LatheCalculator

Home › Threading › Three-wire measurement

Three-wire thread measurement

Check a thread’s pitch diameter with a micrometer and three wires. It gives the best wire size and the reading you should get over the wires.

Pitch diameter is the basic (nominal) value. Look up the class tolerance for the limits you need.

Safety: stop the spindle before measuring. Wires can flick out when the micrometer is removed, so hold them with a little grease or a wire holder.
How it works, guides and notes

How it works

Put one wire in a thread groove on one side and two on the other, then measure over all three with a micrometer. The “best” wire touches the flanks right on the pitch line, so small errors in the thread angle barely change the reading. Any wire that sits below the crest and clear of the root works, as long as you use its real measured diameter.

With half-angle α, pitch P, pitch diameter E and wire diameter W: best wire = P ÷ (2 cos α) and the reading over the wires M = E + W(1 + 1/sin α) − (P/2) cot α. For 60° threads this simplifies to M = E + 3W − 0.866025P. The small effect of the helix (lead) angle is ignored, which is fine for single-start threads; for coarse multi-start or Acme threads the true reading is slightly larger.

Worked example: M10 × 1.5: E = 9.026 mm, best wire 0.866 mm, so M = 9.026 + 3 × 0.866 − 0.866025 × 1.5 = 10.325 mm.

See the thread calculator for the other dimensions and the screwcutting guide.

Spotted a mistake? Report an error.