Unified inch · UNF
#3-56 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
#45
0.0820" · 73% thread
Or 2.1 mm for 70%
Form tap
#43
0.0890" · 82% thread
Or 2.3 mm for 70%
Pick your material for its % of thread, tapping RPM and the rigid-tap code.
Thread dimensions
- Major Ø
- 0.0990" (2.515 mm)
- Threads per inch
- 56 (pitch 0.0179")
- Pitch Ø (basic)
- 0.0874" (2.220 mm)
- Minor Ø (internal, basic)
- 0.0797" (2.024 mm)
- Thread depth (internal)
- 0.0097" (0.246 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 |
|---|---|---|---|
| #46 | 0.0810 | 2.06 | 78% |
| #45 | 0.0820 | 2.08 | 73% |
| 2.1 mmsuggested | 0.0827 | 2.10 | 70% |
| #44 | 0.0860 | 2.18 | 56% |
| 2.2 mm | 0.0866 | 2.20 | 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 |
|---|---|---|---|
| #43 | 0.0890 | 2.26 | 82% |
| 2.3 mmsuggested | 0.0906 | 2.30 | 70% |
3-wire measurement
- Best wire size
- 0.0103" (0.262 mm)
- Over wires, at basic pitch Ø
- 0.1029" (2.613 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