Unified inch · UNC
#12-24 UNC 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
#15
0.1800" · 67% thread
Form tap
#9
0.1960" · 71% thread
Or 5 mm for 68%
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
- 24 (pitch 0.0417")
- Pitch Ø (basic)
- 0.1889" (4.799 mm)
- Minor Ø (internal, basic)
- 0.1709" (4.341 mm)
- Thread depth (internal)
- 0.0226" (0.573 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 |
|---|---|---|---|
| 11/64 | 0.1719 | 4.37 | 82% |
| #17 | 0.1730 | 4.39 | 79% |
| 4.4 mm | 0.1732 | 4.40 | 79% |
| #16 | 0.1770 | 4.50 | 72% |
| 4.5 mm | 0.1772 | 4.50 | 72% |
| #15suggested | 0.1800 | 4.57 | 67% |
| 4.6 mm | 0.1811 | 4.60 | 64% |
| #14 | 0.1820 | 4.62 | 63% |
| #13 | 0.1850 | 4.70 | 57% |
| 4.7 mm | 0.1850 | 4.70 | 57% |
| 3/16 | 0.1875 | 4.76 | 53% |
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 |
|---|---|---|---|
| 4.9 mm | 0.1929 | 4.90 | 81% |
| #10 | 0.1935 | 4.91 | 79% |
| #9 | 0.1960 | 4.98 | 71% |
| 5 mmsuggested | 0.1969 | 5.00 | 68% |
| #8 | 0.1990 | 5.05 | 60% |
| 5.1 mm | 0.2008 | 5.10 | 54% |
| #7 | 0.2010 | 5.11 | 53% |
3-wire measurement
- Best wire size
- 0.0241" (0.611 mm)
- Over wires, at basic pitch Ø
- 0.2250" (5.716 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