Thread Depth Calculator

Where 0.613 × pitch comes from

For a standard 60° thread form, the full theoretical depth from major diameter down to minor diameter works out to 0.61343 times the pitch — this is the standard constant published across machining references for single-point external threading depth of cut, and it's what this calculator uses directly: depth = 0.61343 × pitch, with pitch = 1÷TPI for imperial threads.

Why the compound rest angle matters

Feeding straight in (perpendicular to the workpiece axis) cuts on both flanks of the thread at once, which loads both cutting edges simultaneously and tends to produce a rougher finish and more chatter. Feeding in at a compound angle — 29.5° is the standard for a 60° thread — cuts mostly on the leading flank instead, which most machinists find gives a cleaner finish and easier chip control. Because that angled feed travels further to reach the same actual depth, the compound's dial reading needs to be the perpendicular depth divided by cos(29.5°), not the perpendicular depth itself.

This is the full depth, not a single pass

The number this calculator gives is the total depth of cut across the whole threading operation, not one pass — single-point threading is always done in multiple progressively lighter passes (often 8–12 for a fine finish thread), with the first passes taking more material and the last couple taking only a few thousandths for finish. Most lathes' threading charts or CAM posts handle the pass-by-pass breakdown; this number is the target to work down to across all of them.