On my fishtail (center?) gauge are printed some numbers labeled "double depth
of sharp thread" for various thread pitches.
I did some math and verified that these numbers are indeed twice the thread
depth, measured perpendicular to the thread axis, for the indicated pitches.
My question is...
What use are these numbers? Since most machinists angle the compound when
cutting threads a number related to feeding straight in seems less useful than
say the compound infeed for some standard compound angle (e.g. 29 deg).
I've never seen a fishtail gauge without these numbers so I have to presume
that they're there for a good reason and 'real' machinists have some use for
them. Perhaps some 'real' machinist can explain?