#21 drill size
A #21 number (wire-gauge) drill is 0.1590 in (4.039 mm). It is the cut tap, 75% thread drill for 10-32.
- Decimal inch
- 0.1590
- Millimeters
- 4.039
- Nearest 1/64
- 5/32" (+0.0028)
- Type
- number (wire-gauge) drill
Nearest equivalents in other systems
Speeds and feeds: in mild steel an HSS drill starts at about 2,160 RPM and 0.0039 in/rev (8.42 IPM); in 6061 aluminum about 6,010 RPM.
#21 drill speeds and feeds by material
Jobber-length twist drill, D = 0.1590 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 | 6,010 | 600 | 14,410 | 0.0037 | 22.2 | 53.3 |
| Brass / free-machining | 150 | 3,600 | 400 | 9,610 | 0.0032 | 11.5 | 30.8 |
| Low-carbon steel (1018) | 90 | 2,160 | 300 | 7,210 | 0.0039 | 8.42 | 28.1 |
| Alloy steel (4140, annealed) | 70 | 1,680 | 250 | 6,010 | 0.0039 | 6.55 | 23.4 |
| Stainless 304 | 40 | 960 | 150 | 3,600 | 0.0026 | 2.50 | 9.36 |
| Gray cast iron | 80 | 1,920 | 250 | 6,010 | 0.0025 | 4.80 | 15.0 |
| Titanium 6Al-4V | 25 | 600 | 100 | 2,400 | 0.0019 | 1.14 | 4.56 |
| Plastics (acetal) | 300 | 7,210 | 500 | 12,010 | 0.0047 | 33.9 | 56.5 |
Metric: Vc (m/min) = SFM × 0.3048; feed (mm/rev) = in/rev × 25.4. In steel, 2,160 RPM × 0.099 mm/rev = 214 mm/min.
Worked example: #21 drill in 1018 steel
- HSS drill in low-carbon steel: 90 SFM.
- RPM = 3.82 × 90 ÷ 0.1590 ≈ 2,160 RPM.
- Feed for a 0.159 in drill in mild steel ≈ 0.0039 in/rev.
- IPM = 2,160 × 0.0039 = 8.42 IPM.
Peck cycle by hole depth
| Depth | Cycle | Peck | Note |
|---|---|---|---|
| 3×D = 0.477 in (12.1 mm) | G81 (no peck) | None | Up to 3×D a sharp drill clears chips on its own. |
| 5×D = 0.795 in (20.2 mm) | G73 chip-break | 1 × D | Chip-break pecks; full retract if chips pack (soft aluminum, stainless). |
| 8×D = 1.272 in (32.3 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 drill
- 10-32 — cut tap, 75% thread · tap speeds and feeds
Nearest drills
- 4.0 mm: 0.1575 in / 4.000 mm (-0.0015 in)
- #22: 0.1570 in / 3.988 mm (-0.0020 in)
- #20: 0.1610 in / 4.089 mm (+0.0020 in)
- 4.1 mm: 0.1614 in / 4.100 mm (+0.0024 in)
Common questions
- What is a #21 drill in decimal and mm?
- 0.1590 in, or 4.039 mm; nearest fractional drill: 5/32" (0.1563 in, -0.0027 in); nearest metric drill: 4.0 mm (0.1575 in, -0.0015 in).
- What RPM for a #21 drill in steel?
- In low-carbon steel (1018) an HSS #21 drill (0.1590 in) runs about 2,160 RPM at 90 SFM; a carbide drill about 7,210 RPM at 300 SFM. RPM = 3.82 × SFM ÷ D.
- What feed for a #21 drill?
- About 0.0039 in/rev in mild steel, 0.0037 in/rev in aluminum and 0.0026 in/rev in 304 stainless. At 2,160 RPM that is 8.42 IPM in steel (IPM = RPM × IPR).
- What RPM for a #21 drill in aluminum?
- 6,010 RPM with HSS (250 SFM) or up to 14,410 RPM with carbide (600 SFM), feeding about 0.0037 in/rev.
- What tap uses a #21 drill?
- 10-32 (cut tap, 75% 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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