Thread, tap drill and rod blank chart
Metric (M), unified inch (UN), pipe (G/BSPP) — tap drill for internal threads and rod blank for external threads
| Designation | Major Ø, mm | Tap drill, mm (for internal thread) | Rod blank, mm (for external thread) |
|---|---|---|---|
| M2 | 2.00 | 1.60 | 1.95 |
| M2.5 | 2.50 | 2.05 | 2.45 |
| M3 | 3.00 | 2.50 | 2.95 |
| M4 | 4.00 | 3.30 | 3.95 |
| M5 | 5.00 | 4.20 | 4.95 |
| M6 | 6.00 | 5.00 | 5.90 |
| M8 | 8.00 | 6.80 | 7.90 |
| M10 | 10.00 | 8.50 | 9.90 |
| M12 | 12.00 | 10.20 | 11.90 |
| M14 | 14.00 | 12.00 | 13.87 |
| M16 | 16.00 | 14.00 | 15.87 |
| M18 | 18.00 | 15.50 | 17.87 |
| M20 | 20.00 | 17.50 | 19.87 |
| M22 | 22.00 | 19.50 | 21.87 |
| M24 | 24.00 | 21.00 | 23.82 |
| M27 | 27.00 | 24.00 | 26.82 |
| M30 | 30.00 | 26.50 | 29.82 |
| M33 | 33.00 | 29.50 | 32.82 |
| M36 | 36.00 | 32.00 | 35.82 |
| M39 | 39.00 | 35.00 | 38.75 |
| M42 | 42.00 | 37.50 | 41.75 |
| M45 | 45.00 | 40.50 | 44.75 |
| M48 | 48.00 | 43.00 | 47.75 |
| M52 | 52.00 | 47.00 | 51.75 |
| M56 | 56.00 | 50.50 | 55.75 |
| M60 | 60.00 | 54.50 | 59.75 |
| M64 | 64.00 | 58.00 | 63.75 |
Calculators — FAQ
What is a tap drill (for internal thread)?+
The hole diameter drilled before tapping an internal thread. Sized to the thread's minor diameter with a small allowance for tap clearance. M10×1.5 needs 8.5 mm; UNC 1/4-20 needs 5.10 mm.
What rod diameter do I need for an external thread?+
Turn the rod slightly under the nominal major to get a clean crest with no burr. Typical undersize: 0.05 mm for small threads (≤M5), 0.10 mm for M6-M12, 0.13 mm for M14-M22, 0.18 mm for M24-M36, 0.25 mm for larger. So M10 → ⌀9.90 mm; M20 → ⌀19.87 mm. This is the recommendation for die-cutting; on CNC lathes you can leave the rod at full nominal and form the thread profile directly with a finishing insert.
Coarse vs fine thread — what's the difference?+
Coarse threads have larger pitch — easier to cut, more tolerant to dirt and damage, but lower load per turn. Fine threads (e.g. M10×1) carry more axial load, work better in thin walls, but are more sensitive to contamination.
Can you cut any thread from this chart?+
Yes — CNC turning handles all standard metric, unified inch, and pipe threads, both internal and external. For oversize or non-standard (trapezoidal, buttress, lead-screws) — agreed per drawing.
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