Unified inch · UNF
#12-28 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
#13
0.1850" · 67% thread
Form tap
#8
0.1990" · 70% thread
Pick your material for its % of thread, tapping RPM and the rigid-tap code.
Thread dimensions
- Major Ø
- 0.2160" (5.486 mm)
- Threads per inch
- 28 (pitch 0.0357")
- Pitch Ø (basic)
- 0.1928" (4.897 mm)
- Minor Ø (internal, basic)
- 0.1773" (4.504 mm)
- Thread depth (internal)
- 0.0193" (0.491 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 |
|---|---|---|---|
| #16 | 0.1770 | 4.50 | 84% |
| 4.5 mm | 0.1772 | 4.50 | 84% |
| #15 | 0.1800 | 4.57 | 78% |
| 4.6 mm | 0.1811 | 4.60 | 75% |
| #14 | 0.1820 | 4.62 | 73% |
| #13suggested | 0.1850 | 4.70 | 67% |
| 4.7 mmsuggested | 0.1850 | 4.70 | 67% |
| 3/16 | 0.1875 | 4.76 | 61% |
| 4.8 mm | 0.1890 | 4.80 | 58% |
| #12 | 0.1890 | 4.80 | 58% |
| #11 | 0.1910 | 4.85 | 54% |
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 |
|---|---|---|---|
| #9 | 0.1960 | 4.98 | 82% |
| 5 mm | 0.1969 | 5.00 | 79% |
| #8suggested | 0.1990 | 5.05 | 70% |
| 5.1 mm | 0.2008 | 5.10 | 63% |
| #7 | 0.2010 | 5.11 | 62% |
| 13/64 | 0.2031 | 5.16 | 53% |
3-wire measurement
- Best wire size
- 0.0206" (0.524 mm)
- Over wires, at basic pitch Ø
- 0.2237" (5.683 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