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How to Cut Square Threads on a Lathe
Guide · Updated October 28, 2026
Square threads carry heavy loads in vises and presses with little friction. The flanks are straight, so the tool plunges straight in with the cross slide rather than feeding at an angle. The tool width is half the pitch and the sides need clearance for the helix angle.
Check the minor and major diameters
Measure the major diameter with a mic checked on gage blocks before you start threading. The free Gage Block Calculator builds the stack.
Step by step
- Work out pitch and depth: Pitch P = 1 ÷ TPI. Thread depth = P ÷ 2, tool width = P ÷ 2.
- Find the helix angle: tan(helix) = lead ÷ (π × mean diameter). Grind side clearance a few degrees more on the leading side.
- Grind the tool: Square nose at exactly P ÷ 2 wide (or slightly less), sides relieved for the helix.
- Set the lathe: Gearbox to the TPI, tool on center and square to the work, compound at 0° or parallel to the axis.
- Plunge in steps: Feed straight in with the cross slide, 0.002–0.005 in per pass, slow speed with cutting oil.
- Use a narrower tool for big pitches: On coarse threads, rough with a narrow tool and finish each flank by shifting the compound.
- Check fit: Measure the minor diameter and test with the mating nut.
Worked example
A 1 in diameter, 5 TPI square thread: P = 1 ÷ 5 = 0.200 in. Tool width = 0.100 in, depth = 0.100 in, so minor diameter = 1.000 − 2 × 0.100 = 0.800 in. Mean diameter = 0.900 in. tan(helix) = 0.200 ÷ (3.1416 × 0.900) = 0.0707, so the helix is 4.0°. Grind the leading side about 4° + 3° = 7° clearance and the trailing side about 3°. At 50 SFM, RPM = 3.82 × 50 ÷ 1.000 = 191 RPM; drop lower for control near the shoulder.
Square vs Acme
Acme threads have 29° flanks, are easier to cut and self-center in the nut. Square threads are slightly more efficient but harder to make and to fit; many shops use Acme instead.
Common problems
Rubbing or torn flanks mean not enough helix clearance. A tight nut means too wide a tool or not enough depth. Chatter means too much tool width engaged at once; rough narrower.
Tips
- Alternative: cut Acme threads.
- Basics: cut threads on a lathe.
- Tool height: set tool height.
FAQ
How wide is a square thread tool?
Half the pitch, or a hair less for clearance.
How deep is a square thread?
Half the pitch.
Do I set the compound at 29°?
No, square threads plunge straight in; the compound stays parallel to the axis.
Why do the flanks rub?
The helix angle needs side clearance on the tool.
Why use Acme instead?
Acme is easier to cut, measure and fit and is the common standard.
Related guides and calculators
- How to Cut Acme Threads on a Lathe
- How to Cut Threads on a Lathe
- How to Cut Internal Threads on a Lathe
- How to Set Tool Height on a Lathe
- Thread Pitch Gage Basics
- How to Use a Tool Post Grinder
- How to Use a Quick Change Tool Post
- How to Cut Buttress Threads on a Lathe
- How to Cut Internal Acme Threads
- How to Cut Multiple-Start Threads on a Lathe
- How to Cut Internal Square Threads on a Lathe
- All free machinist calculators
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