3-wire thread calculator. Mic it over the wires.
Enter the pitch and the pitch diameter limits from your thread table. Get the micrometer reading over three wires that means the thread is in spec, plus the right wire size to use.
Measurement over wires
- M at max pitch diameter
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- M at min pitch diameter
- —
- Pitch
- —
- Wire used
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| Wire | Size | Factor |
|---|
Simplified formula, no lead-angle (helix) correction. Fine for most shop checks with best-size wires. For gauge-grade work use ASME B1.2 or the full formula.
Three wires, one micrometer reading.
Lay one wire in a thread groove on one side and two on the other, then mic over the wires. The reading M converts back to pitch diameter.
M = E − 0.866025 × p + 3 × G
E is pitch diameter, p is pitch (1 / TPI for inch threads), and G is the wire diameter. This is the 60° thread form used by UN and ISO metric threads.
Best wire G = 0.57735 × p
The best wire touches the flanks right at the pitch diameter, so small errors in thread angle matter least. Any wire between 0.505p and 1.010p works with the formula.
Your thread is in spec if the reading falls between M at min PD and M at max PD.
Good to know
Where do I get the pitch diameter limits?
From the thread standard for your class of fit: ASME B1.1 for UN threads (for example 1/4-20 UNC-2A) and ISO 965 for metric threads (for example M10x1.5-6g). Machinery's Handbook has both tables.
My wires aren't the best size. Can I still use them?
Yes, as long as they're between 0.505 and 1.010 times the pitch. Enter your actual wire diameter in the wire field and the reading is worked out for that wire.
How far off is the simplified formula?
For common coarse and fine threads with best-size wires, the lead-angle error is usually a few tenths or less. On large leads, multi-start threads or gauge calibration, use the full ASME B1.2 method.
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