Three-wire thread measurement
What a micrometer over three wires should read when the thread is right — and what your reading means: pitch diameter, deviation and how much deeper to cut.
Quick answer
Best wire W = 0.57735 × P, measurement over wires M = E − 0.866025 × P + 3W, E = pitch diameter. For 1/2-13 UNC: W ≈ 0.0444″, M ≈ 0.5167″. For M12×1.75: W ≈ 1.010 mm, M ≈ 12.379 mm.
Result
- Nominal pitch diameter E
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- Best wire size
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- Usable wire range
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- Measurement over wires M
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Compared with the nominal pitch diameter, without a tolerance class (6g, 2A etc.): look the tolerance up in the standard's tables. No lead-angle correction — for a single-start thread it is small.
How to measure
Two wires of the same size go into the thread grooves on one side, the third on the opposite side, and you measure over them with a micrometer. The wires touch the flanks, so the reading tells you the pitch diameter — the one the thread actually fits on — not the major diameter.
The full method — with metric and UNC/UNF charts, a CNC recut example and measuring tips — is in the article Three-wire thread measurement.
Who needs it and why
- No mating nut or ring gauge — especially for less common and inch threads: wires and a micrometer are everywhere.
- Recut without removing the part — measure, let the calculator tell you how much to reduce X of the last pass, and run it again in the same groove.
- First part of a batch — set the depth from a measurement instead of by feel, then cut to size.
- Checking an existing thread — see what shape the thread on a repair part is in.
The formulas (60° thread)
- pitch diameter
E = D − 0.649519 · P - best wire
W = 0.57735 · P, usable from 0.505·P to 1.010·P - over wires
M = E − 0.866025 · P + 3W - from a reading
E = M + 0.866025 · P − 3W
Thread depth and the diameter of every pass are in the thread calculator.