Letter C drill size
A letter C drill is 0.2420 in (6.147 mm).
- Decimal inch
- 0.2420
- Millimeters
- 6.147
- Nearest 1/64
- 15/64" (+0.0076)
- Type
- letter drill
Nearest equivalents in other systems
| System | Drill | Inch | mm | Difference |
|---|---|---|---|---|
| Fractional | 15/64" | 0.2344 | 5.953 | -0.0076 in |
| Metric | 6.1 mm | 0.2402 | 6.100 | -0.0018 in |
Speeds and feeds: in mild steel an HSS drill starts at about 1,420 RPM and 0.0063 in/rev (8.95 IPM); in 6061 aluminum about 3,950 RPM.
Letter C drill speeds and feeds by material
Jobber-length twist drill, D = 0.2420 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 | 3,950 | 600 | 9,470 | 0.0058 | 22.9 | 54.9 |
| Brass / free-machining | 150 | 2,370 | 400 | 6,310 | 0.0051 | 12.1 | 32.2 |
| Low-carbon steel (1018) | 90 | 1,420 | 300 | 4,740 | 0.0063 | 8.95 | 29.9 |
| Alloy steel (4140, annealed) | 70 | 1,100 | 250 | 3,950 | 0.0063 | 6.93 | 24.9 |
| Stainless 304 | 40 | 630 | 150 | 2,370 | 0.0041 | 2.58 | 9.72 |
| Gray cast iron | 80 | 1,260 | 250 | 3,950 | 0.0043 | 5.42 | 17.0 |
| Titanium 6Al-4V | 25 | 390 | 100 | 1,580 | 0.0029 | 1.13 | 4.58 |
| Plastics (acetal) | 300 | 4,740 | 500 | 7,890 | 0.0041 | 19.4 | 32.4 |
Metric: Vc (m/min) = SFM × 0.3048; feed (mm/rev) = in/rev × 25.4. In steel, 1,420 RPM × 0.160 mm/rev = 227 mm/min.
Worked example: letter C drill in 1018 steel
- HSS drill in low-carbon steel: 90 SFM.
- RPM = 3.82 × 90 ÷ 0.2420 ≈ 1,420 RPM.
- Feed for a 0.242 in drill in mild steel ≈ 0.0063 in/rev.
- IPM = 1,420 × 0.0063 = 8.95 IPM.
Peck cycle by hole depth
| Depth | Cycle | Peck | Note |
|---|---|---|---|
| 3×D = 0.726 in (18.4 mm) | G81 (no peck) | None | Up to 3×D a sharp drill clears chips on its own. |
| 5×D = 1.210 in (30.7 mm) | G73 chip-break | 1 × D | Chip-break pecks; full retract if chips pack (soft aluminum, stainless). |
| 8×D = 1.936 in (49.2 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 C drill
No standard UNC, UNF or metric tap in our tables calls for a letter C drill. Use it for clearance, pilot or reamer prep holes.
Nearest drills
- 6.1 mm: 0.2402 in / 6.100 mm (-0.0018 in)
- 6.2 mm: 0.2441 in / 6.200 mm (+0.0021 in)
- Letter B: 0.2380 in / 6.045 mm (-0.0040 in)
- Letter D: 0.2460 in / 6.248 mm (+0.0040 in)
Common questions
- What is a letter C drill in decimal and mm?
- 0.2420 in, or 6.147 mm; nearest fractional drill: 15/64" (0.2344 in, -0.0076 in); nearest metric drill: 6.1 mm (0.2402 in, -0.0018 in).
- What RPM for a letter C drill in steel?
- In low-carbon steel (1018) an HSS letter C drill (0.2420 in) runs about 1,420 RPM at 90 SFM; a carbide drill about 4,740 RPM at 300 SFM. RPM = 3.82 × SFM ÷ D.
- What feed for a letter C drill?
- About 0.0063 in/rev in mild steel, 0.0058 in/rev in aluminum and 0.0041 in/rev in 304 stainless. At 1,420 RPM that is 8.95 IPM in steel (IPM = RPM × IPR).
- What RPM for a letter C drill in aluminum?
- 3,950 RPM with HSS (250 SFM) or up to 9,470 RPM with carbide (600 SFM), feeding about 0.0058 in/rev.
- What size is a letter C drill?
- 0.2420 in / 6.147 mm. The nearest other drills are 6.1 mm (0.2402 in), 6.2 mm (0.2441 in), Letter B (0.2380 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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