Letter A drill size
A letter A drill is 0.2340 in (5.944 mm).
- Decimal inch
- 0.2340
- Millimeters
- 5.944
- Nearest 1/64
- 15/64" (-0.0004)
- Type
- letter drill
Nearest equivalents in other systems
| System | Drill | Inch | mm | Difference |
|---|---|---|---|---|
| Fractional | 15/64" | 0.2344 | 5.953 | +0.0004 in |
| Metric | 5.9 mm | 0.2323 | 5.900 | -0.0017 in |
Speeds and feeds: in mild steel an HSS drill starts at about 1,470 RPM and 0.0060 in/rev (8.82 IPM); in 6061 aluminum about 4,080 RPM.
Letter A drill speeds and feeds by material
Jobber-length twist drill, D = 0.2340 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 | 4,080 | 600 | 9,790 | 0.0056 | 22.9 | 54.8 |
| Brass / free-machining | 150 | 2,450 | 400 | 6,530 | 0.0049 | 12.0 | 32.0 |
| Low-carbon steel (1018) | 90 | 1,470 | 300 | 4,900 | 0.0060 | 8.82 | 29.4 |
| Alloy steel (4140, annealed) | 70 | 1,140 | 250 | 4,080 | 0.0060 | 6.84 | 24.5 |
| Stainless 304 | 40 | 650 | 150 | 2,450 | 0.0040 | 2.60 | 9.80 |
| Gray cast iron | 80 | 1,310 | 250 | 4,080 | 0.0042 | 5.50 | 17.1 |
| Titanium 6Al-4V | 25 | 410 | 100 | 1,630 | 0.0028 | 1.15 | 4.56 |
| Plastics (acetal) | 300 | 4,900 | 500 | 8,160 | 0.0041 | 20.1 | 33.5 |
Metric: Vc (m/min) = SFM × 0.3048; feed (mm/rev) = in/rev × 25.4. In steel, 1,470 RPM × 0.152 mm/rev = 224 mm/min.
Worked example: letter A drill in 1018 steel
- HSS drill in low-carbon steel: 90 SFM.
- RPM = 3.82 × 90 ÷ 0.2340 ≈ 1,470 RPM.
- Feed for a 0.234 in drill in mild steel ≈ 0.0060 in/rev.
- IPM = 1,470 × 0.0060 = 8.82 IPM.
Peck cycle by hole depth
| Depth | Cycle | Peck | Note |
|---|---|---|---|
| 3×D = 0.702 in (17.8 mm) | G81 (no peck) | None | Up to 3×D a sharp drill clears chips on its own. |
| 5×D = 1.170 in (29.7 mm) | G73 chip-break | 1 × D | Chip-break pecks; full retract if chips pack (soft aluminum, stainless). |
| 8×D = 1.872 in (47.6 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 A drill
No standard UNC, UNF or metric tap in our tables calls for a letter A drill. Use it for clearance, pilot or reamer prep holes.
Nearest drills
- 15/64": 0.2344 in / 5.953 mm (+0.0004 in)
- 5.9 mm: 0.2323 in / 5.900 mm (-0.0017 in)
- 6.0 mm: 0.2362 in / 6.000 mm (+0.0022 in)
- Letter B: 0.2380 in / 6.045 mm (+0.0040 in)
Common questions
- What is a letter A drill in decimal and mm?
- 0.2340 in, or 5.944 mm; nearest fractional drill: 15/64" (0.2344 in, +0.0004 in); nearest metric drill: 5.9 mm (0.2323 in, -0.0017 in).
- What RPM for a letter A drill in steel?
- In low-carbon steel (1018) an HSS letter A drill (0.2340 in) runs about 1,470 RPM at 90 SFM; a carbide drill about 4,900 RPM at 300 SFM. RPM = 3.82 × SFM ÷ D.
- What feed for a letter A drill?
- About 0.0060 in/rev in mild steel, 0.0056 in/rev in aluminum and 0.0040 in/rev in 304 stainless. At 1,470 RPM that is 8.82 IPM in steel (IPM = RPM × IPR).
- What RPM for a letter A drill in aluminum?
- 4,080 RPM with HSS (250 SFM) or up to 9,790 RPM with carbide (600 SFM), feeding about 0.0056 in/rev.
- What size is a letter A drill?
- 0.2340 in / 5.944 mm. The nearest other drills are 15/64" (0.2344 in), 5.9 mm (0.2323 in), 6.0 mm (0.2362 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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