Infinite Engineering
Workshop tools · thread wires
Workshop tools · thread wires
Pick the thread, the tolerance class and the wires on the bench. It gives the measurement over wires (M) you should see for the top and bottom of the pitch diameter tolerance, and turns your micrometer reading into an actual pitch diameter. External threads only. Click any answer to copy it.
Enter the major Ø and pitch in mm (25.4 ÷ tpi for an inch thread).
From the drawing, a thread table or the gauge certificate.
Micrometer reading with the three wires in place. Leave blank to see the target figures only.
| Pitch Ø | M over wires |
|---|
Three wires of the same diameter sit in the thread, two on one side and one on the other, and the micrometer reads across them. For a thread of half-angle α (30° for metric and Unified, 27.5° for Whitworth), pitch P and wire Ø w: M = E − P·cot α / 2 + w·(1 + cosec α) + c, where E is the pitch (effective) diameter. For 60° threads that is M = E − 0.86603 P + 3 w + c; for 55° it is M = E − 0.96049 P + 3.1657 w + c. Working back from your reading, E = M + P·cot α / 2 − w·(1 + cosec α) − c.
c is the lead-angle correction, c = (w / 2) · tan²λ · cos α · cot α with tan λ = P / (π E). It is a few microns on single-start threads and is included in every figure here. Wire compression under the measuring force is not included (it is a micron or two with a standard micrometer; for fine work use the measuring force and wire certificate).
The best wire, w = P / (2 cos α) (0.57735 P at 60°, 0.56369 P at 55°), touches the flank exactly at the pitch line, so any error in the thread angle cancels out. Any wire that beds on the flanks works: this page accepts 0.505 P to 1.010 P at 60° and 0.505 P to 0.853 P at 55°, and warns outside that.
Pitch diameter limits: ISO metric from ISO 965-1 (grade 6 tolerance Td2 = 90 P^0.4 D^0.1 µm rounded to the R40 series, grades 4 and 8 at 0.63× and 1.6×; fundamental deviations e = −(50 + 11P), f = −(30 + 11P), g = −(15 + 11P), h = 0 µm). On small sizes the rounded tolerance can differ from the printed ISO table by one R40 step (a few microns); below M4 check against the table. Unified from ASME B1.1 (class 2A Td2 = 0.0015 ∛D + 0.0015 √LE + 0.015 ∛P² in, LE = D; 3A at 0.75×, 1A at 1.5×; allowance 0.3 Td2(2A) on 1A and 2A, none on 3A). BSW, BSF and BSP are given at basic effective diameter only, with a box for the BS 84 or ISO 228 limits from your table, because those class tolerances are tabulated rather than calculated here. All of this is Infinite Engineering's own working from the published standards; it is not any manufacturer's catalogue data. Check critical threads with a thread gauge.
The speeds, feeds, sizes and other figures on this site are a starting guide only. They come from published catalogue data and standards and from general workshop practice, and they do not allow for your machine, tooling, workholding or material condition. Check everything before cutting, prove the first part, and use thread and plug gauges where it matters. All information is used entirely at your own risk. Infinite Engineering Ltd accepts no liability for any loss, damage or injury arising from its use.
Spotted an error, or want something added? Email info@infiniteengineering.co.uk.