What is a letter B drill in decimal and mm?

0.2380 in, or 6.045 mm; nearest fractional drill: 15/64" (0.2344 in, -0.0036 in); nearest metric drill: 6.0 mm (0.2362 in, -0.0018 in).

What RPM for a letter B drill in steel?

In low-carbon steel (1018) an HSS letter B drill (0.2380 in) runs about 1,440 RPM at 90 SFM; a carbide drill about 4,810 RPM at 300 SFM. RPM = 3.82 × SFM ÷ D.

What feed for a letter B drill?

About 0.0062 in/rev in mild steel, 0.0057 in/rev in aluminum and 0.0040 in/rev in 304 stainless. At 1,440 RPM that is 8.93 IPM in steel (IPM = RPM × IPR).

What RPM for a letter B drill in aluminum?

4,010 RPM with HSS (250 SFM) or up to 9,630 RPM with carbide (600 SFM), feeding about 0.0057 in/rev.

What size is a letter B drill?

0.2380 in / 6.045 mm. The nearest other drills are 6.0 mm (0.2362 in), 6.1 mm (0.2402 in), 15/64" (0.2344 in).

Letter B drill size

A letter B drill is 0.2380 in (6.045 mm).

Decimal inch: 0.2380
Millimeters: 6.045
Nearest 1/64: 15/64" (+0.0036)
Type: letter drill

Nearest equivalents in other systems

Drills closest to Letter B
System	Drill	Inch	mm	Difference
Fractional	15/64"	0.2344	5.953	-0.0036 in
Metric	6.0 mm	0.2362	6.000	-0.0018 in

Speeds and feeds: in mild steel an HSS drill starts at about 1,440 RPM and 0.0062 in/rev (8.93 IPM); in 6061 aluminum about 4,010 RPM.

Letter B drill speeds and feeds by material

Jobber-length twist drill, D = 0.2380 in. RPM = 3.82 × SFM ÷ D, rounded to 10. Feed per rev scales with diameter and material; IPM = RPM × IPR.

Letter B drilling speeds
Material	HSS SFM	HSS RPM	Carbide SFM	Carbide RPM	Feed (in/rev)	HSS IPM	Carbide IPM
Aluminum (6061)	250	4,010	600	9,630	0.0057	22.9	54.9
Brass / free-machining	150	2,410	400	6,420	0.0050	12.1	32.1
Low-carbon steel (1018)	90	1,440	300	4,810	0.0062	8.93	29.8
Alloy steel (4140, annealed)	70	1,120	250	4,010	0.0062	6.94	24.9
Stainless 304	40	640	150	2,410	0.0040	2.56	9.64
Gray cast iron	80	1,280	250	4,010	0.0042	5.38	16.8
Titanium 6Al-4V	25	400	100	1,600	0.0029	1.16	4.64
Plastics (acetal)	300	4,810	500	8,020	0.0041	19.7	32.9

Metric: Vc (m/min) = SFM × 0.3048; feed (mm/rev) = in/rev × 25.4. In steel, 1,440 RPM × 0.157 mm/rev = 227 mm/min.

Worked example: letter B drill in 1018 steel

HSS drill in low-carbon steel: 90 SFM.
RPM = 3.82 × 90 ÷ 0.2380 ≈ 1,440 RPM.
Feed for a 0.238 in drill in mild steel ≈ 0.0062 in/rev.
IPM = 1,440 × 0.0062 = 8.93 IPM.

Peck cycle by hole depth

Letter B drill: depth and canned cycle
Depth	Cycle	Peck	Note
3×D = 0.714 in (18.1 mm)	G81 (no peck)	None	Up to 3×D a sharp drill clears chips on its own.
5×D = 1.190 in (30.2 mm)	G73 chip-break	1 × D	Chip-break pecks; full retract if chips pack (soft aluminum, stainless).
8×D = 1.904 in (48.4 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 B drill

No standard UNC, UNF or metric tap in our tables calls for a letter B drill. Use it for clearance, pilot or reamer prep holes.

Nearest drills

6.0 mm: 0.2362 in / 6.000 mm (-0.0018 in)
6.1 mm: 0.2402 in / 6.100 mm (+0.0022 in)
15/64": 0.2344 in / 5.953 mm (-0.0036 in)
Letter A: 0.2340 in / 5.944 mm (-0.0040 in)

Drill speed calculator, prefilled for letter B Drill finder: sizes near letter B Letter drill chart Decimal equivalent chart

Common questions

What is a letter B drill in decimal and mm?: 0.2380 in, or 6.045 mm; nearest fractional drill: 15/64" (0.2344 in, -0.0036 in); nearest metric drill: 6.0 mm (0.2362 in, -0.0018 in).
What RPM for a letter B drill in steel?: In low-carbon steel (1018) an HSS letter B drill (0.2380 in) runs about 1,440 RPM at 90 SFM; a carbide drill about 4,810 RPM at 300 SFM. RPM = 3.82 × SFM ÷ D.
What feed for a letter B drill?: About 0.0062 in/rev in mild steel, 0.0057 in/rev in aluminum and 0.0040 in/rev in 304 stainless. At 1,440 RPM that is 8.93 IPM in steel (IPM = RPM × IPR).
What RPM for a letter B drill in aluminum?: 4,010 RPM with HSS (250 SFM) or up to 9,630 RPM with carbide (600 SFM), feeding about 0.0057 in/rev.
What size is a letter B drill?: 0.2380 in / 6.045 mm. The nearest other drills are 6.0 mm (0.2362 in), 6.1 mm (0.2402 in), 15/64" (0.2344 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.

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