Letter G drill size
A letter G drill is 0.2610 in (6.629 mm).
- Decimal inch
- 0.2610
- Millimeters
- 6.629
- Nearest 1/64
- 17/64" (-0.0046)
- Type
- letter drill
Nearest equivalents in other systems
Speeds and feeds: in mild steel an HSS drill starts at about 1,320 RPM and 0.0068 in/rev (8.98 IPM); in 6061 aluminum about 3,660 RPM.
Letter G drill speeds and feeds by material
Jobber-length twist drill, D = 0.2610 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 | 3,660 | 600 | 8,780 | 0.0062 | 22.7 | 54.4 |
| Brass / free-machining | 150 | 2,200 | 400 | 5,850 | 0.0053 | 11.7 | 31.0 |
| Low-carbon steel (1018) | 90 | 1,320 | 300 | 4,390 | 0.0068 | 8.98 | 29.9 |
| Alloy steel (4140, annealed) | 70 | 1,020 | 250 | 3,660 | 0.0069 | 7.04 | 25.3 |
| Stainless 304 | 40 | 590 | 150 | 2,200 | 0.0044 | 2.60 | 9.68 |
| Gray cast iron | 80 | 1,170 | 250 | 3,660 | 0.0047 | 5.50 | 17.2 |
| Titanium 6Al-4V | 25 | 370 | 100 | 1,460 | 0.0031 | 1.15 | 4.53 |
| Plastics (acetal) | 300 | 4,390 | 500 | 7,320 | 0.0041 | 18.0 | 30.0 |
Metric: Vc (m/min) = SFM × 0.3048; feed (mm/rev) = in/rev × 25.4. In steel, 1,320 RPM × 0.173 mm/rev = 228 mm/min.
Worked example: letter G drill in 1018 steel
- HSS drill in low-carbon steel: 90 SFM.
- RPM = 3.82 × 90 ÷ 0.2610 ≈ 1,320 RPM.
- Feed for a 0.261 in drill in mild steel ≈ 0.0068 in/rev.
- IPM = 1,320 × 0.0068 = 8.98 IPM.
Peck cycle by hole depth
| Depth | Cycle | Peck | Note |
|---|---|---|---|
| 3×D = 0.783 in (19.9 mm) | G81 (no peck) | None | Up to 3×D a sharp drill clears chips on its own. |
| 5×D = 1.305 in (33.1 mm) | G73 chip-break | 1 × D | Chip-break pecks; full retract if chips pack (soft aluminum, stainless). |
| 8×D = 2.088 in (53 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 G drill
No standard UNC, UNF or metric tap in our tables calls for a letter G drill. Use it for clearance, pilot or reamer prep holes.
Nearest drills
- 6.6 mm: 0.2598 in / 6.600 mm (-0.0012 in)
- 6.7 mm: 0.2638 in / 6.700 mm (+0.0028 in)
- Letter F: 0.2570 in / 6.528 mm (-0.0040 in)
- 17/64": 0.2656 in / 6.747 mm (+0.0046 in)
Common questions
- What is a letter G drill in decimal and mm?
- 0.2610 in, or 6.629 mm; nearest fractional drill: 17/64" (0.2656 in, +0.0046 in); nearest metric drill: 6.6 mm (0.2598 in, -0.0012 in).
- What RPM for a letter G drill in steel?
- In low-carbon steel (1018) an HSS letter G drill (0.2610 in) runs about 1,320 RPM at 90 SFM; a carbide drill about 4,390 RPM at 300 SFM. RPM = 3.82 × SFM ÷ D.
- What feed for a letter G drill?
- About 0.0068 in/rev in mild steel, 0.0062 in/rev in aluminum and 0.0044 in/rev in 304 stainless. At 1,320 RPM that is 8.98 IPM in steel (IPM = RPM × IPR).
- What RPM for a letter G drill in aluminum?
- 3,660 RPM with HSS (250 SFM) or up to 8,780 RPM with carbide (600 SFM), feeding about 0.0062 in/rev.
- What size is a letter G drill?
- 0.2610 in / 6.629 mm. The nearest other drills are 6.6 mm (0.2598 in), 6.7 mm (0.2638 in), Letter F (0.2570 in).
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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