Thread Milling Passes Calculator
Multiple radial passes with a thread mill follow the same constant-chip-area ladder as single-point threading: cumulative depth after pass n = total depth × √(n/N), so every pass sweeps the same V cross-section. The working infeed calculator returns the whole ladder for any depth and pass count.
The ladder
For total thread depth d and N radial passes, the cumulative depth after pass n holds chip area constant:
d_n = d × √(n/N) — each pass removes the same V cross-section
Worked numbers — M10 × 1.5, three passes
Total external depth ≈ 0.6134 × 1.5 = 0.92 mm. Three constant-area radial passes run 0.531 / 0.220 / 0.169 mm — 57.7%, 23.9% and 18.4% of depth, close to the 60/25/15 split threadmill vendors quote. The first-to-last ratio is 3.15:1, yet every pass loads the flutes equally.
Why not equal passes
Equal radial steps grow the swept V every pass — the last and widest cut lands on the finished thread. The √(n/N) ladder front-loads depth while the work is rigid and finishes light; the calculator also flags passes that fall under your minimum chip thickness.
Frequently asked questions
How many passes should a thread mill take?
One pass works in free-machining material with a rigid setup; two to four constant-area passes are the norm in tough alloys, small mills or long reach. More passes trade cycle time for flute load and deflection — the ladder keeps each pass equal work whatever count you choose.
Does the G76 lathe math really apply to thread milling?
Yes — both remove a 60° V in radial steps, so holding chip area constant gives the identical √(n/N) cumulative-depth series. Only the motion differs: helical interpolation instead of a longitudinal threading pass. Cutter-radius compensation per pass comes from your CAM or control, not the depth ladder.
Related pages
- Threading infeed calculator - the same constant-area ladder, any depth and pass count.
- Tap drill / % thread calculator - minor diameter and engagement for the hole before the thread mill.
- Speeds & feeds calculator - RPM and feed for the thread mill itself.