CNC / machinist calculator
ACME Thread Calculator (General Purpose, ASME B1.5)
ACME threads are the 29-degree trapezoidal threads used on lead screws, vises, presses and jacks, where a square-ish flank carries load better than a vee thread. Enter the nominal major diameter and the threads per inch, choose a General Purpose class, and this calculator returns the basic form, pitch diameter and minor diameter, and the full external and internal limits, so you can cut, gauge or inspect a General Purpose ACME thread to ASME B1.5 without pulling the tables.
| External (screw), in | Max | Min |
|---|---|---|
| Major diameter | - | - |
| Pitch diameter | - | - |
| Minor diameter | - | - |
| Internal (nut), in | Min | Max |
|---|---|---|
| Minor diameter | - | - |
| Pitch diameter | - | - |
| Major diameter | - | clearance |
How it works
A General Purpose ACME thread has a 29-degree included flank angle and a thread height of half the pitch. The pitch is one divided by the threads per inch, the basic pitch diameter is the major diameter minus half the pitch, and the basic minor diameter is the major diameter minus one whole pitch. General Purpose threads add a clearance at the crest and root, 0.010 inch for ten threads per inch and coarser and 0.005 inch for finer pitches, so the flanks carry the load rather than the tips.
The fit between screw and nut is set by an allowance and a tolerance. Only the external (screw) pitch diameter carries the allowance, 0.008 times the square root of the diameter, which is subtracted so the screw is a touch small. The pitch-diameter tolerance depends on the class: 2G is the general case, 3G and 4G are progressively tighter for less backlash. Major and minor diameters carry their own tolerances derived from the pitch and the pitch-diameter tolerance.
The limits here are computed from the ASME B1.5 formulas and were checked against the published B1.5 dimension tables. On sizes whose pitch is a repeating fraction, such as 3/4-6, a limit can differ from a printed table by 0.0001 inch because the table rounds the basic size first; the values here carry full precision.
Worked example
A 1/2-10 General Purpose 2G ACME screw: pitch 0.100 in, thread depth 0.050 in, and an external pitch diameter of 0.4443 to 0.4306 in after the 0.008 x square-root allowance. The mating nut pitch diameter runs 0.4500 to 0.4637 in.
Frequently asked questions
What is an ACME thread?
An ACME thread is a 29-degree trapezoidal thread used for motion and power transmission, such as lead screws and clamps. The wide flat flanks carry axial load and wear better than a vee thread, and are easier to cut and inspect than a square thread.
What is the difference between General Purpose classes 2G, 3G and 4G?
They set how tight the fit is. 2G is the general-purpose default with the most clearance and backlash; 3G and 4G have progressively smaller pitch-diameter tolerances for tighter, lower-backlash assemblies at higher cost.
How deep is an ACME thread?
The basic thread height is half the pitch. General Purpose threads add a clearance of 0.010 inch for 10 TPI and coarser, or 0.005 inch for finer pitches, so the cut external minor is the major diameter minus one pitch minus twice that clearance.
Does this handle metric trapezoidal (Tr) threads?
No. This calculator is for inch General Purpose ACME threads to ASME B1.5. Metric trapezoidal threads follow the 30-degree Tr / DIN 103 standard, which uses different angles and tables and is not covered here.
How accurate are the limits?
They are computed from the ASME B1.5 formulas and cross-checked against the published B1.5 external and internal dimension tables, matching to about 0.0001 inch. The small differences are table rounding on repeating-fraction pitches.
Related calculators
Coverage
Implements ASME/ANSI B1.5 (General Purpose ACME): General Purpose ACME, 29 degree form, inch series, Classes 2G, 3G, 4G, Basic form plus external and internal limits. Deliberately not offered yet:
- Centralizing (C) and Stub ACME threads - different form and tolerance rules from General Purpose; not yet transcribed and verified (revisit: when the Centralizing and Stub tables are sourced and golden-vector tested)
- Metric trapezoidal (Tr / DIN 103, 30 degree) threads - a separate metric standard with a different flank angle and tables
Sources
- ACME screw threads - ASME B1.5 (ASME)
- General Purpose ACME thread dimensions (Machining Doctor)
- Acme thread form (Wikipedia)
Every formula on this page is shown and sourced. See how we verify.
These calculators are for planning and as a starting point. Recommended speeds and feeds are published starting values that vary with your specific tool, coating, machine rigidity, workholding and coolant. Always start conservative, listen to the cut, and follow your tool maker data sheet.