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How to Cut Internal Square Threads on a Lathe

Guide · Updated October 28, 2026

Square threads carry heavy loads with little friction, which is why they show up in vises, jacks and presses. Cutting the nut is harder than the screw: the tool works blind inside a bore, it must be ground for the helix, and it plunges straight in with no compound to help.

Check the bore first

Size the minor bore with a bore gage set on gage blocks. The free Gage Block Calculator builds the stack.

Step by step

  1. Work out the nut sizes: Basic square-thread depth and groove width are each half the pitch; the nut minor bore is the screw major minus one pitch, plus a little clearance.
  2. Bore the minor diameter: Bore to the minor size plus clearance, with a chamfer at the start.
  3. Grind the tool: Grind a square-nosed boring tool to just over half the pitch wide, with side clearance tilted to the helix angle.
  4. Set the gearbox: Set for the lead of the screw.
  5. Plunge feed: Feed straight in with the cross slide, about 0.002–0.004 in per pass, lighter near depth.
  6. Watch the end: Mark the thread length on the bar; stop and back out on time in a blind nut.
  7. Fit to the screw: Try the screw; take spring passes until it turns freely without shake.

Worked example: 1 in, 5 TPI square thread nut

P = 1 ÷ 5 = 0.200 in. Thread depth = 0.5P = 0.100 in. Basic nut minor = 1.000 − 2 × 0.100 = 0.800 in; bore it a few thousandths over (for example 0.805 in) so the screw crests do not bind. Groove width = 0.5P = 0.100 in; grind the tool to about 0.101–0.102 in for clearance. Helix angle at the mean diameter (0.900 in): atan(0.200 ÷ (π × 0.900)) = atan(0.0707) = 4.05°. Grind the leading side clearance at least that much more than the trailing side.

Why the helix matters

At 4° on a coarse thread, a tool ground straight rubs on its leading side and tears the flank. Coarser leads and smaller diameters need more helix clearance.

Fitting the nut

Make the screw first and use it as the gage. A square thread should run with little end play; check with an indicator on the screw end while rocking the nut.

Common problems

Chatter: long, thin boring bar. Torn flanks: not enough helix clearance. Tight fit: tool too narrow or depth short. Crashing in blind nuts: run at low speed and use a relief groove at the bottom.

Tips

FAQ

How deep is a square thread?

Basic depth is half the pitch.

What is the nut minor?

Major minus one pitch, plus a little clearance.

How wide should the tool be?

Just over half the pitch.

Why grind for the helix?

So the tool does not rub its leading side.

Compound or plunge?

Plunge straight in with the cross slide.

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