Inspecting an ISO metric thread with wires and gauges
On the drawing of a stud for a pressurised assembly, the end thread calls for M16 x 1.5 - 6g. The CNC lathe setter has just machined the first part. The GO thread ring screws on without effort, but the pitch diameter has to be confirmed to the thousandth with calibrated wires, to guarantee the joint holds under load without stripping the thread.
Measuring method and inspection rules with gauges and wires
Inspecting an ISO metric thread with gauges and wires rests on three separate checks: the major diameter over the crests (measured with an ordinary micrometer), the pitch diameter d2 (measured by the three-wire method) and overall assembly conformity, confirmed with GO and NO GO ring gauges.
| Thread element | Inspection means | Acceptance criterion |
|---|---|---|
| Major diameter (crest) | Flat-anvil outside micrometer | Between the limits d min and d max |
| Pitch diameter d2 | Three-wire method with a micrometer | Measurement over wires M calculated to ISO 965 |
| Overall thread conformity | Full-form GO thread ring (6g) | The ring must screw on freely along the whole length |
| Rejecting a thin flank | Truncated NO GO thread ring | The ring must not engage more than 2 turns |
Only the combination of the dimensional pitch diameter check and the gauge check guarantees ISO 965-1 interchangeability without rejection at assembly.
Formula for the pitch diameter of a 60° external thread
For an ISO metric thread with a 60 degree profile angle, measuring the pitch diameter d2 by the three-wire method means placing two calibrated wires in two thread grooves on one side, and one wire on the opposite side.
The theoretical dimension measured over the wires, M, is expressed from the pitch diameter sought d2, the thread pitch P and the wire diameter W:
M = d2 + 3 × W - 0.86603 × P
To find the actual pitch diameter d2 from a reading over wires M taken on the outside micrometer, use the inverse formula:
d2 = M - 3 × W + 0.86603 × P
The best wire size depends on the pitch: W = 0.57735 × P. For a 1.5 mm pitch, the ideal wire is 0.866 mm in diameter.
Dimensional limits of an M16 x 1.5 class 6g thread
For an M16 x 1.5 - 6g external thread, ISO 965-2 sets the fundamental deviation position g, i.e. a negative upper deviation es of -0.032 mm.
The limits on the pitch diameter d2 are as follows.
| Quantity | Value |
|---|---|
| Nominal pitch diameter, d2 nom | 15.026 mm |
| Upper deviation, es | -0.032 mm |
| Upper limit, d2 max | 14.994 mm |
| Class 6 tolerance, Td2 | 0.118 mm |
| Lower limit, d2 min | 14.876 mm |
The acceptable range for the pitch diameter therefore runs from 14.876 mm to 14.994 mm.
To go back over the rules for calculating fundamental deviations, read our guide on ISO 286 fits and numerical limits.
Full worked example: a 42CrMo4 steel stud
An inspector checks a batch of M16 x 1.5 - 6g studs fresh from the lathe. The measurement takes place in a metrology room held at 20 °C, with an outside micrometer whose anvils are at least 6 mm in diameter and three calibrated 0.866 mm wires.
The reading over the three wires on the stud gives M = 16.305 mm.
| Step | Value |
|---|---|
| Reading over wires, M | 16.305 mm |
| Wire diameter, W | 0.866 mm |
| Three wires, 3 × W | 2.598 mm |
| Pitch correction, 0.86603 × 1.5 | 1.299 mm |
| Effective pitch diameter, d2 | 15.006 mm |
| ISO 965-2 limits for 6g | 14.876 to 14.994 mm |
Verdict: the calculated pitch diameter, 15.006 mm, exceeds the upper limit by 0.012 mm, i.e. 12 microns. The thread is too full on the flanks. The GO thread ring will refuse to screw on all the way. The part has to be reworked with an extra finishing pass on the lathe.
Classic pitfalls when inspecting metric threads
The first pitfall is using wires whose diameter isn’t certified, or that have a worn flat. A wire worn by 2 microns throws the measurement over wires out by 6 microns on the calculated pitch diameter, which leads to accepting a thread that is too thin and may strip under tightening torque.
The second comes from forgetting to check the major diameter over the crests. If the machinist leaves a major diameter of 16.15 mm, above the 15.968 mm maximum for an M16 6g, the thread crest will jam in the root of the internal thread at assembly, even if the pitch diameter d2 conformed.
The third is forcing the NO GO thread ring. If the operator screws the NO GO ring on with a wrench and too much torque, they destroy the first lead threads of the gauge and wrongly accept a stripped thread whose strength is compromised.
To handle these inspection requirements without errors and export compliant reports, CoteCote’s dimensional quality inspection modules make entering metrology readings reliable, and our support and questions page answers your technical questions.