How do you measure thread pitch diameter when a part fails the go/no-go gauge?

What does it mean when a thread passes the GO gauge but the NOT GO gauge enters?

It means the thread assembles at the maximum-material limit but its pitch diameter is probably past the minimum-material limit: a tapped hole is too large, or an external thread is too small. It is only a probable result until the gauge, the drag rule and the pitch diameter itself have been checked.

The two gauge members check different things. The GO member is a full-form thread that checks functional size, the cumulative effect of pitch diameter, lead, flank angle, taper and roundness, at the tight end of the tolerance. The NOT GO member, which the inch standards call HI on a plug and LO on a ring, has truncated crests and a short length so it touches the flanks near the pitch line and checks pitch diameter alone at the loose end. The NBS Handbook H28 describes the GO plug as a cumulative check of all thread elements except the minor diameter, and explains that elements can only be checked singly at the minimum-material limit.

That gives four outcomes. GO passes and NOT GO stops: accept. GO passes and NOT GO enters: pitch diameter is likely loose, which weakens engagement and strip strength but rarely stops assembly. GO fails and NOT GO stops: the thread is tight, either because the pitch diameter is small (or large on an external thread) or because lead, angle or burrs make the functional size tight while the pitch diameter itself is fine. GO fails and NOT GO enters: the thread is usually distorted, out of round or tapered, or the wrong gauges are being used. The second and third outcomes are the ones where a direct pitch diameter measurement settles the argument.

Remember the drag rule before calling a reject. Under the inch practice in NBS Handbook H28, the predecessor of ASME B1.2, a HI plug that enters a tapped hole still accepts it if a definite drag from contact with the product results on or before the third turn of entry, and the gauge is not forced after that; the same rule applies to a LO ring on an external thread. The reason given is that the entering threads of the product are incomplete and the starting threads of the gauge wear first. Metric gauging under ISO 1502 sets its own entry allowance for NOT GO gauges, so check that standard rather than applying the inch three-turn rule to metric parts. Inspectors who expect the NOT GO end to stop dead reject good parts; the go/no-go thread gauge guide walks through the full gauge procedure.

How do you measure external thread pitch diameter with the three-wire method?

Place two calibrated wires in adjacent grooves on one side of the thread and a third wire in the opposite groove, measure over the wires with a micrometer or bench comparator, and convert the reading to pitch diameter with the wire formula. For a 60-degree thread the simplified formula is E = M − 3W + 0.86603P, where M is the measurement over wires, W the wire diameter and P the pitch.

The wire size matters. The best wire for a 60-degree thread is 0.57735P, the size that touches the flanks at mid-slope, where the reading is least affected by any error in flank angle. The three-wire appendix of NBS Handbook H28 (Appendix 4) gives that best-wire formula, the simplified pitch diameter formula and a table of best wire sizes by threads per inch; it also notes that any wire size may be used as long as the wires rest on the flanks and project above the crests. The current wire-method practice for gauges sits in ASME B1.2 for inch threads and ASME B1.16M for metric threads. With best-size wires the formula reduces to M = E + 0.86603P, which makes a quick sanity check on the reading. Commercial wire sets rarely match the best size exactly, so always put the calibrated diameter of the wire actually used into the formula, not the nominal size.

A repeatable procedure looks like this:

1. Clean the thread and the wires. Oil, chips and plating residue sit exactly where the wires bear.
2. Use a matched, calibrated set. The three wires should come from one calibrated set; H28 specifies that the three wires of a set share the same diameter within 0.00001 in and warns against mixing wires from different sets of the same nominal size unless they have been calibrated. Thread measuring wires have their own calibration practice in ASME B89.1.17.
3. Seat the wires square. Two wires go in adjacent grooves on one side, one wire in the groove between them on the other side, all at the same axial position.
4. Control the measuring force. Wires deform elastically at the contacts, and a different force gives a different reading. For hardened thread gauges NBS Handbook H28 (1969) recommends 2.5 lb for 20 threads per inch or coarser and 1 lb for finer than 20 up to 40 tpi, each held within 10 percent; use the force in the governing standard, on a bench comparator or a micrometer with a ratchet or friction thimble, and agree the same force with the supplier if readings are to be compared. Do not hold the wires with rubber bands, which H28 says stop them seating and give false readings.
5. Read at more than one place. Take readings at several angular positions and at both ends of the thread; spread between them reveals taper and out-of-round.
6. Convert and compare. Calculate pitch diameter and compare it with the class limits from ASME B1.1 for inch threads or ISO 965 for metric threads.

The simplified formula ignores the small effect of the lead angle, which is acceptable for most coarse and fine series threads with best-size wires; for large pitches, multi-start threads or gauge calibration work, use the full formula with the lead-angle correction from the standard. Also remember what the number is: three wires give a single-element pitch diameter, not functional size. A thread can measure within pitch diameter limits and still refuse a GO ring because of lead or angle error, and that is a real defect, just a different one.

How do you decide whether to reject, rework or accept the lot?

Decide on the measured pitch diameter and functional size against the limits of the gauging system named on the drawing, not on a single gauge reading. Under ASME B1.3, a thread is acceptable if any one approved gauge in the specified system, in current calibration, accepts it.

That acceptance rule is the one most often missed in supplier disputes. A trade column on fastener inspection quotes it directly: whenever any one approved and calibrated gauge accepts a thread, it is acceptable, even if another approved gauge rejects it (Fasteners Clearinghouse, thread acceptability Q&A). So if the supplier's calibrated ring accepts and the receiving gauge rejects, check both gauges before calling the parts bad; a split usually means the parts sit close to a limit.

A practical decision sequence for a suspect lot:

1. Confirm the gauges. Right class, right tolerance position, current calibration, NOT GO checked against its setting plug or ring.
2. Confirm the condition. Was the part gauged before or after coating, and which state does the drawing control?
3. Measure a sample. Take pitch diameter on the failing parts and on several passing parts to see whether the lot is centered near the limit or split into two populations, which points to a tool change or a second machine.
4. Check function. A hole that is slightly loose in pitch diameter still assembles; whether it carries load depends on engagement length and material, which the thread engagement guide covers. Use a deviation only with engineering approval and in writing.
5. Choose the disposition. Tight external threads can sometimes be rechased or the coating stripped and redone; tight tapped holes can be retapped with a sharp tap of the correct limit. Oversize threads cannot be reworked to size, so they are scrapped, used under an approved deviation, or repaired with a thread insert if the design allows it.

How is thread pitch diameter handled on a Saola order?

Threads are cut, tapped and checked where the parts are machined, at our lead CNC plant, and the gauging method for each thread should be settled when the order is quoted. Plating and anodizing are done by partner plants we manage, so the drawing's before-or-after-coating note decides which step owns the thread size.

Saola Manufacturing is the brand of Aximora Global Trading Co., Ltd., which places and manages metal-parts work. CNC turning and CNC milling run at our lead CNC plant, T.K Viet Nam Precision Company Limited, in operation since 2017, whose published inspection equipment includes a 2D vision measuring machine, a height gauge and a hardness tester. That list does not include thread measuring wires, indicating thread gauges or a coordinate measuring machine, so if your drawing calls for gauging system 22 or a recorded pitch diameter, say so in the RFQ and the quote will state how and where that measurement will be made. Surface finishing, die casting and stamping are done by partner plants we manage, and every quote names the production site and the country of origin. ISO 9001 certification is in progress, not held. To get a threaded part quoted with the gauging requirement spelled out, send the drawing through the quote form.