Find the right drill bit for any thread. Select thread standard and size, get the recommended drill diameter and expected thread percentage.
Calculate ↓Formula: Drill Dia = Major Dia − (0.013 × Pitch × %Thread) / 1.299
Thread strength and tapping difficulty are a direct trade-off. A 65% thread is standard for general use — strong enough for most applications without breaking taps. For soft materials like aluminum, 75% thread is common. For hardened steel, 50-60% thread reduces tap breakage risk. The formula is simple: drill diameter = major diameter − (0.013 × pitch × %thread). Get it right the first time and you avoid broken taps and scrapped parts.
On a Haas or Mazak machining center, a broken tap in a $500 part is a bad day. The cost of the tap is nothing compared to the rework time. Use the recommended drill size above and you'll be in the safe range.
| Thread | Pitch | 65% Drill | 75% Drill |
|---|---|---|---|
| M3 | 0.5 | 2.5 | 2.4 |
| M4 | 0.7 | 3.3 | 3.2 |
| M5 | 0.8 | 4.2 | 4.1 |
| M6 | 1.0 | 5.0 | 4.9 |
| M8 | 1.25 | 6.8 | 6.6 |
| M10 | 1.5 | 8.5 | 8.3 |
| M12 | 1.75 | 10.2 | 10.0 |
What happens if I use the wrong tap drill size? An undersized drill increases tapping torque, causes oversized threads, and risks tap breakage. An oversized drill reduces thread engagement below the acceptable minimum, risking thread stripping under load. A ±0.1 mm error in tap drill for M10 changes thread engagement by approximately 5%.
Should I use a letter, number, or fractional drill for tapping? For inch-series threads, letter drills are specified for most sizes (e.g., 'Q' for 3/8-16). Number drills are specified for small sizes (e.g., #7 for 1/4-20). Fractional drills are used for larger diameters. Always check the Machinery's Handbook for the recommended drill size when converting between systems.
Why do some tap drill charts recommend different sizes? Different charts may target different thread engagement percentages. Commercial production charts often target 65% for ease of tapping, while military or aerospace charts target 75% for maximum strength. Always verify the target engagement before selecting a drill size from any chart.
⚡ Calculated for Carbide-Tooling.com High-Performance Series. View Catalog →
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