Unified inch · UNF
#2-64 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
#49
0.0730" · 64% thread
Form tap
#47
0.0785" · 71% thread
Or 2 mm for 68%
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
- 64 (pitch 0.0156")
- Pitch Ø (basic)
- 0.0759" (1.927 mm)
- Minor Ø (internal, basic)
- 0.0691" (1.755 mm)
- Thread depth (internal)
- 0.0085" (0.215 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 |
|---|---|---|---|
| #50 | 0.0700 | 1.78 | 79% |
| 1.8 mm | 0.0709 | 1.80 | 75% |
| #49 | 0.0730 | 1.85 | 64% |
| 1.9 mm | 0.0748 | 1.90 | 55% |
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 |
|---|---|---|---|
| 5/64 | 0.0781 | 1.98 | 74% |
| #47 | 0.0785 | 1.99 | 71% |
| 2 mmsuggested | 0.0787 | 2.00 | 68% |
3-wire measurement
- Best wire size
- 0.0090" (0.229 mm)
- Over wires, at basic pitch Ø
- 0.0894" (2.270 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