1/8" drill size
A 1/8" fractional drill is 0.1250 in (3.175 mm). It is the form (roll) tap, 65% thread drill for 6-32.
- Decimal inch
- 0.1250
- Millimeters
- 3.175
- Nearest 1/64
- 1/8" (+0.0000)
- Type
- fractional drill
Nearest equivalents in other systems
Speeds and feeds: in mild steel an HSS drill starts at about 2,750 RPM and 0.0030 in/rev (8.25 IPM); in 6061 aluminum about 7,640 RPM.
1/8" drill speeds and feeds by material
Jobber-length twist drill, D = 0.1250 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 | 7,640 | 600 | 18,330 | 0.0029 | 22.2 | 53.2 |
| Brass / free-machining | 150 | 4,580 | 400 | 12,220 | 0.0024 | 11.0 | 29.3 |
| Low-carbon steel (1018) | 90 | 2,750 | 300 | 9,170 | 0.0030 | 8.25 | 27.5 |
| Alloy steel (4140, annealed) | 70 | 2,140 | 250 | 7,640 | 0.0030 | 6.42 | 22.9 |
| Stainless 304 | 40 | 1,220 | 150 | 4,580 | 0.0020 | 2.44 | 9.16 |
| Gray cast iron | 80 | 2,440 | 250 | 7,640 | 0.0018 | 4.39 | 13.8 |
| Titanium 6Al-4V | 25 | 760 | 100 | 3,060 | 0.0016 | 1.22 | 4.90 |
| Plastics (acetal) | 300 | 9,170 | 500 | 15,280 | 0.0050 | 45.9 | 76.4 |
Metric: Vc (m/min) = SFM × 0.3048; feed (mm/rev) = in/rev × 25.4. In steel, 2,750 RPM × 0.076 mm/rev = 210 mm/min.
Worked example: 1/8" drill in 1018 steel
- HSS drill in low-carbon steel: 90 SFM.
- RPM = 3.82 × 90 ÷ 0.1250 ≈ 2,750 RPM.
- Feed for a 0.125 in drill in mild steel ≈ 0.0030 in/rev.
- IPM = 2,750 × 0.0030 = 8.25 IPM.
Peck cycle by hole depth
| Depth | Cycle | Peck | Note |
|---|---|---|---|
| 3×D = 0.375 in (9.5 mm) | G81 (no peck) | None | Up to 3×D a sharp drill clears chips on its own. |
| 5×D = 0.625 in (15.9 mm) | G73 chip-break | 1 × D | Chip-break pecks; full retract if chips pack (soft aluminum, stainless). |
| 8×D = 1.000 in (25.4 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 1/8" drill
- 6-32 — form (roll) tap, 65% thread · tap speeds and feeds
Nearest drills
- 3.2 mm: 0.1260 in / 3.200 mm (+0.0010 in)
- 3.1 mm: 0.1221 in / 3.100 mm (-0.0029 in)
- #30: 0.1285 in / 3.264 mm (+0.0035 in)
- 3.3 mm: 0.1299 in / 3.300 mm (+0.0049 in)
Common questions
- What is a 1/8" drill in decimal and mm?
- 0.1250 in, or 3.175 mm; nearest number / letter drill: #30 (0.1285 in, +0.0035 in); nearest metric drill: 3.2 mm (0.1260 in, +0.0010 in).
- What RPM for a 1/8" drill in steel?
- In low-carbon steel (1018) an HSS 1/8" drill (0.1250 in) runs about 2,750 RPM at 90 SFM; a carbide drill about 9,170 RPM at 300 SFM. RPM = 3.82 × SFM ÷ D.
- What feed for a 1/8" drill?
- About 0.0030 in/rev in mild steel, 0.0029 in/rev in aluminum and 0.0020 in/rev in 304 stainless. At 2,750 RPM that is 8.25 IPM in steel (IPM = RPM × IPR).
- What RPM for a 1/8" drill in aluminum?
- 7,640 RPM with HSS (250 SFM) or up to 18,330 RPM with carbide (600 SFM), feeding about 0.0029 in/rev.
- What tap uses a 1/8" drill?
- 6-32 (form (roll) tap, 65% 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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