rapid movement of the nut or other attached parts is
desired and when weakening of the thread must be
avoided. A single thread having the same lead as a
multiple thread would be very deep compared to the
multiple thread. The depth of the thread is calculated
according to the pitch of the thread.
The tool selected for cutting multiple threads has
the same shape as that of the thread to be cut and is
similar to the tool used for cutting a single thread
except that greater side clearance is necessary. The
helix angle of the thread increases as the number of
threads increases. The general method for cutting
multiple threads is about the same as for single screw
threads, except that the lathe gearing must be based on
the lead of the thread (number of single threads per
Figure 6-100.--Cutting multiple threads using the
thread-chasing dial.
Provisions must also be made to obtain the correct
spacing of the different thread grooves. You can get
the proper spacing by using the thread-chasing dial,
to the lathe apron. To cut 5 threads per inch, on most
setting the compound rest parallel to the ways, using a
lathes, engage the half-nut at any numbered line on
faceplate, or using the change gear box mechanism.
The second groove of a double thread lies in the
the most desirable method for cutting 60 multiple
middle of the flat surface between the grooves of the
threads. With each setting for depth of cut with the
first thread. Engage the half-nut to begin cutting the
compound, you can take successive cuts on each of
second thread when an unnumbered line passes the
the multiple threads so that you can use thread
you cut each thread to the same depth, engage the
half-nut first at one of the numbered positions and cut
To determine the possibility of using the
in the first groove. Then, engage the half-nut at an
thread-chasing dial, first find out if the lathe can be
unnumbered position so that alternate cuts bring both
geared to cut a thread identical to one of the multiple
thread grooves down to size together. To cut a
threads. For example, if you want to cut 10 threads
multiple thread with an even number of threads, first
per inch, double threaded, divide the number of
use the thread-chasing dial to cut the first thread.
threads per inch (10) by the multiple (2) to get the
Then, use one of the other multiple thread-cutting
number of single threads per inch (5). Then, gear the
procedures to cut the second thread.
lathe for 5 threads per inch.
Cutting of multiple threads by positioning the
To use the thread-chasing dial on a specific
compound rest parallel to the ways should be limited
machine, refer to instructions usually found attached
to square and Acme threads. To use this method, set
the compound rest parallel to the ways of the lathe and
cut the first thread to the finished size. Then, feed the
compound rest and tool forward, parallel to the thread
axis a distance equal to the pitch of the thread and cut
the next thread.
The faceplate method of cutting multiple threads
involves changing the position of the work between
centers for each groove of the multiple thread. One
method is to cut the first thread groove in the
conventional manner. Then, remove the work from
between centers and replace it between centers so the
Figure 6-99.--Comparison of single- and multiple-lead
tail of the dog is in another slot of the drive plate, as
threads.
6-59