flat
should be ground at the point of the tool bit (Figure 7-76).
perpendicular to the center line of the 600 thread angle. See
the thread form table for the appropriate thread to determine
the width of the Sat. For unified thread forms. the tip of the
tool bit should be ground with a radius formed to fit the size
of the root of the thread. Internal unified threads have a flat
on the tip of the tool bit. In all threads listed above. the tool bit
should be ground with enough side relief angle and enough
front clearance angle (Figure 7-76). Figure 7-77 illustrates the
correct steps involved in grinding a thread-cutting tool bit.
7-47

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TC 9-524
For Acme and 29° worm screw threads, the cutter bit must
be ground to form a point angle of 29°. Side clearances must
be sufficient to prevent rubbing on threads of steep pitch. The
end of the bit is then ground to a flat which agrees with the
width of the root for the specific pitch being cut. Thread-
cutting tool gages (Figure 7-78) are available to simplify the
procedure and make computations unnecessary.
To cut square threads, a special thread-cutter bit is required.
Before the square thread-cutter bit can be ground, it is
necessary to compute the helix angle of the thread to be cut
(Figure 7-79). Compute the helix angle by drawing a line
equal in length to the thread circumference at its minor
diameter (this is accomplished by multiplying the minor
diameter by 3.1416 [pi]). Next, draw a line perpendicular to
and at one end of the first line, equal in length to the lead of
the thread. If the screw is to have a single thread, the lead will
be equal to the pitch. Connect the ends of the angle so formed
to obtain the helix angle.
The tool bit should be ground to the helix angle. The
clearance angles for the sides should be within the helix angle.
Note that the sides are also ground in toward the shank to
provide additional clearance.
The end of the tool should be ground flat, the flat being
equal to one-half the pitch of the thread to produce equal flats
and spaces on the threaded part.
When positioning the thread-cutter bit for use, place it
exactly on line horizontally with the axis of the workpiece.
This is especially important for thread-cutter bits since a slight
variation in the vertical position of the bit will change the
thread angle being cut.