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Three-Wire Thread Measurement (M Value)
The classic metrology method for verifying pitch diameter on external threads.
Formula (60Ā° metric / unified)
M = dā + 3G ā 0.86603 Ć P
Best wire size: G = P/2 Ć cos30Ā° = 0.57735 Ć P
Best Wire Sizes (metric coarse)
| Thread | Pitch P | Best wire G |
|---|---|---|
| M6 | 1 | 0.5774 |
| M8 | 1.25 | 0.7217 |
| M10 | 1.5 | 0.8660 |
| M12 | 1.75 | 1.0104 |
| M16 | 2 | 1.1547 |
| M20 | 2.5 | 1.4434 |
| M24 | 3 | 1.7321 |
FAQ
Why three wires?
Two on one flank, one on the other ā the micrometer measures over all three, and the geometry back-calculates the pitch diameter.
Can I use any wire size?
Preferably the best-wire size, which contacts the thread at the pitch line. Wires must be within certain limits to touch the flanks.