THREAD-CUTTING TOOLS

For each of the types of thread a tool bit to form that particular thread must be ground. Since the form of
thread is the end product the grinding of the tool bit must take into consideration the clearance angle
side rake form and other factors to achieve the result desired.

CLEARANCE

Because of the rapid advance of the cutting tool across the work piece, the clearance angle is an
important factor. Clean, accurate threads are impossible unless both sides and front of the tool are
given enough clearance to permit the tool to move freely in the groove being formed. When the tool is
fed into the work at an angle, as for form threads, the tool should have 3 to 5 degrees of side clearance.

V thread tools are ground flat across the top, with about a 5-degree side-clearance angle. If the top of
the tool is at an angle, the thread will be incorrect.

The center gage is a tool used for checking the points of cutter bits for the American standard screw
threads, which must be ground to an angle of 60 degrees. The 60-degree included angle on one end of
the gage is used when grinding the tool to the exact angle. This same tool is useful in checking the angle
of lathe centers.