21/64" drill size
A 21/64" fractional drill is 0.3281 in (8.334 mm).
- Decimal inch
- 0.3281
- Millimeters
- 8.334
- Nearest 1/64
- 21/64" (+0.0000)
- Type
- fractional drill
Nearest equivalents in other systems
| System | Drill | Inch | mm | Difference |
|---|---|---|---|---|
| Number / letter | Letter Q | 0.3320 | 8.433 | +0.0039 in |
| Metric | 8.3 mm | 0.3268 | 8.300 | -0.0014 in |
Speeds and feeds: in mild steel an HSS drill starts at about 1,050 RPM and 0.0084 in/rev (8.82 IPM); in 6061 aluminum about 2,910 RPM.
21/64" drill speeds and feeds by material
Jobber-length twist drill, D = 0.3281 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 | 2,910 | 600 | 6,980 | 0.0076 | 22.1 | 53.0 |
| Brass / free-machining | 150 | 1,750 | 400 | 4,660 | 0.0059 | 10.3 | 27.5 |
| Low-carbon steel (1018) | 90 | 1,050 | 300 | 3,490 | 0.0084 | 8.82 | 29.3 |
| Alloy steel (4140, annealed) | 70 | 810 | 250 | 2,910 | 0.0090 | 7.29 | 26.2 |
| Stainless 304 | 40 | 470 | 150 | 1,750 | 0.0055 | 2.59 | 9.63 |
| Gray cast iron | 80 | 930 | 250 | 2,910 | 0.0058 | 5.39 | 16.9 |
| Titanium 6Al-4V | 25 | 290 | 100 | 1,160 | 0.0038 | 1.10 | 4.41 |
| Plastics (acetal) | 300 | 3,490 | 500 | 5,820 | 0.0046 | 16.1 | 26.8 |
Metric: Vc (m/min) = SFM × 0.3048; feed (mm/rev) = in/rev × 25.4. In steel, 1,050 RPM × 0.213 mm/rev = 224 mm/min.
Worked example: 21/64" drill in 1018 steel
- HSS drill in low-carbon steel: 90 SFM.
- RPM = 3.82 × 90 ÷ 0.3281 ≈ 1,050 RPM.
- Feed for a 0.328 in drill in mild steel ≈ 0.0084 in/rev.
- IPM = 1,050 × 0.0084 = 8.82 IPM.
Peck cycle by hole depth
| Depth | Cycle | Peck | Note |
|---|---|---|---|
| 3×D = 0.984 in (25 mm) | G81 (no peck) | None | Up to 3×D a sharp drill clears chips on its own. |
| 5×D = 1.641 in (41.7 mm) | G73 chip-break | 1 × D | Chip-break pecks; full retract if chips pack (soft aluminum, stainless). |
| 8×D = 2.625 in (66.7 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 21/64" drill
No standard UNC, UNF or metric tap in our tables calls for a 21/64" drill. Use it for clearance, pilot or reamer prep holes.
Nearest drills
- 8.3 mm: 0.3268 in / 8.300 mm (-0.0014 in)
- 8.4 mm: 0.3307 in / 8.400 mm (+0.0026 in)
- Letter Q: 0.3320 in / 8.433 mm (+0.0039 in)
- Letter P: 0.3230 in / 8.204 mm (-0.0051 in)
Common questions
- What is a 21/64" drill in decimal and mm?
- 0.3281 in, or 8.334 mm; nearest number / letter drill: Letter Q (0.3320 in, +0.0039 in); nearest metric drill: 8.3 mm (0.3268 in, -0.0014 in).
- What RPM for a 21/64" drill in steel?
- In low-carbon steel (1018) an HSS 21/64" drill (0.3281 in) runs about 1,050 RPM at 90 SFM; a carbide drill about 3,490 RPM at 300 SFM. RPM = 3.82 × SFM ÷ D.
- What feed for a 21/64" drill?
- About 0.0084 in/rev in mild steel, 0.0076 in/rev in aluminum and 0.0055 in/rev in 304 stainless. At 1,050 RPM that is 8.82 IPM in steel (IPM = RPM × IPR).
- What RPM for a 21/64" drill in aluminum?
- 2,910 RPM with HSS (250 SFM) or up to 6,980 RPM with carbide (600 SFM), feeding about 0.0076 in/rev.
- What size is a 21/64" drill?
- 0.3281 in / 8.334 mm. The nearest other drills are 8.3 mm (0.3268 in), 8.4 mm (0.3307 in), Letter Q (0.3320 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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