Letter O drill size
A letter O drill is 0.3160 in (8.026 mm).
- Decimal inch
- 0.3160
- Millimeters
- 8.026
- Nearest 1/64
- 5/16" (+0.0035)
- Type
- letter drill
Nearest equivalents in other systems
Speeds and feeds: in mild steel an HSS drill starts at about 1,090 RPM and 0.0081 in/rev (8.83 IPM); in 6061 aluminum about 3,020 RPM.
Letter O drill speeds and feeds by material
Jobber-length twist drill, D = 0.3160 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 | 3,020 | 600 | 7,250 | 0.0073 | 22.1 | 52.9 |
| Brass / free-machining | 150 | 1,810 | 400 | 4,840 | 0.0058 | 10.5 | 28.1 |
| Low-carbon steel (1018) | 90 | 1,090 | 300 | 3,630 | 0.0081 | 8.83 | 29.4 |
| Alloy steel (4140, annealed) | 70 | 850 | 250 | 3,020 | 0.0086 | 7.31 | 26.0 |
| Stainless 304 | 40 | 480 | 150 | 1,810 | 0.0053 | 2.54 | 9.59 |
| Gray cast iron | 80 | 970 | 250 | 3,020 | 0.0056 | 5.43 | 16.9 |
| Titanium 6Al-4V | 25 | 300 | 100 | 1,210 | 0.0037 | 1.11 | 4.48 |
| Plastics (acetal) | 300 | 3,630 | 500 | 6,040 | 0.0045 | 16.3 | 27.2 |
Metric: Vc (m/min) = SFM × 0.3048; feed (mm/rev) = in/rev × 25.4. In steel, 1,090 RPM × 0.206 mm/rev = 224 mm/min.
Worked example: letter O drill in 1018 steel
- HSS drill in low-carbon steel: 90 SFM.
- RPM = 3.82 × 90 ÷ 0.3160 ≈ 1,090 RPM.
- Feed for a 0.316 in drill in mild steel ≈ 0.0081 in/rev.
- IPM = 1,090 × 0.0081 = 8.83 IPM.
Peck cycle by hole depth
| Depth | Cycle | Peck | Note |
|---|---|---|---|
| 3×D = 0.948 in (24.1 mm) | G81 (no peck) | None | Up to 3×D a sharp drill clears chips on its own. |
| 5×D = 1.580 in (40.1 mm) | G73 chip-break | 1 × D | Chip-break pecks; full retract if chips pack (soft aluminum, stainless). |
| 8×D = 2.528 in (64.2 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 O drill
No standard UNC, UNF or metric tap in our tables calls for a letter O drill. Use it for clearance, pilot or reamer prep holes.
Nearest drills
- 8.0 mm: 0.3150 in / 8.000 mm (-0.0010 in)
- 8.1 mm: 0.3189 in / 8.100 mm (+0.0029 in)
- 5/16": 0.3125 in / 7.938 mm (-0.0035 in)
- 7.9 mm: 0.3110 in / 7.900 mm (-0.0050 in)
Common questions
- What is a letter O drill in decimal and mm?
- 0.3160 in, or 8.026 mm; nearest fractional drill: 5/16" (0.3125 in, -0.0035 in); nearest metric drill: 8.0 mm (0.3150 in, -0.0010 in).
- What RPM for a letter O drill in steel?
- In low-carbon steel (1018) an HSS letter O drill (0.3160 in) runs about 1,090 RPM at 90 SFM; a carbide drill about 3,630 RPM at 300 SFM. RPM = 3.82 × SFM ÷ D.
- What feed for a letter O drill?
- About 0.0081 in/rev in mild steel, 0.0073 in/rev in aluminum and 0.0053 in/rev in 304 stainless. At 1,090 RPM that is 8.83 IPM in steel (IPM = RPM × IPR).
- What RPM for a letter O drill in aluminum?
- 3,020 RPM with HSS (250 SFM) or up to 7,250 RPM with carbide (600 SFM), feeding about 0.0073 in/rev.
- What size is a letter O drill?
- 0.3160 in / 8.026 mm. The nearest other drills are 8.0 mm (0.3150 in), 8.1 mm (0.3189 in), 5/16" (0.3125 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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