1/4-28 tap drill size
Use a #3 drill (0.2130 in) for a 1/4-28 cutting tap at 75% thread.
| Tap type | Drill | Decimal (in) |
|---|---|---|
| Cutting tap, 75% thread | #3 | 0.2130 |
| Cutting tap, 50% thread (hard material) | #1 | 0.2280 |
| Roll / form tap, 65% thread | 15/64 | 0.2344 |
| Helicoil / STI insert (1/4-28×1D) | H | 0.2660 |
- Pitch
- 0.9071 mm (28 TPI)
- Major Ø
- 0.2500
- Pitch Ø
- 0.2268
- Minor Ø
- 0.2062
1/4-28 tapping speeds and feeds
Feed = pitch (0.0357 in per rev). Starting points for a general-purpose HSS cut tap:
- Aluminum 6061 / 7075: 1,070 RPM · 38.2 IPM
- Free-machining / mild steel: 310 RPM · 11.1 IPM
- 304 / 316 stainless: 150 RPM · 5.36 IPM
1/4-28 tap speeds and feeds: all materials, form taps and G84 code · #3 drill size and speeds
Common questions
- What size drill for a 1/4-28 tap?
- Use a #3 drill (0.2130 in) for a 1/4-28 cutting tap at 75% thread.
- What drill for a 1/4-28 roll (form) tap?
- About 0.2342 in; the nearest standard drill is 15/64. Check your tap maker's table.
- What drill for a 1/4-28 Helicoil?
- Drill H (0.2660 in), then tap with the 1/4-28 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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