6-40 tap drill size
Use a #33 drill (0.1130 in) for a 6-40 cutting tap at 75% thread.
| Tap type | Drill | Decimal (in) |
|---|---|---|
| Cutting tap, 75% thread | #33 | 0.1130 |
| Cutting tap, 50% thread (hard material) | #31 | 0.1200 |
| Roll / form tap, 65% thread | #30 | 0.1285 |
| Helicoil / STI insert (#6-40×1D) | #19 | 0.1660 |
- Pitch
- 0.635 mm (40 TPI)
- Major Ø
- 0.1380
- Pitch Ø
- 0.1218
- Minor Ø
- 0.1073
6-40 tapping speeds and feeds
Feed = pitch (0.0250 in per rev). Starting points for a general-purpose HSS cut tap:
- Aluminum 6061 / 7075: 1,940 RPM · 48.5 IPM
- Free-machining / mild steel: 550 RPM · 13.8 IPM
- 304 / 316 stainless: 280 RPM · 7.00 IPM
6-40 tap speeds and feeds: all materials, form taps and G84 code · #33 drill size and speeds
Common questions
- What size drill for a 6-40 tap?
- Use a #33 drill (0.1130 in) for a 6-40 cutting tap at 75% thread.
- What drill for a 6-40 roll (form) tap?
- About 0.1270 in; the nearest standard drill is #30. Check your tap maker's table.
- What drill for a 6-40 Helicoil?
- Drill #19 (0.1660 in), then tap with the 6-40 STI tap.
Other UNF sizes
Drill sizes from Machinery's Handbook and ASME B1.1/B1.13M; roll tap sizes from the form-tap formula. Tap and insert makers' tables are authoritative.
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