ISO metric · coarse
M30 × 3.5 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
27 mm
1.0630" · 66% thread
Form tap
28.5 mm
1.1220" · 63% thread
Pick your material for its % of thread, tapping RPM and the rigid-tap code.
Thread dimensions
- Major Ø
- 30.000 mm (1.1811")
- Pitch
- 3.5 mm
- Pitch Ø (basic)
- 27.727 mm (1.0916")
- Minor Ø (internal, basic)
- 26.211 mm (1.0319")
- Thread depth (internal)
- 1.894 mm (0.0746")
Basic sizes of the 60° form. Tolerance classes (6H) 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 |
|---|---|---|---|
| 1-1/32 | 1.0313 | 26.19 | 84% |
| 26.5 mm | 1.0433 | 26.50 | 77% |
| 1-3/64 | 1.0469 | 26.59 | 75% |
| 1-1/16suggested | 1.0625 | 26.99 | 66% |
| 27 mmsuggested | 1.0630 | 27.00 | 66% |
| 1-5/64 | 1.0781 | 27.38 | 58% |
| 27.5 mm | 1.0827 | 27.50 | 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 |
|---|---|---|---|
| 28 mm | 1.1024 | 28.00 | 84% |
| 1-7/64 | 1.1094 | 28.18 | 77% |
| 28.5 mm | 1.1220 | 28.50 | 63% |
| 1-1/8 | 1.1250 | 28.58 | 60% |
3-wire measurement
- Best wire size
- 2.021 mm (0.0796")
- Over wires, at basic pitch Ø
- 30.758 mm (1.2109")
For the external thread (the screw). The best wire touches the flanks at the pitch diameter.
Sources
- ISO 261 / ISO 262 — ISO general purpose metric screw threads: general plan and selected sizes
- 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