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How to Cut Buttress Threads on a Lathe
Guide · Updated October 28, 2026
A buttress thread carries heavy load in one direction, like a vise screw, jack or breech. The pressure flank is nearly square (7° from perpendicular) and the back flank is 45°. It cuts like a single-point thread, but the tool is asymmetric and must face the right way.
Measure the major diameter
Check the blank OD with a mic proved on gage blocks before threading. The free Gage Block Calculator builds the stack.
Step by step
- Know the load direction: The steep 7° flank faces the load. Get this right before grinding.
- Grind the tool: 7° on the pressure side, 45° on the back side, with a small flat at the tip.
- Set the compound: Set the compound to about 5–6° (just under 7°) so the tool feeds down the steep flank and the back side does the cutting.
- Set tool on center: Square it to the work using a thread gage ground for the form.
- Find depth: Depth ≈ 0.6 × pitch for the common 7°/45° form.
- Cut passes: 0.003–0.006 in per pass with oil, reducing near final depth.
- Check fit: Measure and test with the nut or a gauge.
Worked example
A 1.500 in, 6 TPI buttress: pitch P = 1 ÷ 6 = 0.1667 in. Thread depth ≈ 0.6 × 0.1667 = 0.100 in, so minor diameter ≈ 1.500 − 2 × 0.100 = 1.300 in. At 60 SFM with HSS, RPM = 3.82 × 60 ÷ 1.500 = 153 RPM. With the compound at 6° and 0.005 in per pass on the compound, you need about 0.100 ÷ cos 6° ÷ 0.005 ≈ 20 passes plus spring passes.
Buttress vs Acme
Acme is symmetric at 29° and good for both directions. Buttress takes higher one-way load with less radial push on the nut.
Common problems
A nut that only fits one way usually means the flanks were reversed. Tearing on the back flank means too much depth per pass. Tight fit means not enough depth; take spring passes.
Tips
- Acme: cut Acme threads.
- Square: cut square threads.
- Basics: cut threads on a lathe.
Second worked example: 8 TPI buttress depth
ANSI B1.9 buttress threads use a 7° pressure flank and a 45° clearance flank. At 8 TPI the pitch is P = 1 ÷ 8 = 0.125 in, and the basic height of thread engagement is h = 0.6P = 0.6 × 0.125 = 0.075 in. At 4 TPI (P = 0.250 in) the same rule gives 0.150 in, so depth scales directly with pitch.
Plan passes by total depth: at 0.075 in, about 8–10 passes of 0.008–0.010 in early and 0.002–0.003 in near finish is typical on steel. Check the 7° flank orientation against the load direction on the drawing before the first pass; a buttress cut backwards carries load on the 45° flank and fails the part.
Quick shop check
Common problems: the load flank must face the direction of the load. Mark the part before cutting so the steep flank faces the correct way.
FAQ
What angles are buttress threads?
Commonly 7° pressure flank and 45° back flank.
Which way does it face?
The steep 7° flank faces the load.
How deep is a buttress thread?
About 0.6 × pitch for the common form.
What compound angle?
A little under the steep flank angle, about 5–6°.
Why buttress instead of Acme?
Higher load in one direction.
Related guides and calculators
- How to Cut Acme Threads on a Lathe
- How to Cut Square Threads on a Lathe
- How to Cut Threads on a Lathe
- Thread Pitch Gage Basics
- How to Set Tool Height on a Lathe
- How to Use a Dead Center
- How to Cut a Face Groove on a Lathe
- All free machinist calculators
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