Letter F drill size
A letter F drill is 0.2570 in (6.528 mm). It is the cut tap, 75% thread drill for 5/16-18.
- Decimal inch
- 0.2570
- Millimeters
- 6.528
- Nearest 1/64
- 1/4" (+0.0070)
- Type
- letter drill
Nearest equivalents in other systems
Speeds and feeds: in mild steel an HSS drill starts at about 1,340 RPM and 0.0067 in/rev (8.98 IPM); in 6061 aluminum about 3,720 RPM.
Letter F drill speeds and feeds by material
Jobber-length twist drill, D = 0.2570 in. RPM = 3.82 × SFM ÷ D, rounded to 10. Feed per rev scales with diameter and material; IPM = RPM × IPR.
| Material | HSS SFM | HSS RPM | Carbide SFM | Carbide RPM | Feed (in/rev) | HSS IPM | Carbide IPM |
|---|---|---|---|---|---|---|---|
| Aluminum (6061) | 250 | 3,720 | 600 | 8,920 | 0.0061 | 22.7 | 54.4 |
| Brass / free-machining | 150 | 2,230 | 400 | 5,950 | 0.0053 | 11.8 | 31.5 |
| Low-carbon steel (1018) | 90 | 1,340 | 300 | 4,460 | 0.0067 | 8.98 | 29.9 |
| Alloy steel (4140, annealed) | 70 | 1,040 | 250 | 3,720 | 0.0067 | 6.97 | 24.9 |
| Stainless 304 | 40 | 590 | 150 | 2,230 | 0.0044 | 2.60 | 9.81 |
| Gray cast iron | 80 | 1,190 | 250 | 3,720 | 0.0046 | 5.47 | 17.1 |
| Titanium 6Al-4V | 25 | 370 | 100 | 1,490 | 0.0031 | 1.15 | 4.62 |
| Plastics (acetal) | 300 | 4,460 | 500 | 7,430 | 0.0041 | 18.3 | 30.5 |
Metric: Vc (m/min) = SFM × 0.3048; feed (mm/rev) = in/rev × 25.4. In steel, 1,340 RPM × 0.170 mm/rev = 228 mm/min.
Worked example: letter F drill in 1018 steel
- HSS drill in low-carbon steel: 90 SFM.
- RPM = 3.82 × 90 ÷ 0.2570 ≈ 1,340 RPM.
- Feed for a 0.257 in drill in mild steel ≈ 0.0067 in/rev.
- IPM = 1,340 × 0.0067 = 8.98 IPM.
Peck cycle by hole depth
| Depth | Cycle | Peck | Note |
|---|---|---|---|
| 3×D = 0.771 in (19.6 mm) | G81 (no peck) | None | Up to 3×D a sharp drill clears chips on its own. |
| 5×D = 1.285 in (32.6 mm) | G73 chip-break | 1 × D | Chip-break pecks; full retract if chips pack (soft aluminum, stainless). |
| 8×D = 2.056 in (52.2 mm) | G83 full retract | 0.5-1 × D, shrinking with depth | Retract to clear chips; reduce feed ~10-20% past 5×D. |
Taps that use a letter F drill
- 5/16-18 — cut tap, 75% thread · tap speeds and feeds
- 1/4-20 — Helicoil / STI insert (1/4-20×1D) · tap speeds and feeds
Nearest drills
- 6.5 mm: 0.2559 in / 6.500 mm (-0.0011 in)
- 6.6 mm: 0.2598 in / 6.600 mm (+0.0028 in)
- Letter G: 0.2610 in / 6.629 mm (+0.0040 in)
- 6.4 mm: 0.2520 in / 6.400 mm (-0.0050 in)
Common questions
- What is a letter F drill in decimal and mm?
- 0.2570 in, or 6.528 mm; nearest fractional drill: 1/4" (0.2500 in, -0.0070 in); nearest metric drill: 6.5 mm (0.2559 in, -0.0011 in).
- What RPM for a letter F drill in steel?
- In low-carbon steel (1018) an HSS letter F drill (0.2570 in) runs about 1,340 RPM at 90 SFM; a carbide drill about 4,460 RPM at 300 SFM. RPM = 3.82 × SFM ÷ D.
- What feed for a letter F drill?
- About 0.0067 in/rev in mild steel, 0.0061 in/rev in aluminum and 0.0044 in/rev in 304 stainless. At 1,340 RPM that is 8.98 IPM in steel (IPM = RPM × IPR).
- What RPM for a letter F drill in aluminum?
- 3,720 RPM with HSS (250 SFM) or up to 8,920 RPM with carbide (600 SFM), feeding about 0.0061 in/rev.
- What tap uses a letter F drill?
- 5/16-18 (cut tap, 75% thread); 1/4-20 (Helicoil / STI insert (1/4-20×1D)).
Nearby drill sizes
Sources
- Machinery's Handbook, 31st ed., Speeds and Feeds for Drilling, Reaming and Threading (Tables 17-23) — HSS and carbide drilling SFM (rounded).
- RedLine Tools (Productivity Inc.) catalog p. 340, Cobalt & HSS Drills Speeds & Feeds — feed per rev by drill diameter and material (middle of the catalog range).
- ASME B94.11M twist drills: number (#1-#80) and letter (A-Z) drill sizes; drill size reference.
Material feed multipliers (aluminum 1.3×, brass 1.1×, 4140 0.85×, stainless and titanium 0.7× the mild-steel feed) are JobLine estimates. Peck guidance is JobLine shop practice. Use your drill maker's data first.
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