Question 1

What wire diameter should I use if the 'best size' is not available?

Accepted Answer

Any wire diameter between the minimum (contacts at thread root) and maximum (contacts above the pitch line) can be used, but accuracy decreases as you move from the best size. The acceptable range for 60° threads is roughly 0.505P to 0.577P. Using non-best wires requires a correction factor for the helical angle effect. Always measure wire diameter with a micrometer to ±1 μm before use.

Question 2

How do I account for thread angle errors in wire measurement?

Accepted Answer

The standard formula assumes a perfect 60° thread angle. If the actual thread angle deviates (common in worn or poorly ground threads), a correction is needed: ΔM = W × (1/cos(α₁/2) − 1/cos(30°)) where α₁ is the actual half-angle. For a 0.5° half-angle error with 1.45 mm wire: ΔM ≈ 0.002 mm. For precision work, measure both flanks separately using profile projector or CMM.