28
Threads
per inch
Dial
divisions
18
1, 5
19
1
20
1, 3, 5, 7
22
1, 5
24
Any
26
1, 5
28
1, 3, 5, 7
Threads
per inch
Dial
divisions
32
Any
36
1, 3, 5, 7
38
1, 5
40
Any
44
1, 3, 5, 7
48
Any
52
1, 3, 5, 7
Tool Bit
For threading, the tool bit is ground to the profile of the thread. For most
threads, this is a point with a 60° included angle. The front of the tool should
have about 10° of relief. No back rake is used. The left side should have about
8° of relief, and the right side should have about 10° of relief. The tip of the
tool should have a flat that is 1/8 of the thread pitch.
Compound Angle
Set the compound rest at a 29.5° angle from a line perpendicular to the axis of
the lathe. This allows you to advance the tool with the compound rest. At this
angle the tool cuts only on the left side of the thread form. This helps prevent
chatter that might result from cutting the entire V form of the thread at once.
It is best to use an accurate protractor when setting the compound rest. The
protractor on the lathe is not accurate enough.
