Unified inch · UNF
1-1/8-12 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
1-3/64
1.0469" · 72% thread
Form tap
1-3/32
1.0938" · 55% thread
Pick your material for its % of thread, tapping RPM and the rigid-tap code.
Thread dimensions
- Major Ø
- 1.1250" (28.575 mm)
- Threads per inch
- 12 (pitch 0.0833")
- Pitch Ø (basic)
- 1.0709" (27.200 mm)
- Minor Ø (internal, basic)
- 1.0348" (26.284 mm)
- Thread depth (internal)
- 0.0451" (1.146 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 |
|---|---|---|---|
| 26.5 mm | 1.0433 | 26.50 | 75% |
| 1-3/64 | 1.0469 | 26.59 | 72% |
| 1-1/16 | 1.0625 | 26.99 | 58% |
| 27 mm | 1.0630 | 27.00 | 57% |
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 |
|---|---|---|---|
| 1-5/64 | 1.0781 | 27.38 | 83% |
| 27.5 mm | 1.0827 | 27.50 | 75% |
| 1-3/32 | 1.0938 | 27.78 | 55% |
3-wire measurement
- Best wire size
- 0.0481" (1.222 mm)
- Over wires, at basic pitch Ø
- 1.1430" (29.033 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