Fig. 28. Diagram Illustrating Method of Cutting a Thread
to Compensate for the Error in Pitch due to Shrinkage in Hardening]

As is well known, it is objectionable to cut a thread with the tailstock
center offset, because the work is not rotated at a uniform velocity,
owing to the fact that the driving dog is at an angle with the
faceplate. For a small angle such as 1 degree, however, the error
resulting from this cause would be very small.

If a thread having a pitch slightly less than standard is needed to fit
a threaded part which has contracted in hardening, the taper attachment
can also be used provided the lathe is equipped with special gears to
cut a little less than the required pitch. Suppose a screw having a
pitch of 0.198 inch is required to fit the thread of a nut the pitch of
which has been reduced from 0.200 inch to 0.198 inch. If gears having 83
and 84 teeth are available, these can be inserted in a compound train,
so as to reduce the 0.200 inch pitch that would be obtained with the
regular gearing, to 83/84 of 0.200 or 0.19762 inch. This pitch, which is
less than the 0.198 inch pitch required, is then increased by using the
taper attachment as previously described. (This method was described by
Mr. G. H. Gardner in MACHINERY, February, 1914.)

=Calculating Change Gears for Thread Cutting.=--As previously mentioned,
the change gears for cutting threads of various pitches are shown by a
table or "index plate" attached to the lathe. The proper gears to be
used can be calculated, but the use of the table saves time and tends to
avoid mistakes. Every machinist, however, should know how to determine
the size of gears used for cutting any number of threads to the inch.
Before referring to any rules, let us first consider why a lathe cuts a
certain number of threads to the inch and how this number is changed by
the use of different gears.

As the carriage _C_ and the tool are moved by the lead-screw _S_ (see
Fig. 2), which is geared to the spindle, the number of threads to the
inch that are cut depends, in every case, upon the number of turns the
work makes while the lead-screw is moving the carriage one inch. If the
lead-screw has six threads per inch, it will make six revolutions while
the carriage and the thread tool travel one inch along the piece to be
threaded. Now if the change gears _a_ and _c_ (see also sketch _A_, Fig.
29) are so proportioned that the spindle makes the same number of
revolutions as