Unified inch · UNF
#6-40 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
#32
0.1160" · 68% thread
Form tap
1/8
0.1250" · 76% thread
Pick your material for its % of thread, tapping RPM and the rigid-tap code.
Thread dimensions
- Major Ø
- 0.1380" (3.505 mm)
- Threads per inch
- 40 (pitch 0.0250")
- Pitch Ø (basic)
- 0.1218" (3.093 mm)
- Minor Ø (internal, basic)
- 0.1109" (2.818 mm)
- Thread depth (internal)
- 0.0135" (0.344 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 |
|---|---|---|---|
| #34 | 0.1110 | 2.82 | 83% |
| #33 | 0.1130 | 2.87 | 77% |
| 2.9 mm | 0.1142 | 2.90 | 73% |
| #32suggested | 0.1160 | 2.95 | 68% |
| 3 mm | 0.1181 | 3.00 | 61% |
| #31 | 0.1200 | 3.05 | 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 |
|---|---|---|---|
| 1/8 | 0.1250 | 3.18 | 76% |
| 3.2 mm | 0.1260 | 3.20 | 71% |
| #30 | 0.1285 | 3.26 | 56% |
3-wire measurement
- Best wire size
- 0.0144" (0.367 mm)
- Over wires, at basic pitch Ø
- 0.1434" (3.643 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