ISO metric · coarse
M1 × 0.25 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
0.8 mm
0.0315" · 62% thread
Or #68 (0.0310") for 65%
Form tap
0.9 mm
0.0354" · 59% thread
Or #65 (0.0350") for 65%
Pick your material for its % of thread, tapping RPM and the rigid-tap code.
Thread dimensions
- Major Ø
- 1.000 mm (0.0394")
- Pitch
- 0.25 mm
- Pitch Ø (basic)
- 0.838 mm (0.0330")
- Minor Ø (internal, basic)
- 0.729 mm (0.0287")
- Thread depth (internal)
- 0.135 mm (0.0053")
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 |
|---|---|---|---|
| #69 | 0.0292 | 0.74 | 80% |
| #68suggested | 0.0310 | 0.79 | 65% |
| 1/32 | 0.0313 | 0.79 | 64% |
| 0.8 mm | 0.0315 | 0.80 | 62% |
| #67 | 0.0320 | 0.81 | 58% |
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 |
|---|---|---|---|
| #65suggested | 0.0350 | 0.89 | 65% |
| 0.9 mm | 0.0354 | 0.90 | 59% |
| #64 | 0.0360 | 0.91 | 50% |
3-wire measurement
- Best wire size
- 0.144 mm (0.0057")
- Over wires, at basic pitch Ø
- 1.054 mm (0.0415")
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