Unified inch · UNC
#3-48 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
#46
0.0810" · 67% thread
Form tap
#43
0.0890" · 71% thread
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
- 48 (pitch 0.0208")
- Pitch Ø (basic)
- 0.0855" (2.171 mm)
- Minor Ø (internal, basic)
- 0.0764" (1.942 mm)
- Thread depth (internal)
- 0.0113" (0.286 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 |
|---|---|---|---|
| #48 | 0.0760 | 1.93 | 85% |
| 5/64 | 0.0781 | 1.98 | 77% |
| #47 | 0.0785 | 1.99 | 76% |
| 2 mm | 0.0787 | 2.00 | 75% |
| #46suggested | 0.0810 | 2.06 | 67% |
| #45 | 0.0820 | 2.08 | 63% |
| 2.1 mm | 0.0827 | 2.10 | 60% |
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 | 71% |
| 2.3 mm | 0.0906 | 2.30 | 60% |
3-wire measurement
- Best wire size
- 0.0120" (0.306 mm)
- Over wires, at basic pitch Ø
- 0.1035" (2.629 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