The cross-slide is then adjusted to its zero reading and the compound advanced a distance equal to the
next cut. The operation is repeated until the proper depth of thread is obtained.
The gears to use for each thread are shown on a plate attached to the lathe by the maker. You should
know how to gear the lathe without the plate. There is a gear of a known number of teeth permanently
attached to the head stock spindle.
If the lead screw can be made to advance the cutting tool one thread-space, or thread-pitch, per
revolution of the work, the result will be a thread of that pitch.
The rule for finding the ratio of teeth in the lead-screw gear to the teeth in the stud gear is to multiply the
pitch of the thread to be cut by the teeth in the stud gear and divide by the pitch of the lead screw, which
will give the number of teeth in the lead-screw gear.
A gear with 40 teeth on the screw will cut a thread with a pitch of 10 threads per inch.
Pitch of lead screw, multiplied by the product of all the driving gears, divided by the product of all the
driven gears will give the pitch of the thread to be cut.
With lathes equipped with quick-change gears, most common threads can be set up by moving the
required gears. When it is necessary to cut a thread of an odd pitch, the train of gears between the stud
gear and the quick change-gear box will have to be changed. Metric threads can be cut on a lathe
having a lead screw with the pitch in inches by using the ratio of 50 on the stud to 127 on the screw. This
is because there are 127 centimeters in 50 inches.
The cutting tool is ground to the shape required for the form of screw thread being cut. For cutting 60" V
threads, a center gage is used for checking the angle when grinding the tool to shape.
In cutting a right-hand exterior thread, the compound is turned in the direction of the head stock and set
at an angle of 29".
The tool is set so its centerline is at a right angle to the axis of the work piece. This setting can be
obtained by the use of the center gage as shown.
When the tool point fits uniformly into the v notch of the gage, the tool is at a 90" angle.
NOTE: The point of the tool should be at the same elevation as the centerline of the work piece.
The compound slide is set to an angle of 60", and the tool is set square with the work, using the "V"
notch of the thread gauge to set the tool. The point of the tool must be at the same height as the lathe
centers. The tool is run up to the work with the cross feed, and the cross-feed stop is set to always bring
the cross feed back to the same position after backing out the tool to return for another cut.