Letter S drill size
A letter S drill is 0.3480 in (8.839 mm). It is the form (roll) tap, 65% thread drill for 3/8-16.
- Decimal inch
- 0.3480
- Millimeters
- 8.839
- Nearest 1/64
- 11/32" (+0.0042)
- Type
- letter drill
Nearest equivalents in other systems
Speeds and feeds: in mild steel an HSS drill starts at about 990 RPM and 0.0089 in/rev (8.81 IPM); in 6061 aluminum about 2,740 RPM.
Letter S drill speeds and feeds by material
Jobber-length twist drill, D = 0.3480 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 | 2,740 | 600 | 6,590 | 0.0080 | 21.9 | 52.7 |
| Brass / free-machining | 150 | 1,650 | 400 | 4,390 | 0.0060 | 9.90 | 26.3 |
| Low-carbon steel (1018) | 90 | 990 | 300 | 3,290 | 0.0089 | 8.81 | 29.3 |
| Alloy steel (4140, annealed) | 70 | 770 | 250 | 2,740 | 0.0096 | 7.39 | 26.3 |
| Stainless 304 | 40 | 440 | 150 | 1,650 | 0.0058 | 2.55 | 9.57 |
| Gray cast iron | 80 | 880 | 250 | 2,740 | 0.0061 | 5.37 | 16.7 |
| Titanium 6Al-4V | 25 | 270 | 100 | 1,100 | 0.0040 | 1.08 | 4.40 |
| Plastics (acetal) | 300 | 3,290 | 500 | 5,490 | 0.0048 | 15.8 | 26.4 |
Metric: Vc (m/min) = SFM × 0.3048; feed (mm/rev) = in/rev × 25.4. In steel, 990 RPM × 0.226 mm/rev = 224 mm/min.
Worked example: letter S drill in 1018 steel
- HSS drill in low-carbon steel: 90 SFM.
- RPM = 3.82 × 90 ÷ 0.3480 ≈ 990 RPM.
- Feed for a 0.348 in drill in mild steel ≈ 0.0089 in/rev.
- IPM = 990 × 0.0089 = 8.81 IPM.
Peck cycle by hole depth
| Depth | Cycle | Peck | Note |
|---|---|---|---|
| 3×D = 1.044 in (26.5 mm) | G81 (no peck) | None | Up to 3×D a sharp drill clears chips on its own. |
| 5×D = 1.740 in (44.2 mm) | G73 chip-break | 1 × D | Chip-break pecks; full retract if chips pack (soft aluminum, stainless). |
| 8×D = 2.784 in (70.7 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 letter S drill
- 3/8-16 — form (roll) tap, 65% thread · tap speeds and feeds
- 3/8-24 — cut tap, 50% thread · tap speeds and feeds
- 5/16-24 — Helicoil / STI insert (5/16-24×1D) · tap speeds and feeds
Nearest drills
- 8.8 mm: 0.3465 in / 8.800 mm (-0.0015 in)
- 8.9 mm: 0.3504 in / 8.900 mm (+0.0024 in)
- 11/32": 0.3438 in / 8.731 mm (-0.0043 in)
- 8.7 mm: 0.3425 in / 8.700 mm (-0.0055 in)
Common questions
- What is a letter S drill in decimal and mm?
- 0.3480 in, or 8.839 mm; nearest fractional drill: 11/32" (0.3438 in, -0.0043 in); nearest metric drill: 8.8 mm (0.3465 in, -0.0015 in).
- What RPM for a letter S drill in steel?
- In low-carbon steel (1018) an HSS letter S drill (0.3480 in) runs about 990 RPM at 90 SFM; a carbide drill about 3,290 RPM at 300 SFM. RPM = 3.82 × SFM ÷ D.
- What feed for a letter S drill?
- About 0.0089 in/rev in mild steel, 0.0080 in/rev in aluminum and 0.0058 in/rev in 304 stainless. At 990 RPM that is 8.81 IPM in steel (IPM = RPM × IPR).
- What RPM for a letter S drill in aluminum?
- 2,740 RPM with HSS (250 SFM) or up to 6,590 RPM with carbide (600 SFM), feeding about 0.0080 in/rev.
- What tap uses a letter S drill?
- 3/8-16 (form (roll) tap, 65% thread); 3/8-24 (cut tap, 50% thread); 5/16-24 (Helicoil / STI insert (5/16-24×1D)).
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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