Letter K drill size
A letter K drill is 0.2810 in (7.137 mm).
- Decimal inch
- 0.2810
- Millimeters
- 7.137
- Nearest 1/64
- 9/32" (-0.0002)
- Type
- letter drill
Nearest equivalents in other systems
| System | Drill | Inch | mm | Difference |
|---|---|---|---|---|
| Fractional | 9/32" | 0.2813 | 7.144 | +0.0003 in |
| Metric | 7.1 mm | 0.2795 | 7.100 | -0.0015 in |
Speeds and feeds: in mild steel an HSS drill starts at about 1,220 RPM and 0.0072 in/rev (8.78 IPM); in 6061 aluminum about 3,400 RPM.
Letter K drill speeds and feeds by material
Jobber-length twist drill, D = 0.2810 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 | 3,400 | 600 | 8,160 | 0.0066 | 22.4 | 53.9 |
| Brass / free-machining | 150 | 2,040 | 400 | 5,440 | 0.0055 | 11.2 | 29.9 |
| Low-carbon steel (1018) | 90 | 1,220 | 300 | 4,080 | 0.0072 | 8.78 | 29.4 |
| Alloy steel (4140, annealed) | 70 | 950 | 250 | 3,400 | 0.0075 | 7.13 | 25.5 |
| Stainless 304 | 40 | 540 | 150 | 2,040 | 0.0047 | 2.54 | 9.59 |
| Gray cast iron | 80 | 1,090 | 250 | 3,400 | 0.0050 | 5.45 | 17.0 |
| Titanium 6Al-4V | 25 | 340 | 100 | 1,360 | 0.0033 | 1.12 | 4.49 |
| Plastics (acetal) | 300 | 4,080 | 500 | 6,800 | 0.0042 | 17.1 | 28.6 |
Metric: Vc (m/min) = SFM × 0.3048; feed (mm/rev) = in/rev × 25.4. In steel, 1,220 RPM × 0.183 mm/rev = 223 mm/min.
Worked example: letter K drill in 1018 steel
- HSS drill in low-carbon steel: 90 SFM.
- RPM = 3.82 × 90 ÷ 0.2810 ≈ 1,220 RPM.
- Feed for a 0.281 in drill in mild steel ≈ 0.0072 in/rev.
- IPM = 1,220 × 0.0072 = 8.78 IPM.
Peck cycle by hole depth
| Depth | Cycle | Peck | Note |
|---|---|---|---|
| 3×D = 0.843 in (21.4 mm) | G81 (no peck) | None | Up to 3×D a sharp drill clears chips on its own. |
| 5×D = 1.405 in (35.7 mm) | G73 chip-break | 1 × D | Chip-break pecks; full retract if chips pack (soft aluminum, stainless). |
| 8×D = 2.248 in (57.1 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 K drill
No standard UNC, UNF or metric tap in our tables calls for a letter K drill. Use it for clearance, pilot or reamer prep holes.
Nearest drills
- 9/32": 0.2813 in / 7.144 mm (+0.0003 in)
- 7.1 mm: 0.2795 in / 7.100 mm (-0.0015 in)
- 7.2 mm: 0.2835 in / 7.200 mm (+0.0025 in)
- Letter J: 0.2770 in / 7.036 mm (-0.0040 in)
Common questions
- What is a letter K drill in decimal and mm?
- 0.2810 in, or 7.137 mm; nearest fractional drill: 9/32" (0.2813 in, +0.0003 in); nearest metric drill: 7.1 mm (0.2795 in, -0.0015 in).
- What RPM for a letter K drill in steel?
- In low-carbon steel (1018) an HSS letter K drill (0.2810 in) runs about 1,220 RPM at 90 SFM; a carbide drill about 4,080 RPM at 300 SFM. RPM = 3.82 × SFM ÷ D.
- What feed for a letter K drill?
- About 0.0072 in/rev in mild steel, 0.0066 in/rev in aluminum and 0.0047 in/rev in 304 stainless. At 1,220 RPM that is 8.78 IPM in steel (IPM = RPM × IPR).
- What RPM for a letter K drill in aluminum?
- 3,400 RPM with HSS (250 SFM) or up to 8,160 RPM with carbide (600 SFM), feeding about 0.0066 in/rev.
- What size is a letter K drill?
- 0.2810 in / 7.137 mm. The nearest other drills are 9/32" (0.2813 in), 7.1 mm (0.2795 in), 7.2 mm (0.2835 in).
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.
Put your skills on a free talent profile
Machines, controls, certs and saved calculations in one profile shops can find.
Create free profileBring shift handoffs to your shop
Stop losing job status between shifts. Every station's notes, counts and issues, handed off cleanly.
See shift handoffs