Letter W drill size
A letter W drill is 0.3860 in (9.804 mm).
- Decimal inch
- 0.3860
- Millimeters
- 9.804
- Nearest 1/64
- 25/64" (-0.0046)
- Type
- letter drill
Nearest equivalents in other systems
| System | Drill | Inch | mm | Difference |
|---|---|---|---|---|
| Fractional | 25/64" | 0.3906 | 9.922 | +0.0046 in |
| Metric | 9.8 mm | 0.3858 | 9.800 | -0.0002 in |
Speeds and feeds: in mild steel an HSS drill starts at about 890 RPM and 0.0096 in/rev (8.54 IPM); in 6061 aluminum about 2,470 RPM.
Letter W drill speeds and feeds by material
Jobber-length twist drill, D = 0.3860 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 | 2,470 | 600 | 5,940 | 0.0086 | 21.2 | 51.1 |
| Brass / free-machining | 150 | 1,480 | 400 | 3,960 | 0.0064 | 9.47 | 25.3 |
| Low-carbon steel (1018) | 90 | 890 | 300 | 2,970 | 0.0096 | 8.54 | 28.5 |
| Alloy steel (4140, annealed) | 70 | 690 | 250 | 2,470 | 0.0106 | 7.31 | 26.2 |
| Stainless 304 | 40 | 400 | 150 | 1,480 | 0.0064 | 2.56 | 9.47 |
| Gray cast iron | 80 | 790 | 250 | 2,470 | 0.0067 | 5.29 | 16.6 |
| Titanium 6Al-4V | 25 | 250 | 100 | 990 | 0.0043 | 1.08 | 4.26 |
| Plastics (acetal) | 300 | 2,970 | 500 | 4,950 | 0.0051 | 15.2 | 25.3 |
Metric: Vc (m/min) = SFM × 0.3048; feed (mm/rev) = in/rev × 25.4. In steel, 890 RPM × 0.244 mm/rev = 217 mm/min.
Worked example: letter W drill in 1018 steel
- HSS drill in low-carbon steel: 90 SFM.
- RPM = 3.82 × 90 ÷ 0.3860 ≈ 890 RPM.
- Feed for a 0.386 in drill in mild steel ≈ 0.0096 in/rev.
- IPM = 890 × 0.0096 = 8.54 IPM.
Peck cycle by hole depth
| Depth | Cycle | Peck | Note |
|---|---|---|---|
| 3×D = 1.158 in (29.4 mm) | G81 (no peck) | None | Up to 3×D a sharp drill clears chips on its own. |
| 5×D = 1.930 in (49 mm) | G73 chip-break | 1 × D | Chip-break pecks; full retract if chips pack (soft aluminum, stainless). |
| 8×D = 3.088 in (78.4 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 W drill
No standard UNC, UNF or metric tap in our tables calls for a letter W drill. Use it for clearance, pilot or reamer prep holes.
Nearest drills
- 9.8 mm: 0.3858 in / 9.800 mm (-0.0002 in)
- 9.9 mm: 0.3898 in / 9.900 mm (+0.0038 in)
- 9.7 mm: 0.3819 in / 9.700 mm (-0.0041 in)
- 25/64": 0.3906 in / 9.922 mm (+0.0046 in)
Common questions
- What is a letter W drill in decimal and mm?
- 0.3860 in, or 9.804 mm; nearest fractional drill: 25/64" (0.3906 in, +0.0046 in); nearest metric drill: 9.8 mm (0.3858 in, -0.0002 in).
- What RPM for a letter W drill in steel?
- In low-carbon steel (1018) an HSS letter W drill (0.3860 in) runs about 890 RPM at 90 SFM; a carbide drill about 2,970 RPM at 300 SFM. RPM = 3.82 × SFM ÷ D.
- What feed for a letter W drill?
- About 0.0096 in/rev in mild steel, 0.0086 in/rev in aluminum and 0.0064 in/rev in 304 stainless. At 890 RPM that is 8.54 IPM in steel (IPM = RPM × IPR).
- What RPM for a letter W drill in aluminum?
- 2,470 RPM with HSS (250 SFM) or up to 5,940 RPM with carbide (600 SFM), feeding about 0.0086 in/rev.
- What size is a letter W drill?
- 0.3860 in / 9.804 mm. The nearest other drills are 9.8 mm (0.3858 in), 9.9 mm (0.3898 in), 9.7 mm (0.3819 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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