Threading and thread measuring
Acme thread calculator
Acme basic thread height is 0.5 x P; a 1-5 Acme has 0.9000 in basic pitch diameter and a 0.1033 in best wire.
Technical review pending. Formulas and values are cited. This notice comes down after a machinist review.
Acme general purpose thread dimensions
Basic pitch diameter (E)
in
E = D - 0.5 x P = 1.0000 - 0.5 x 0.2000 = 0.9000 in
22.860 mm. Basic, not a limit: take the class 2G external limits from ASME B1.5.
Measurement over best wires at basic E
in
M = E + 4.9939 x W - 1.9334 x P = 0.9000 + 4.9939 x 0.10329 - 1.9334 x 0.2000 = 1.0291 in
Best wire 0.1033 in (2.624 mm). Single start, lead angle ignored.
| Dimension | Formula | Inch (in) | Metric (mm) |
|---|---|---|---|
| Pitch (P) | P = 1 / TPI | 0.2000 | 5.080 |
| Lead | lead = P x starts | 0.2000 | 5.080 |
| Basic thread height | h = 0.5 x P | 0.1000 | 2.540 |
| Basic pitch diameter (E) | E = D - 0.5 x P | 0.9000 | 22.860 |
| External minor diameter, 2G | D - (P + 0.020 in) | 0.7800 | 19.812 |
| Cutting depth, radial | 0.5 x P + 0.010 in | 0.1100 | 2.794 |
| Basic crest flat | 0.3707 x P | 0.0741 | 1.883 |
| Best wire (W) | W = 0.51645 x P | 0.1033 | 2.624 |
| Over best wires at basic E (M) | M = E + 4.9939 x W - 1.9334 x P | 1.0291 | 26.140 |
General purpose (2G) clearance: 0.020 in on the minor diameter for 10 TPI and coarser, 0.010 in for finer pitches.
Source: ASME B1.5 (Acme screw threads); Machinery's Handbook (Acme threads; measuring threads over wires)
How it works
Acme general purpose dimensions
P = 1 / TPI; lead = P x starts; h = 0.5 x P; E = D - 0.5 x P; minor = D - (P + c); depth = 0.5 x P + c / 2
| Symbol | Meaning | Unit |
|---|---|---|
| D | Major diameter (nominal size) | in |
| TPI | Threads per inch | 1/in |
| P | Pitch | in |
| h | Basic thread height | in |
| E | Basic pitch diameter | in |
| c | Minor diameter clearance: 0.020 in for 10 TPI and coarser, 0.010 in finer | in |
| depth | Radial cutting depth for the external thread | in |
Best wire and measurement over wires
W = 0.51645 x P; M = E + 4.9939 x W - 1.9334 x P
| Symbol | Meaning | Unit |
|---|---|---|
| W | Wire diameter; 0.51645 x P touches the flanks at the pitch line | in |
| M | Micrometer reading over three wires | in |
| 4.9939, 1.9334 | Constants for the 29° thread angle (14.5° half angle) |
This page gives the dimensions. The Acme 3-wire measuring calculator turns a reading over wires back into pitch diameter and flags a wire that is too large or too small for the pitch.
Worked example: 1-5 Acme
1-5 Acme, class 2G, single start.
- Pitch: P = 1 / 5 = 0.2000 in.
- Basic height: h = 0.5 x 0.2000 = 0.1000 in.
- Pitch diameter: E = 1.0000 - 0.5 x 0.2000 = 1.0000 - 0.1000 = 0.9000 in.
- Minor diameter, 5 TPI is coarser than 10 TPI: 1.0000 - (0.2000 + 0.020) = 0.7800 in.
- Best wire: W = 0.51645 x 0.2000 = 0.10329 in, about 0.1033 in.
- Over wires: M = 0.9000 + 4.9939 x 0.10329 - 1.9334 x 0.2000 = 0.9000 + 0.5158 - 0.3867 = 1.0291 in.
Result: E 0.9000 in, minor 0.7800 in, best wire 0.1033 in, M 1.0291 in at basic E. Check E against the 2G external limits in ASME B1.5.
Worked example: 3/4-6 Acme
3/4-6 Acme, class 2G, single start.
- Pitch: P = 1 / 6 = 0.1667 in.
- Pitch diameter: E = 0.7500 - 0.5 x 0.1667 = 0.7500 - 0.0833 = 0.6667 in.
- Minor diameter: 0.7500 - (0.1667 + 0.020) = 0.5633 in.
- Best wire: W = 0.51645 x 0.1667 = 0.0861 in.
- Over wires: M = 0.6667 + 0.4298 - 0.3222 = 0.7743 in.
Result: E 0.6667 in, minor 0.5633 in, best wire 0.0861 in, M 0.7743 in at basic E.
Shop notes
- Basic E is not a limit. Class 2G, 3G and 4G pitch diameter tolerances come from ASME B1.5, with separate external and internal values.
- The wire formula is the simplified one for single-start threads with a small lead angle. On multi-start threads, apply the lead angle correction in the Handbook.
- On a multi-start thread the lead is P x starts, and the feed per revolution on the lathe equals the lead.
- An Acme wire set is smaller than a 60° set for the same TPI: 0.51645 x P against 0.57735 x P.
FAQ
How do you measure an Acme thread with three wires?
Best wire W = 0.51645 x P, then M = E + 4.9939 x W - 1.9334 x P. For 1-5 Acme at the basic pitch diameter, M is 1.0291 in over 0.1033 in wires.
How deep is a 1-5 Acme thread?
Basic height is 0.5 x P = 0.1000 in. Cut to 0.1100 in radial with the general purpose clearance, which leaves a 0.7800 in external minor diameter.
What is the included angle of an Acme thread?
29°, 14.5° per flank, per ASME B1.5.
Acme or metric trapezoidal: what is the difference?
Acme is an inch thread with a 29° included angle (ASME B1.5). Metric trapezoidal (Tr) is a 30° thread sized in mm (ISO 2904). Wire sizes and constants differ, so do not swap them.
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