Letter P drill size
A letter P drill is 0.3230 in (8.204 mm).
- Decimal inch
- 0.3230
- Millimeters
- 8.204
- Nearest 1/64
- 21/64" (-0.0051)
- Type
- letter drill
Nearest equivalents in other systems
| System | Drill | Inch | mm | Difference |
|---|---|---|---|---|
| Fractional | 21/64" | 0.3281 | 8.334 | +0.0051 in |
| Metric | 8.2 mm | 0.3228 | 8.200 | -0.0002 in |
Speeds and feeds: in mild steel an HSS drill starts at about 1,060 RPM and 0.0083 in/rev (8.80 IPM); in 6061 aluminum about 2,960 RPM.
Letter P drill speeds and feeds by material
Jobber-length twist drill, D = 0.3230 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 | 2,960 | 600 | 7,100 | 0.0075 | 22.2 | 53.3 |
| Brass / free-machining | 150 | 1,770 | 400 | 4,730 | 0.0058 | 10.3 | 27.4 |
| Low-carbon steel (1018) | 90 | 1,060 | 300 | 3,550 | 0.0083 | 8.80 | 29.5 |
| Alloy steel (4140, annealed) | 70 | 830 | 250 | 2,960 | 0.0088 | 7.30 | 26.1 |
| Stainless 304 | 40 | 470 | 150 | 1,770 | 0.0054 | 2.54 | 9.56 |
| Gray cast iron | 80 | 950 | 250 | 2,960 | 0.0057 | 5.42 | 16.9 |
| Titanium 6Al-4V | 25 | 300 | 100 | 1,180 | 0.0037 | 1.11 | 4.37 |
| Plastics (acetal) | 300 | 3,550 | 500 | 5,910 | 0.0046 | 16.3 | 27.2 |
Metric: Vc (m/min) = SFM × 0.3048; feed (mm/rev) = in/rev × 25.4. In steel, 1,060 RPM × 0.211 mm/rev = 223 mm/min.
Worked example: letter P drill in 1018 steel
- HSS drill in low-carbon steel: 90 SFM.
- RPM = 3.82 × 90 ÷ 0.3230 ≈ 1,060 RPM.
- Feed for a 0.323 in drill in mild steel ≈ 0.0083 in/rev.
- IPM = 1,060 × 0.0083 = 8.80 IPM.
Peck cycle by hole depth
| Depth | Cycle | Peck | Note |
|---|---|---|---|
| 3×D = 0.969 in (24.6 mm) | G81 (no peck) | None | Up to 3×D a sharp drill clears chips on its own. |
| 5×D = 1.615 in (41 mm) | G73 chip-break | 1 × D | Chip-break pecks; full retract if chips pack (soft aluminum, stainless). |
| 8×D = 2.584 in (65.6 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 P drill
No standard UNC, UNF or metric tap in our tables calls for a letter P drill. Use it for clearance, pilot or reamer prep holes.
Nearest drills
- 8.2 mm: 0.3228 in / 8.200 mm (-0.0002 in)
- 8.3 mm: 0.3268 in / 8.300 mm (+0.0038 in)
- 8.1 mm: 0.3189 in / 8.100 mm (-0.0041 in)
- 21/64": 0.3281 in / 8.334 mm (+0.0051 in)
Common questions
- What is a letter P drill in decimal and mm?
- 0.3230 in, or 8.204 mm; nearest fractional drill: 21/64" (0.3281 in, +0.0051 in); nearest metric drill: 8.2 mm (0.3228 in, -0.0002 in).
- What RPM for a letter P drill in steel?
- In low-carbon steel (1018) an HSS letter P drill (0.3230 in) runs about 1,060 RPM at 90 SFM; a carbide drill about 3,550 RPM at 300 SFM. RPM = 3.82 × SFM ÷ D.
- What feed for a letter P drill?
- About 0.0083 in/rev in mild steel, 0.0075 in/rev in aluminum and 0.0054 in/rev in 304 stainless. At 1,060 RPM that is 8.80 IPM in steel (IPM = RPM × IPR).
- What RPM for a letter P drill in aluminum?
- 2,960 RPM with HSS (250 SFM) or up to 7,100 RPM with carbide (600 SFM), feeding about 0.0075 in/rev.
- What size is a letter P drill?
- 0.3230 in / 8.204 mm. The nearest other drills are 8.2 mm (0.3228 in), 8.3 mm (0.3268 in), 8.1 mm (0.3189 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