Thread measuring wires
The best wire touches the flank at the pitch line: w = 0.57735 × pitch for 60° threads. For an M10×1.5 that is a 0.866 mm wire, and over best wires a 6g thread reads 10.163 to 10.295 mm: the class’s pitch diameter limits put through the same sum. Type or pick the external thread below and open the Check tab: it gives the reading for the class, the basic size and the thread as drawn, and takes the wire you actually have.
Three-wire measurement is the shop-floor way to verify pitch diameter without a thread micrometer; the same geometry underlies both. Wire sets are certified to a size — measure with the size you actually have, not the ideal.
How does the three-wire method work?
The three-wire method measures a thread’s pitch diameter without a thread micrometer. Three identical wires sit in the thread grooves — two on one side, one opposite — and a plain micrometer measures over them. Pick the best-wire size, w = 0.57735 × pitch for a 60° thread, and it contacts the flanks at the pitch line so the reading maps cleanly to pitch diameter.
Any wire from 0.505 to 1.010 times the pitch rests on both flanks (NBS Handbook H28, Table 4.1), so measure with the wires in your set and enter their certified size: the expected reading moves with the wire. Past a 5° lead angle the wire’s own tilt matters and the simple formula no longer holds.