Unified inch · UNC
#2-56 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
#50
0.0700" · 69% thread
Form tap
5/64
0.0781" · 65% thread
Pick your material for its % of thread, tapping RPM and the rigid-tap code.
Thread dimensions
- Major Ø
- 0.0860" (2.184 mm)
- Threads per inch
- 56 (pitch 0.0179")
- Pitch Ø (basic)
- 0.0744" (1.890 mm)
- Minor Ø (internal, basic)
- 0.0667" (1.693 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 |
|---|---|---|---|
| 1.7 mm | 0.0669 | 1.70 | 82% |
| #51 | 0.0670 | 1.70 | 82% |
| #50suggested | 0.0700 | 1.78 | 69% |
| 1.8 mmsuggested | 0.0709 | 1.80 | 65% |
| #49 | 0.0730 | 1.85 | 56% |
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 |
|---|---|---|---|
| #48 | 0.0760 | 1.93 | 82% |
| 5/64suggested | 0.0781 | 1.98 | 65% |
| #47 | 0.0785 | 1.99 | 62% |
| 2 mm | 0.0787 | 2.00 | 60% |
3-wire measurement
- Best wire size
- 0.0103" (0.262 mm)
- Over wires, at basic pitch Ø
- 0.0899" (2.283 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