Unified inch · UNF
3/4-16 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
11/16
0.6875" · 77% thread
Form tap
23/32
0.7188" · 74% thread
Pick your material for its % of thread, tapping RPM and the rigid-tap code.
Thread dimensions
- Major Ø
- 0.7500" (19.050 mm)
- Threads per inch
- 16 (pitch 0.0625")
- Pitch Ø (basic)
- 0.7094" (18.019 mm)
- Minor Ø (internal, basic)
- 0.6823" (17.331 mm)
- Thread depth (internal)
- 0.0338" (0.859 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 |
|---|---|---|---|
| 11/16 | 0.6875 | 17.46 | 77% |
| 17.5 mm | 0.6890 | 17.50 | 75% |
| 45/64 | 0.7031 | 17.86 | 58% |
| 18 mm | 0.7087 | 18.00 | 51% |
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 |
|---|---|---|---|
| 23/32 | 0.7188 | 18.26 | 74% |
| 18.5 mm | 0.7283 | 18.50 | 51% |
3-wire measurement
- Best wire size
- 0.0361" (0.917 mm)
- Over wires, at basic pitch Ø
- 0.7635" (19.394 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