LATHE OPERATIONS - OD1645 - LESSON 1/TASK 3
j. Taper Screw Threads. Tapered screw threads or pipe threads can be cut
on the lathe by setting the tailstock over or by using a taper attachment.
For the National Taper Pipe thread form, the taper is 3/4-inch per foot.
Check the thread cutter bit carefully for clearances before cutting since
the bit will not be entering the workpiece at right angles to the tapered
In cutting a tapered thread, the cutter bit should be
set at right angles to the axis of the workpiece. *Do
not set the thread cutter bit at right angles to the
taper of the thread.
Measuring External Screw Threads.
(1) The fit of the thread is determined by the pitch diameter. The pitch
diameter is the diameter of the thread at an imaginary point on the thread
where the width of the space and the width of the thread is equal. The fact
that the mating parts bear on this point or angle of the thread, and not on
the top of it, makes the pitch diameter an important dimension to use in
measuring screw threads.
(2) Thread micrometers are used to measure or gage the pitch diameter of
threads (figure 47 on the following page). They are graduated and read in
the same manner as ordinary micrometers. The anvil is V-shaped to fit over
the V-thread. The spindle, or movable point, is cone-shaped (pointed to a
V) to fit between the threads. Since the anvil and the spindle both contact
the sides of the threads, the pitch diameter is gaged and the reading is
given on the sleeve and spindle where it can be read by the operator.
(a) Thread micrometers are marked on the frame to specify the pitch
diameters which the micrometer is used to measure. One will be marked, for
instance, to measure from 8 to 13 threads per inch, while others are marked
14 to 20, 22 to 30, 32 to 40, and so on.
(b) This method of thread inspection
department and the lathe operator.