Letter M drill size
A letter M drill is 0.2950 in (7.493 mm). It is the form (roll) tap, 65% thread drill for 5/16-24.
- Decimal inch
- 0.2950
- Millimeters
- 7.493
- Nearest 1/64
- 19/64" (-0.0019)
- Type
- letter drill
Nearest equivalents in other systems
Speeds and feeds: in mild steel an HSS drill starts at about 1,170 RPM and 0.0076 in/rev (8.89 IPM); in 6061 aluminum about 3,240 RPM.
Letter M drill speeds and feeds by material
Jobber-length twist drill, D = 0.2950 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 | 3,240 | 600 | 7,770 | 0.0069 | 22.4 | 53.6 |
| Brass / free-machining | 150 | 1,940 | 400 | 5,180 | 0.0056 | 10.9 | 29.0 |
| Low-carbon steel (1018) | 90 | 1,170 | 300 | 3,880 | 0.0076 | 8.89 | 29.5 |
| Alloy steel (4140, annealed) | 70 | 910 | 250 | 3,240 | 0.0079 | 7.19 | 25.6 |
| Stainless 304 | 40 | 520 | 150 | 1,940 | 0.0050 | 2.60 | 9.70 |
| Gray cast iron | 80 | 1,040 | 250 | 3,240 | 0.0052 | 5.41 | 16.9 |
| Titanium 6Al-4V | 25 | 320 | 100 | 1,290 | 0.0035 | 1.12 | 4.51 |
| Plastics (acetal) | 300 | 3,880 | 500 | 6,470 | 0.0044 | 17.1 | 28.5 |
Metric: Vc (m/min) = SFM × 0.3048; feed (mm/rev) = in/rev × 25.4. In steel, 1,170 RPM × 0.193 mm/rev = 226 mm/min.
Worked example: letter M drill in 1018 steel
- HSS drill in low-carbon steel: 90 SFM.
- RPM = 3.82 × 90 ÷ 0.2950 ≈ 1,170 RPM.
- Feed for a 0.295 in drill in mild steel ≈ 0.0076 in/rev.
- IPM = 1,170 × 0.0076 = 8.89 IPM.
Peck cycle by hole depth
| Depth | Cycle | Peck | Note |
|---|---|---|---|
| 3×D = 0.885 in (22.5 mm) | G81 (no peck) | None | Up to 3×D a sharp drill clears chips on its own. |
| 5×D = 1.475 in (37.5 mm) | G73 chip-break | 1 × D | Chip-break pecks; full retract if chips pack (soft aluminum, stainless). |
| 8×D = 2.360 in (59.9 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 M drill
- 5/16-24 — form (roll) tap, 65% thread · tap speeds and feeds
Nearest drills
- 7.5 mm: 0.2953 in / 7.500 mm (+0.0003 in)
- 19/64": 0.2969 in / 7.541 mm (+0.0019 in)
- 7.4 mm: 0.2913 in / 7.400 mm (-0.0037 in)
- 7.6 mm: 0.2992 in / 7.600 mm (+0.0042 in)
Common questions
- What is a letter M drill in decimal and mm?
- 0.2950 in, or 7.493 mm; nearest fractional drill: 19/64" (0.2969 in, +0.0019 in); nearest metric drill: 7.5 mm (0.2953 in, +0.0003 in).
- What RPM for a letter M drill in steel?
- In low-carbon steel (1018) an HSS letter M drill (0.2950 in) runs about 1,170 RPM at 90 SFM; a carbide drill about 3,880 RPM at 300 SFM. RPM = 3.82 × SFM ÷ D.
- What feed for a letter M drill?
- About 0.0076 in/rev in mild steel, 0.0069 in/rev in aluminum and 0.0050 in/rev in 304 stainless. At 1,170 RPM that is 8.89 IPM in steel (IPM = RPM × IPR).
- What RPM for a letter M drill in aluminum?
- 3,240 RPM with HSS (250 SFM) or up to 7,770 RPM with carbide (600 SFM), feeding about 0.0069 in/rev.
- What tap uses a letter M drill?
- 5/16-24 (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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