Three-Wire Thread Measurement

Scope: external threads: Unified inch and ISO metric 60° threads, 29° Acme and Stub Acme threads (single- or multi-start, up to 4 starts), and 7°/45° Buttress threads (single-start). Classes offered: Unified 2A and 3A; metric normal-length 6g and 6h; Acme 2G, 3G and 4G and centralizing 2C, 3C and 4C (same pitch-diameter limits); Stub Acme one standard class; Buttress 1, 2 and 3. Unified pitch-diameter limits are calculated for Classes 2A and 3A using the standard Unified tolerance formula and standard length-of-engagement basis. Metric pitch-diameter limits are calculated for normal-length Classes 6g and 6h using ISO 965 product-limit data. The implementation preserves the published whole-micrometre tolerance values rather than reconstructing table values from unrounded formulas.

Best wire size is 0.57735 × pitch. The practical wire range used here is 0.56 × pitch through 0.90 × pitch. Measurement over wires uses the National Bureau of Standards / NIST lead-angle-compensated three-wire method rather than the simpler zero-lead-angle approximation. The Unified thread and three-wire basis uses NBS Handbook H28 as the primary public technical source. Metric tolerance limits use the ISO 965 tolerance system.

Acme and Stub Acme limits follow NBS Handbook H28 (1957) Part III, Sections XII and XIII, for the standard diameter-pitch series: basic pitch diameter is the major diameter less the basic thread height (0.5 × pitch for Acme, 0.3 × pitch for Stub Acme), the allowance comes from the size-range table, and the tolerance is the pitch increment plus the diameter increment. Measurement over wires uses the H28 Appendix 13 method for 29° threads, which corrects the flank angle for the lead angle. Wire sizes are 0.48726 × pitch to 0.65001 × pitch, best 0.51645 × pitch. On Stub Acme no wire smaller than the best size is used, the minimum is raised further where a smaller wire would bottom in the shallow root, and the largest wire is held to about 0.597 × pitch so it still touches the flanks below Stub Acme's lower crest. Measuring force is 1 lb for 8 threads per inch and finer and 2 lb for coarser pitches.

Multi-start Acme and Stub Acme use the same pitch-diameter limits as single-start (H28 gives no separate product limits). Their steeper lead angle is handled with the exact H28 Appendix 13 method (Marriner and Wood), which also sets the best wire for that lead angle. The wire range follows the same rule as the single-start range, with the lead angle accounted for: the smallest wire still stands above the crest and the largest still touches the flanks below it. If a wire would touch one flank in two places, H28 says to measure with balls of the same diameter instead, and the page says so.

Buttress limits follow NBS Handbook H28 (1957) Part III, Section XIV: basic pitch diameter is the major diameter less 0.6 × pitch, the allowance on the external thread is the Class 3 tolerance (Table XIV.5), and the pitch-diameter tolerances are H28's printed Tables XIV.2 to XIV.4 for Classes 3 (close), 2 (medium) and 1 (free). Classes 2 and 3 are the same in the current ANSI B1.9 standard; Class 1 is from H28 only, as the later standard dropped it. H28 gives no standard diameter-pitch series for Buttress threads, so the sizes offered are its preferred diameters and the pitches it tabulates for each size range. Measurement over wires uses the H28 Appendix 13 formula for the 7°/45° form, M = E − 0.89064 × pitch + 3.15689 × wire, with the small lead-angle correction neglected as H28 recommends for single-start threads. Best wire is 0.54147 × pitch and the largest is 0.61433 × pitch; the smallest is the wire whose top still clears the crest, 0.47218 × pitch. H28 gives no measuring force for Buttress threads.

The actual calibrated wire diameter is used in the measurement calculation. This tool does not substitute for calibrated wires, controlled measuring force, or the applicable inspection/gaging specification.