Letter J drill size
A letter J drill is 0.2770 in (7.036 mm). It is the cut tap, 50% thread drill for 5/16-18.
- Decimal inch
- 0.2770
- Millimeters
- 7.036
- Nearest 1/64
- 9/32" (-0.0042)
- Type
- letter drill
Nearest equivalents in other systems
Speeds and feeds: in mild steel an HSS drill starts at about 1,240 RPM and 0.0071 in/rev (8.80 IPM); in 6061 aluminum about 3,450 RPM.
Letter J drill speeds and feeds by material
Jobber-length twist drill, D = 0.2770 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 | 3,450 | 600 | 8,270 | 0.0065 | 22.4 | 53.8 |
| Brass / free-machining | 150 | 2,070 | 400 | 5,520 | 0.0055 | 11.4 | 30.4 |
| Low-carbon steel (1018) | 90 | 1,240 | 300 | 4,140 | 0.0071 | 8.80 | 29.4 |
| Alloy steel (4140, annealed) | 70 | 970 | 250 | 3,450 | 0.0074 | 7.18 | 25.5 |
| Stainless 304 | 40 | 550 | 150 | 2,070 | 0.0047 | 2.59 | 9.73 |
| Gray cast iron | 80 | 1,100 | 250 | 3,450 | 0.0049 | 5.39 | 16.9 |
| Titanium 6Al-4V | 25 | 340 | 100 | 1,380 | 0.0033 | 1.12 | 4.55 |
| Plastics (acetal) | 300 | 4,140 | 500 | 6,890 | 0.0042 | 17.4 | 28.9 |
Metric: Vc (m/min) = SFM × 0.3048; feed (mm/rev) = in/rev × 25.4. In steel, 1,240 RPM × 0.180 mm/rev = 224 mm/min.
Worked example: letter J drill in 1018 steel
- HSS drill in low-carbon steel: 90 SFM.
- RPM = 3.82 × 90 ÷ 0.2770 ≈ 1,240 RPM.
- Feed for a 0.277 in drill in mild steel ≈ 0.0071 in/rev.
- IPM = 1,240 × 0.0071 = 8.80 IPM.
Peck cycle by hole depth
| Depth | Cycle | Peck | Note |
|---|---|---|---|
| 3×D = 0.831 in (21.1 mm) | G81 (no peck) | None | Up to 3×D a sharp drill clears chips on its own. |
| 5×D = 1.385 in (35.2 mm) | G73 chip-break | 1 × D | Chip-break pecks; full retract if chips pack (soft aluminum, stainless). |
| 8×D = 2.216 in (56.3 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 letter J drill
- 5/16-18 — cut tap, 50% thread · tap speeds and feeds
Nearest drills
- 7.0 mm: 0.2756 in / 7.000 mm (-0.0014 in)
- 7.1 mm: 0.2795 in / 7.100 mm (+0.0025 in)
- Letter K: 0.2810 in / 7.137 mm (+0.0040 in)
- 9/32": 0.2813 in / 7.144 mm (+0.0043 in)
Common questions
- What is a letter J drill in decimal and mm?
- 0.2770 in, or 7.036 mm; nearest fractional drill: 9/32" (0.2813 in, +0.0043 in); nearest metric drill: 7.0 mm (0.2756 in, -0.0014 in).
- What RPM for a letter J drill in steel?
- In low-carbon steel (1018) an HSS letter J drill (0.2770 in) runs about 1,240 RPM at 90 SFM; a carbide drill about 4,140 RPM at 300 SFM. RPM = 3.82 × SFM ÷ D.
- What feed for a letter J drill?
- About 0.0071 in/rev in mild steel, 0.0065 in/rev in aluminum and 0.0047 in/rev in 304 stainless. At 1,240 RPM that is 8.80 IPM in steel (IPM = RPM × IPR).
- What RPM for a letter J drill in aluminum?
- 3,450 RPM with HSS (250 SFM) or up to 8,270 RPM with carbide (600 SFM), feeding about 0.0065 in/rev.
- What tap uses a letter J drill?
- 5/16-18 (cut tap, 50% 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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