31/64" drill size
A 31/64" fractional drill is 0.4844 in (12.303 mm). It is the cut tap, 75% thread drill for 9/16-12.
- Decimal inch
- 0.4844
- Millimeters
- 12.303
- Nearest 1/64
- 31/64" (+0.0000)
- Type
- fractional drill
Nearest equivalents in other systems
| System | Drill | Inch | mm | Difference |
|---|---|---|---|---|
| Number / letter | Letter Z | 0.4130 | 10.490 | -0.0714 in |
| Metric | 12.3 mm | 0.4843 | 12.300 | -0.0001 in |
Speeds and feeds: in mild steel an HSS drill starts at about 710 RPM and 0.0104 in/rev (7.38 IPM); in 6061 aluminum about 1,970 RPM.
31/64" drill speeds and feeds by material
Jobber-length twist drill, D = 0.4844 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 | 1,970 | 600 | 4,730 | 0.0098 | 19.3 | 46.4 |
| Brass / free-machining | 150 | 1,180 | 400 | 3,150 | 0.0080 | 9.44 | 25.2 |
| Low-carbon steel (1018) | 90 | 710 | 300 | 2,370 | 0.0104 | 7.38 | 24.6 |
| Alloy steel (4140, annealed) | 70 | 550 | 250 | 1,970 | 0.0114 | 6.27 | 22.5 |
| Stainless 304 | 40 | 320 | 150 | 1,180 | 0.0082 | 2.62 | 9.68 |
| Gray cast iron | 80 | 630 | 250 | 1,970 | 0.0083 | 5.23 | 16.4 |
| Titanium 6Al-4V | 25 | 200 | 100 | 790 | 0.0049 | 0.98 | 3.87 |
| Plastics (acetal) | 300 | 2,370 | 500 | 3,940 | 0.0059 | 14.0 | 23.3 |
Metric: Vc (m/min) = SFM × 0.3048; feed (mm/rev) = in/rev × 25.4. In steel, 710 RPM × 0.264 mm/rev = 188 mm/min.
Worked example: 31/64" drill in 1018 steel
- HSS drill in low-carbon steel: 90 SFM.
- RPM = 3.82 × 90 ÷ 0.4844 ≈ 710 RPM.
- Feed for a 0.484 in drill in mild steel ≈ 0.0104 in/rev.
- IPM = 710 × 0.0104 = 7.38 IPM.
Peck cycle by hole depth
| Depth | Cycle | Peck | Note |
|---|---|---|---|
| 3×D = 1.453 in (36.9 mm) | G81 (no peck) | None | Up to 3×D a sharp drill clears chips on its own. |
| 5×D = 2.422 in (61.5 mm) | G73 chip-break | 1 × D | Chip-break pecks; full retract if chips pack (soft aluminum, stainless). |
| 8×D = 3.875 in (98.4 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 31/64" drill
- 9/16-12 — cut tap, 75% thread · tap speeds and feeds
- 1/2-20 — form (roll) tap, 65% thread · tap speeds and feeds
- 1/2-28 — form (roll) tap, 65% thread
- 1/2-28 — cut tap, 50% thread
- 7/16-20 — Helicoil / STI insert (7/16-20×1D) · tap speeds and feeds
Nearest drills
- 12.3 mm: 0.4843 in / 12.300 mm (-0.0001 in)
- 12.4 mm: 0.4882 in / 12.400 mm (+0.0038 in)
- 12.2 mm: 0.4803 in / 12.200 mm (-0.0041 in)
- 12.5 mm: 0.4921 in / 12.500 mm (+0.0078 in)
Common questions
- What is a 31/64" drill in decimal and mm?
- 0.4844 in, or 12.303 mm; nearest number / letter drill: Letter Z (0.4130 in, -0.0714 in); nearest metric drill: 12.3 mm (0.4843 in, -0.0001 in).
- What RPM for a 31/64" drill in steel?
- In low-carbon steel (1018) an HSS 31/64" drill (0.4844 in) runs about 710 RPM at 90 SFM; a carbide drill about 2,370 RPM at 300 SFM. RPM = 3.82 × SFM ÷ D.
- What feed for a 31/64" drill?
- About 0.0104 in/rev in mild steel, 0.0098 in/rev in aluminum and 0.0082 in/rev in 304 stainless. At 710 RPM that is 7.38 IPM in steel (IPM = RPM × IPR).
- What RPM for a 31/64" drill in aluminum?
- 1,970 RPM with HSS (250 SFM) or up to 4,730 RPM with carbide (600 SFM), feeding about 0.0098 in/rev.
- What tap uses a 31/64" drill?
- 9/16-12 (cut tap, 75% thread); 1/2-20 (form (roll) tap, 65% thread); 1/2-28 (form (roll) tap, 65% thread); 1/2-28 (cut tap, 50% thread).
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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