The V on the edge of the gage is used for setting the tool in the tool post of the lathe. The centerline of
the V-point is perpendicular to the axis of rotation of the cylinder to be threaded. This setting will give the
correct thread form.
The two opposite sides of the gage are parallel and have graduations on the edges for checking the
number of threads per inch. One side of the gage at the point is engraved to show the sizes of tap drills
for 60-degree V-threads (internal threads), and to show, in thousandths of an inch, the double depth of
commonly used threads.
The 60-degree angle at the point is useful to gage the thread after it is cut.
Before the threading operation is begun, the work piece needs to be turned to the maximum or major
diameter of the thread specified on the blueprint or other specifications. This size is usually given as
O.D. (outside diameter) expressed in inches and decimal parts of an inch. When cutting V type threads,
it is also good practice, to chamfer or bevel the ends of the shaft to be threaded. The chamfer can be at
any angle from 30 to 4-5 degrees. For instructions on straight turning, see the section on that operation.
SETUP FOR CUTTING AN EXTERNAL (V) THREAD
The setup for external threading will depend largely upon the shape and size of the work piece. Threads
are often turned on shafts mounted between centers and revolved by a lathe dog.