LATHE OPERATIONS - OD1645 - LESSON 1/TASK 3
cutting tool gages (figure 42 on the previous page)
(4) To cut square threads, a special thread cutter bit is required.
(a) Before the square thread cutter bit can be ground it is necessary to
compute the helix angle of the thread to be cut (figure 43). Compute the
helix by drawing a line equal in length to the thread circumference at its
minor diameter (this is accomplished by multiplying the minor diameter by
Next, draw a line perpendicular to and at one end of the
first line, equal to its 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.
THREAD CUTTER BIT FOR SQUARE THREADS.
(b) The tool bit should be ground to the helix angle (figure 43) and 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
(c) The end of the tool (figure 43) should be ground flat, the flat
being equal to one-half the