#78 drill size
A #78 number (wire-gauge) drill is 0.0160 in (0.406 mm).
- Decimal inch
- 0.0160
- Millimeters
- 0.406
- Nearest 1/64
- 1/64" (+0.0004)
- Type
- number (wire-gauge) drill
Nearest equivalents in other systems
| System | Drill | Inch | mm | Difference |
|---|---|---|---|---|
| Fractional | 1/64" | 0.0156 | 0.397 | -0.0004 in |
| Metric | 0.4 mm | 0.0158 | 0.400 | -0.0003 in |
Speeds and feeds: in mild steel an HSS drill starts at about 21,490 RPM and 0.0007 in/rev (15.0 IPM); in 6061 aluminum about 59,680 RPM.
#78 drill speeds and feeds by material
Jobber-length twist drill, D = 0.0160 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 | 59,680 | 600 | 143,240 | 0.0007 | 41.8 | 100.3 |
| Brass / free-machining | 150 | 35,810 | 400 | 95,490 | 0.0006 | 21.5 | 57.3 |
| Low-carbon steel (1018) | 90 | 21,490 | 300 | 71,620 | 0.0007 | 15.0 | 50.1 |
| Alloy steel (4140, annealed) | 70 | 16,710 | 250 | 59,680 | 0.0007 | 11.7 | 41.8 |
| Stainless 304 | 40 | 9,550 | 150 | 35,810 | 0.0005 | 4.78 | 17.9 |
| Gray cast iron | 80 | 19,100 | 250 | 59,680 | 0.0005 | 9.55 | 29.8 |
| Titanium 6Al-4V | 25 | 5,970 | 100 | 23,870 | 0.0004 | 2.39 | 9.55 |
| Plastics (acetal) | 300 | 71,620 | 500 | 119,370 | 0.0013 | 93.1 | 155.2 |
Small drills: the computed RPM is above most spindles. Run at your spindle's maximum and keep the feed per rev; feed drops in proportion.
Metric: Vc (m/min) = SFM × 0.3048; feed (mm/rev) = in/rev × 25.4. In steel, 21,490 RPM × 0.018 mm/rev = 382 mm/min.
Worked example: #78 drill in 1018 steel
- HSS drill in low-carbon steel: 90 SFM.
- RPM = 3.82 × 90 ÷ 0.0160 ≈ 21,490 RPM.
- Feed for a 0.016 in drill in mild steel ≈ 0.0007 in/rev.
- IPM = 21,490 × 0.0007 = 15.0 IPM.
Peck cycle by hole depth
| Depth | Cycle | Peck | Note |
|---|---|---|---|
| 3×D = 0.048 in (1.2 mm) | G81 (no peck) | None | Up to 3×D a sharp drill clears chips on its own. |
| 5×D = 0.080 in (2 mm) | G73 chip-break | 1 × D | Chip-break pecks; full retract if chips pack (soft aluminum, stainless). |
| 8×D = 0.128 in (3.2 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 #78 drill
No standard UNC, UNF or metric tap in our tables calls for a #78 drill. Use it for clearance, pilot or reamer prep holes.
Nearest drills
- 0.4 mm: 0.0158 in / 0.400 mm (-0.0003 in)
- 1/64": 0.0156 in / 0.397 mm (-0.0004 in)
- #79: 0.0145 in / 0.368 mm (-0.0015 in)
- 0.45 mm: 0.0177 in / 0.450 mm (+0.0017 in)
Common questions
- What is a #78 drill in decimal and mm?
- 0.0160 in, or 0.406 mm; nearest fractional drill: 1/64" (0.0156 in, -0.0004 in); nearest metric drill: 0.4 mm (0.0158 in, -0.0003 in).
- What RPM for a #78 drill in steel?
- In low-carbon steel (1018) an HSS #78 drill (0.0160 in) runs about 21,490 RPM at 90 SFM; a carbide drill about 71,620 RPM at 300 SFM. RPM = 3.82 × SFM ÷ D.
- What feed for a #78 drill?
- About 0.0007 in/rev in mild steel, 0.0007 in/rev in aluminum and 0.0005 in/rev in 304 stainless. At 21,490 RPM that is 15.0 IPM in steel (IPM = RPM × IPR).
- What RPM for a #78 drill in aluminum?
- 59,680 RPM with HSS (250 SFM) or up to 143,240 RPM with carbide (600 SFM), feeding about 0.0007 in/rev.
- What size is a #78 drill?
- 0.0160 in / 0.406 mm. The nearest other drills are 0.4 mm (0.0158 in), 1/64" (0.0156 in), #79 (0.0145 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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