Unified inch · UNF
#10-32 UNF 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
#20
0.1610" · 71% thread
Or 4.1 mm for 70%
Form tap
#16
0.1770" · 61% thread
Pick your material for its % of thread, tapping RPM and the rigid-tap code.
Thread dimensions
- Major Ø
- 0.1900" (4.826 mm)
- Threads per inch
- 32 (pitch 0.0313")
- Pitch Ø (basic)
- 0.1697" (4.310 mm)
- Minor Ø (internal, basic)
- 0.1562" (3.967 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 |
|---|---|---|---|
| 5/32 | 0.1563 | 3.97 | 83% |
| #22 | 0.1570 | 3.99 | 81% |
| 4 mm | 0.1575 | 4.00 | 80% |
| #21 | 0.1590 | 4.04 | 76% |
| #20 | 0.1610 | 4.09 | 71% |
| 4.1 mmsuggested | 0.1614 | 4.10 | 70% |
| 4.2 mm | 0.1654 | 4.20 | 61% |
| #19 | 0.1660 | 4.22 | 59% |
| 4.3 mm | 0.1693 | 4.30 | 51% |
| #18 | 0.1695 | 4.31 | 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 |
|---|---|---|---|
| #17 | 0.1730 | 4.39 | 80% |
| 4.4 mm | 0.1732 | 4.40 | 79% |
| #16 | 0.1770 | 4.50 | 61% |
| 4.5 mm | 0.1772 | 4.50 | 60% |
3-wire measurement
- Best wire size
- 0.0180" (0.458 mm)
- Over wires, at basic pitch Ø
- 0.1968" (4.998 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