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How to Cut Internal Acme Threads

Guide · Updated October 28, 2026

An Acme nut is cut with a stiff boring-bar threading tool ground to the 29° Acme form. Bore to the nut minor diameter, set the compound at 14.5°, and take light passes out to the nut major diameter. Rigidity matters because the tool is working inside a hole with long overhang.

Size the bore

Measure the minor bore with a bore gage proved on gage blocks before threading. The free Gage Block Calculator builds the stack.

Step by step

Get the thread data: Pitch P = 1 ÷ TPI. For general-purpose (G) Acme, basic thread height = 0.5P.
Bore to the nut minor diameter: Basic internal minor diameter = D − P; the 2G tolerance allows up to +0.05P above that.
Know the nut major diameter: For 10 TPI and coarser the nut major is D + 0.020 in (D + 0.010 in for finer pitches), giving clearance at the screw crest.
Grind the tool: 29° included, tip flat = 0.3707P − 0.259 × major clearance, sides relieved for the helix angle.
Set the compound: 14.5° so the tool feeds down one flank, mirrored for an internal thread.
Cut passes: Light passes with cutting oil, lower speed than external threading, withdraw at the end of each pass.
Fit to the screw: Stop short of full depth and test with the mating screw, then take spring passes.

Worked example

A 3/4-6 general-purpose Acme nut (ASME B1.5 formulas): P = 1 ÷ 6 = 0.1667 in. Basic thread height = 0.5 × 0.1667 = 0.0833 in. Nut minor diameter = 0.750 − 0.1667 = 0.5833 in minimum, with 0.05P = 0.0083 in tolerance, so bore to 0.5833–0.5916 in (aim about 0.587 in). Nut major diameter = 0.750 + 0.020 = 0.770 in. Radial thread depth in the nut = (0.770 − 0.5833) ÷ 2 = 0.0933 in. With the compound at 14.5°, compound travel ≈ 0.0933 ÷ cos 14.5° = 0.0933 ÷ 0.968 = 0.0964 in. Tip flat = 0.3707 × 0.1667 − 0.259 × 0.020 = 0.0618 − 0.0052 = 0.0566 in; grind it to that or a hair under. Check the helix angle at the pitch diameter (about 0.667 in): tan⁻¹(0.1667 ÷ (π × 0.667)) = 4.5°, so relieve the leading side about 4.5° + clearance.

Rigidity and chip control

Use the largest bar that fits the 0.587 in bore, keep overhang under about 4 × bar diameter, and run around half the surface speed you would use for the external thread. Blow chips out between passes so they do not pack and push the tool off.

Common problems

A tight nut almost always means not deep enough; take spring passes or a little more depth. Chatter means too much overhang or too wide a tip flat engaging at once. Torn flanks mean not enough helix clearance on the leading side. A nut that rocks on the screw means the bore was cut oversize or depth went too far.

Tips

External: cut Acme threads.
Internal: cut internal threads.
Boring: boring bar.

FAQ

What is the thread height of a general-purpose Acme?

0.5 × pitch basic; the nut is cut 0.010–0.020 in larger on the major for clearance, so the actual nut depth is a little more.

What size do I bore an Acme nut?

To the minor diameter D − P, within +0.05P; for 3/4-6 that is 0.5833–0.5916 in.

What compound angle for Acme?

14.5°, half the 29° included angle.

How wide is the tool tip?

0.3707P minus 0.259 × the major-diameter clearance; about 0.057 in for a 6 TPI nut.

Why is my nut tight?

The thread is not yet full depth; take spring passes and test with the screw.

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