Unified inch · UNEF
3/8-32 UNEF tap drill size
For general work, OSG suggests 65–70% of full thread. Harder materials and deep holes want less; the calculator sets it from your material.
Cut tap
S
0.3480" · 67% thread
Form tap
23/64
0.3594" · 74% thread
Pick your material for its % of thread, tapping RPM and the rigid-tap code.
Thread dimensions
- Major Ø
- 0.3750" (9.525 mm)
- Threads per inch
- 32 (pitch 0.0313")
- Pitch Ø (basic)
- 0.3547" (9.009 mm)
- Minor Ø (internal, basic)
- 0.3412" (8.666 mm)
- Thread depth (internal)
- 0.0169" (0.430 mm)
Basic sizes of the 60° form. Tolerance classes (2B/3B) set the limits around them.
Cut tap drills
For a cutting (fluted or spiral) tap. Every standard drill from 50% to 85% thread; suggested rows are in the 65–70% band.
| Drill | Ø in | Ø mm | Thread |
|---|---|---|---|
| 8.7 mm | 0.3425 | 8.70 | 80% |
| 11/32 | 0.3438 | 8.73 | 77% |
| 8.8 mmsuggested | 0.3465 | 8.80 | 70% |
| Ssuggested | 0.3480 | 8.84 | 67% |
| 8.9 mm | 0.3504 | 8.90 | 61% |
| 9 mm | 0.3543 | 9.00 | 51% |
Form tap drills
A form (roll) tap displaces metal instead of cutting it, so it needs a bigger hole. Every standard drill from 50% to 85% thread; suggested rows are in the 65–70% band.
| Drill | Ø in | Ø mm | Thread |
|---|---|---|---|
| T | 0.3580 | 9.09 | 80% |
| 9.1 mm | 0.3583 | 9.10 | 79% |
| 23/64 | 0.3594 | 9.13 | 74% |
| 9.2 mm | 0.3622 | 9.20 | 60% |
3-wire measurement
- Best wire size
- 0.0180" (0.458 mm)
- Over wires, at basic pitch Ø
- 0.3818" (9.697 mm)
For the external thread (the screw). The best wire touches the flanks at the pitch diameter.
Sources
- ASME B1.1 — Unified Inch Screw Threads (UN and UNR thread form)
- OSG — Tap drill size and pitch diameter limit chart (formula, suggested % of full thread)
- Cutting Tool Engineering — "Threads will roll": form tap hole size = D − 0.0068 × % / TPI
- ASME B94.11M — Twist drills (fractional, number, letter and metric sizes)
- NIST (NBS) Letter Circular 23 and Pratt & Whitney — three-wire thread measurement, best wire size