solvecraft/docs/thread-catalogue.md

Tapped-hole catalogue

The Hole dialog offers ISO metric coarse sizes, 15 additional fine diameter/pitch pairs, a finite set of 14 UNC and 14 UNF sizes, six sizes each for internal G/BSPP and Rp parallel pipe threads, and the ISO 7-1 Rc (1/16 to 4) and ASME B1.20.1 NPT (1/16 to 3) internal taper threads. This is a verified subset, not complete coverage of these standards. Unified sizes range from #4 through #12 and 1/4 through 1 inch. Include the family in a designation, for example 1/4-20 UNC or #10-32 UNF.

Internal units are millimetres. Unified screw diameters convert using exactly 25.4 mm/in; pitch is 25.4 divided by threads per inch. Numbered screw sizes are nominal screw diameters, not numbered drill diameters. Pipe fractions are nominal labels: G 1/4 has a basic major diameter of 13.157 mm, not 6.35 mm.

For UNC/UNF, the proposed pilot diameter comes from Guhring's CUT TAPS, approximately 75% theoretical thread column. Fractional, letter and numbered drills are converted to millimetres without rounding away the recommendation. These are starting recommendations for cutting taps, not form taps. Material, depth, tolerances, drilling accuracy and the tap manufacturer's process guidance still determine the appropriate drill; the command's explicit diameter can override the recommendation.

Metric holes take the tap maker's drill: Yamawa YEU23's drill size column for cutting taps, printed p.704 (M, coarse) and pp.704 to 706 (MF, fine); M8 drills 6.8 mm, M12 10.3 mm. A custom size the table lacks takes the major diameter less the pitch, to 0.001 mm (M8.5x1: 7.5 mm). This replaces the major-minus-pitch estimate every metric size took before (M8 6.75 mm). A saved design keeps the diameter it stored, so it builds as before; editing such a hole keeps its drill. The Design preference "Metric tapped holes drill the major diameter less the pitch" (metric_tap_drill_major_less_pitch, off by default) gives new metric holes that older drill again, in the Hole dialog and for the solid.hole command given no diameter. Custom metric dimensions remain editable, and Custom stays selected even when its current dimensions match a catalogue entry.

Choosing another drill

The Hole dialog's Tap drill diameter is the tap maker's drill for the size and can be changed; the command's diameter does the same. A drill is accepted from the internal thread's standard minor diameter (ISO metric and Unified D1 = D − 1.082532 P, G and Rp their basic minor, Rc and NPT the minor where the thread ends) up to 5% over the tap maker's drill. Where the tap maker's drill lies below that minor (Rp, Rc, NPT), the drill itself is the lower end. A smaller drill would leave the tap cutting inside the thread's minor diameter; a larger one leaves too little of the thread engaged. M8 accepts 6.647 to 7.14 mm. The field's tooltip gives the minor diameter and the range. A new size takes its own tap maker's drill again.

Tapped holes create the physical pilot bore and the thread, listed (cosmetic) or, with modeled, cut into the wall as the Thread command cuts a modelled thread: the entry is where it starts, and towards the bottom it stops square at the thread depth, clear of a blind hole's bottom, or fades out (ease_out, ISO 3508). A Unified thread is cut with the ISO 68-1 basic profile both share. External R, thread classes and sealing behaviour are outside this catalogue.

Taper Rc and NPT threads

An Rc or NPT hole takes the tap maker's drill for a straight (not reamed) hole: Yamawa YEU23's "maximum size of straight bored hole", for Rc with non-full threads (printed p.709, Rc 1/16 to 4) and for NPT (printed p.710, 1/16 to 3). A size of ISO 7-1 or ASME B1.20.1 outside those (Rc 5 and 6, NPT 3 1/2 to 12) is refused as a tapped hole. The Hole dialog shows the drill and, as its tooltip, the standard's minor diameter where the thread ends: the reference plane's minor (d - 2h for ISO 7-1, E1 - h for NPT) at the face less the 1 in 16 taper over the thread's length (ISO 7-1's useful thread, NPT's effective thread L2). Rc 1/2: drill 17.6 mm, minor 17.807 mm; 1/2 NPT: drill 17.60 mm, minor 17.473 mm. An explicit diameter overrides the drill. Listed, the hole is that cylindrical drill; modelled, the thread's taper is cut into it (the tap's minor cone and the helical groove).

Parallel G/BSPP and Rp threads

Both families use a 55-degree profile and a cylindrical pilot bore. Keep G and Rp separate: their process-drill recommendations differ. The UI shows the nominal pipe designation, actual basic major diameter, pitch/TPI and pilot drill. The command accepts canonical labels such as G 1/4 and Rp 1/4, compact forms G1/4 and Rp1/4, and BSPP 1/4 as an alias for G. The Thread command cuts them too (forms bspp and rp).

Nominal	TPI	Major mm	Pitch diameter mm	Minor mm	G drill mm	Rp drill mm
1/8	28	9.728	9.147	8.566	8.75	8.5
1/4	19	13.157	12.301	11.445	11.75	11.4
3/8	19	16.662	15.806	14.950	15.25	14.8
1/2	14	20.955	19.793	18.631	19.0	18.5
3/4	14	26.441	25.279	24.117	24.5	24.0
1	11	33.249	31.770	30.291	30.75	30.2

Pitch is exactly 25.4/TPI. These basic thread dimensions are separate from process bores. The catalogue uses Yamawa's drill column as its default pilot. Its separate recommended process-bore references, for the six rows above, are G: 8.78, 11.78, 15.28, 19.04, 24.52, 30.77 mm and Rp: 8.55, 11.5, 14.9, 18.55, 24.1, 30.25 mm. A saved explicit diameter remains authoritative. Listed, the geometry is the cylindrical pilot plus the designation; modelled, the helical groove is cut too.

Primary sources

• Yamawa, YEU23 European catalogue, printed page 668: basic G/Rp dimensions; page 707: separate G and Rp drill and recommended bore columns; page 709: Rc drills (maximum straight bored hole); page 710: NPT drills. These supply the finite parallel-pipe entries above.

• Guhring, Tap Drill Sizes, PDF page 2: UNC/UNF pitch and practical CUT TAPS approximately75% drills; page 1: decimal equivalents for named drills. The separate FORM TAPS columns are not used. Also verifies 14 of the additional metric fine pairs.

• PEM, Thread Code Comparison Chart: nominal diameters of #4, #6, #8, #10 and #12 screws.

• Yamawa, European catalogue, printed page159: M12x1.25 diameter/pitch pair. Its product-specific pilot recommendation is not used for the retained metric estimate.

• OSG, Tap Drill Sizes and Pitch Diameter Limits, Vol3 (2024): independent cross-check of UNC/UNF pitches. OSG's calculated percentages and Guhring's practical named drills are not interchangeable columns.

Trial

On a small plate, open Hole, choose Tapped, then switch Profile between ISO metric, UNC, UNF, G / BSPP, Rp, Rc and NPT. Try M8, M8x1, 1/4-20 UNC, #10-32 UNF, G 1/4, Rp 1/4, Rc 1/4 and 1/4-18 NPT, listed and Modeled, with the End Square or Fade. Check the displayed drill, create the hole, save, reopen and check the designation and bore. In ISO metric, choose Custom, edit the nominal diameter and pitch, and verify that Custom stays selected and an invalid draft cannot apply an earlier valid value.