Letter D drill size
A letter D drill is 0.2460 in (6.248 mm).
- Decimal inch
- 0.2460
- Millimeters
- 6.248
- Nearest 1/64
- 1/4" (-0.0040)
- Type
- letter drill
Nearest equivalents in other systems
| System | Drill | Inch | mm | Difference |
|---|---|---|---|---|
| Fractional | 1/4" | 0.2500 | 6.350 | +0.0040 in |
| Metric | 6.2 mm | 0.2441 | 6.200 | -0.0019 in |
Speeds and feeds: in mild steel an HSS drill starts at about 1,400 RPM and 0.0064 in/rev (8.96 IPM); in 6061 aluminum about 3,880 RPM.
Letter D drill speeds and feeds by material
Jobber-length twist drill, D = 0.2460 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 | 3,880 | 600 | 9,320 | 0.0059 | 22.9 | 55.0 |
| Brass / free-machining | 150 | 2,330 | 400 | 6,210 | 0.0052 | 12.1 | 32.3 |
| Low-carbon steel (1018) | 90 | 1,400 | 300 | 4,660 | 0.0064 | 8.96 | 29.8 |
| Alloy steel (4140, annealed) | 70 | 1,090 | 250 | 3,880 | 0.0064 | 6.98 | 24.8 |
| Stainless 304 | 40 | 620 | 150 | 2,330 | 0.0042 | 2.60 | 9.79 |
| Gray cast iron | 80 | 1,240 | 250 | 3,880 | 0.0044 | 5.46 | 17.1 |
| Titanium 6Al-4V | 25 | 390 | 100 | 1,550 | 0.0030 | 1.17 | 4.65 |
| Plastics (acetal) | 300 | 4,660 | 500 | 7,760 | 0.0040 | 18.6 | 31.0 |
Metric: Vc (m/min) = SFM × 0.3048; feed (mm/rev) = in/rev × 25.4. In steel, 1,400 RPM × 0.163 mm/rev = 228 mm/min.
Worked example: letter D drill in 1018 steel
- HSS drill in low-carbon steel: 90 SFM.
- RPM = 3.82 × 90 ÷ 0.2460 ≈ 1,400 RPM.
- Feed for a 0.246 in drill in mild steel ≈ 0.0064 in/rev.
- IPM = 1,400 × 0.0064 = 8.96 IPM.
Peck cycle by hole depth
| Depth | Cycle | Peck | Note |
|---|---|---|---|
| 3×D = 0.738 in (18.7 mm) | G81 (no peck) | None | Up to 3×D a sharp drill clears chips on its own. |
| 5×D = 1.230 in (31.2 mm) | G73 chip-break | 1 × D | Chip-break pecks; full retract if chips pack (soft aluminum, stainless). |
| 8×D = 1.968 in (50 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 D drill
No standard UNC, UNF or metric tap in our tables calls for a letter D drill. Use it for clearance, pilot or reamer prep holes.
Nearest drills
- 6.2 mm: 0.2441 in / 6.200 mm (-0.0019 in)
- 6.3 mm: 0.2480 in / 6.300 mm (+0.0020 in)
- Letter C: 0.2420 in / 6.147 mm (-0.0040 in)
- 1/4": 0.2500 in / 6.350 mm (+0.0040 in)
Common questions
- What is a letter D drill in decimal and mm?
- 0.2460 in, or 6.248 mm; nearest fractional drill: 1/4" (0.2500 in, +0.0040 in); nearest metric drill: 6.2 mm (0.2441 in, -0.0019 in).
- What RPM for a letter D drill in steel?
- In low-carbon steel (1018) an HSS letter D drill (0.2460 in) runs about 1,400 RPM at 90 SFM; a carbide drill about 4,660 RPM at 300 SFM. RPM = 3.82 × SFM ÷ D.
- What feed for a letter D drill?
- About 0.0064 in/rev in mild steel, 0.0059 in/rev in aluminum and 0.0042 in/rev in 304 stainless. At 1,400 RPM that is 8.96 IPM in steel (IPM = RPM × IPR).
- What RPM for a letter D drill in aluminum?
- 3,880 RPM with HSS (250 SFM) or up to 9,320 RPM with carbide (600 SFM), feeding about 0.0059 in/rev.
- What size is a letter D drill?
- 0.2460 in / 6.248 mm. The nearest other drills are 6.2 mm (0.2441 in), 6.3 mm (0.2480 in), Letter C (0.2420 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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