What is a letter R drill in decimal and mm?

0.3390 in, or 8.611 mm; nearest fractional drill: 11/32" (0.3438 in, +0.0047 in); nearest metric drill: 8.6 mm (0.3386 in, -0.0004 in).

What RPM for a letter R drill in steel?

In low-carbon steel (1018) an HSS letter R drill (0.3390 in) runs about 1,010 RPM at 90 SFM; a carbide drill about 3,380 RPM at 300 SFM. RPM = 3.82 × SFM ÷ D.

What feed for a letter R drill?

About 0.0086 in/rev in mild steel, 0.0078 in/rev in aluminum and 0.0057 in/rev in 304 stainless. At 1,010 RPM that is 8.69 IPM in steel (IPM = RPM × IPR).

What RPM for a letter R drill in aluminum?

2,820 RPM with HSS (250 SFM) or up to 6,760 RPM with carbide (600 SFM), feeding about 0.0078 in/rev.

What size is a letter R drill?

0.3390 in / 8.611 mm. The nearest other drills are 8.6 mm (0.3386 in), 8.7 mm (0.3425 in), 8.5 mm (0.3347 in).

Letter R drill size

A letter R drill is 0.3390 in (8.611 mm).

Decimal inch: 0.3390
Millimeters: 8.611
Nearest 1/64: 11/32" (-0.0047)
Type: letter drill

Nearest equivalents in other systems

Drills closest to Letter R
System	Drill	Inch	mm	Difference
Fractional	11/32"	0.3438	8.731	+0.0047 in
Metric	8.6 mm	0.3386	8.600	-0.0004 in

Speeds and feeds: in mild steel an HSS drill starts at about 1,010 RPM and 0.0086 in/rev (8.69 IPM); in 6061 aluminum about 2,820 RPM.

Letter R drill speeds and feeds by material

Jobber-length twist drill, D = 0.3390 in. RPM = 3.82 × SFM ÷ D, rounded to 10. Feed per rev scales with diameter and material; IPM = RPM × IPR.

Letter R drilling speeds
Material	HSS SFM	HSS RPM	Carbide SFM	Carbide RPM	Feed (in/rev)	HSS IPM	Carbide IPM
Aluminum (6061)	250	2,820	600	6,760	0.0078	22.0	52.7
Brass / free-machining	150	1,690	400	4,510	0.0060	10.1	27.1
Low-carbon steel (1018)	90	1,010	300	3,380	0.0086	8.69	29.1
Alloy steel (4140, annealed)	70	790	250	2,820	0.0093	7.35	26.2
Stainless 304	40	450	150	1,690	0.0057	2.57	9.63
Gray cast iron	80	900	250	2,820	0.0059	5.31	16.6
Titanium 6Al-4V	25	280	100	1,130	0.0039	1.09	4.41
Plastics (acetal)	300	3,380	500	5,630	0.0047	15.9	26.5

Metric: Vc (m/min) = SFM × 0.3048; feed (mm/rev) = in/rev × 25.4. In steel, 1,010 RPM × 0.218 mm/rev = 221 mm/min.

Worked example: letter R drill in 1018 steel

HSS drill in low-carbon steel: 90 SFM.
RPM = 3.82 × 90 ÷ 0.3390 ≈ 1,010 RPM.
Feed for a 0.339 in drill in mild steel ≈ 0.0086 in/rev.
IPM = 1,010 × 0.0086 = 8.69 IPM.

Peck cycle by hole depth

Letter R drill: depth and canned cycle
Depth	Cycle	Peck	Note
3×D = 1.017 in (25.8 mm)	G81 (no peck)	None	Up to 3×D a sharp drill clears chips on its own.
5×D = 1.695 in (43.1 mm)	G73 chip-break	1 × D	Chip-break pecks; full retract if chips pack (soft aluminum, stainless).
8×D = 2.712 in (68.9 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 R drill

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

Nearest drills

8.6 mm: 0.3386 in / 8.600 mm (-0.0004 in)
8.7 mm: 0.3425 in / 8.700 mm (+0.0035 in)
8.5 mm: 0.3347 in / 8.500 mm (-0.0043 in)
11/32": 0.3438 in / 8.731 mm (+0.0047 in)

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

Common questions

What is a letter R drill in decimal and mm?: 0.3390 in, or 8.611 mm; nearest fractional drill: 11/32" (0.3438 in, +0.0047 in); nearest metric drill: 8.6 mm (0.3386 in, -0.0004 in).
What RPM for a letter R drill in steel?: In low-carbon steel (1018) an HSS letter R drill (0.3390 in) runs about 1,010 RPM at 90 SFM; a carbide drill about 3,380 RPM at 300 SFM. RPM = 3.82 × SFM ÷ D.
What feed for a letter R drill?: About 0.0086 in/rev in mild steel, 0.0078 in/rev in aluminum and 0.0057 in/rev in 304 stainless. At 1,010 RPM that is 8.69 IPM in steel (IPM = RPM × IPR).
What RPM for a letter R drill in aluminum?: 2,820 RPM with HSS (250 SFM) or up to 6,760 RPM with carbide (600 SFM), feeding about 0.0078 in/rev.
What size is a letter R drill?: 0.3390 in / 8.611 mm. The nearest other drills are 8.6 mm (0.3386 in), 8.7 mm (0.3425 in), 8.5 mm (0.3347 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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