#66 drill size
A #66 number (wire-gauge) drill is 0.0330 in (0.838 mm).
- Decimal inch
- 0.0330
- Millimeters
- 0.838
- Nearest 1/64
- 1/32" (+0.0018)
- Type
- number (wire-gauge) drill
Nearest equivalents in other systems
| System | Drill | Inch | mm | Difference |
|---|---|---|---|---|
| Fractional | 1/32" | 0.0313 | 0.794 | -0.0018 in |
| Metric | 0.85 mm | 0.0335 | 0.850 | +0.0005 in |
Speeds and feeds: in mild steel an HSS drill starts at about 10,420 RPM and 0.0008 in/rev (8.34 IPM); in 6061 aluminum about 28,940 RPM.
#66 drill speeds and feeds by material
Jobber-length twist drill, D = 0.0330 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 | 28,940 | 600 | 69,450 | 0.0008 | 23.1 | 55.6 |
| Brass / free-machining | 150 | 17,360 | 400 | 46,300 | 0.0006 | 10.4 | 27.8 |
| Low-carbon steel (1018) | 90 | 10,420 | 300 | 34,720 | 0.0008 | 8.34 | 27.8 |
| Alloy steel (4140, annealed) | 70 | 8,100 | 250 | 28,940 | 0.0008 | 6.48 | 23.1 |
| Stainless 304 | 40 | 4,630 | 150 | 17,360 | 0.0005 | 2.32 | 8.68 |
| Gray cast iron | 80 | 9,260 | 250 | 28,940 | 0.0005 | 4.63 | 14.5 |
| Titanium 6Al-4V | 25 | 2,890 | 100 | 11,570 | 0.0004 | 1.16 | 4.63 |
| Plastics (acetal) | 300 | 34,720 | 500 | 57,870 | 0.0013 | 45.1 | 75.2 |
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, 10,420 RPM × 0.020 mm/rev = 212 mm/min.
Worked example: #66 drill in 1018 steel
- HSS drill in low-carbon steel: 90 SFM.
- RPM = 3.82 × 90 ÷ 0.0330 ≈ 10,420 RPM.
- Feed for a 0.033 in drill in mild steel ≈ 0.0008 in/rev.
- IPM = 10,420 × 0.0008 = 8.34 IPM.
Peck cycle by hole depth
| Depth | Cycle | Peck | Note |
|---|---|---|---|
| 3×D = 0.099 in (2.5 mm) | G81 (no peck) | None | Up to 3×D a sharp drill clears chips on its own. |
| 5×D = 0.165 in (4.2 mm) | G73 chip-break | 1 × D | Chip-break pecks; full retract if chips pack (soft aluminum, stainless). |
| 8×D = 0.264 in (6.7 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 #66 drill
No standard UNC, UNF or metric tap in our tables calls for a #66 drill. Use it for clearance, pilot or reamer prep holes.
Nearest drills
- 0.85 mm: 0.0335 in / 0.850 mm (+0.0005 in)
- #67: 0.0320 in / 0.813 mm (-0.0010 in)
- 0.8 mm: 0.0315 in / 0.800 mm (-0.0015 in)
- 1/32": 0.0313 in / 0.794 mm (-0.0018 in)
Common questions
- What is a #66 drill in decimal and mm?
- 0.0330 in, or 0.838 mm; nearest fractional drill: 1/32" (0.0313 in, -0.0018 in); nearest metric drill: 0.85 mm (0.0335 in, +0.0005 in).
- What RPM for a #66 drill in steel?
- In low-carbon steel (1018) an HSS #66 drill (0.0330 in) runs about 10,420 RPM at 90 SFM; a carbide drill about 34,720 RPM at 300 SFM. RPM = 3.82 × SFM ÷ D.
- What feed for a #66 drill?
- About 0.0008 in/rev in mild steel, 0.0008 in/rev in aluminum and 0.0005 in/rev in 304 stainless. At 10,420 RPM that is 8.34 IPM in steel (IPM = RPM × IPR).
- What RPM for a #66 drill in aluminum?
- 28,940 RPM with HSS (250 SFM) or up to 69,450 RPM with carbide (600 SFM), feeding about 0.0008 in/rev.
- What size is a #66 drill?
- 0.0330 in / 0.838 mm. The nearest other drills are 0.85 mm (0.0335 in), #67 (0.0320 in), 0.8 mm (0.0315 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.
Put your skills on a free talent profile
Machines, controls, certs and saved calculations in one profile shops can find.
Create free profileBring shift handoffs to your shop
Stop losing job status between shifts. Every station's notes, counts and issues, handed off cleanly.
See shift handoffs