What is a letter U drill in decimal and mm?

0.3680 in, or 9.347 mm; nearest fractional drill: 3/8" (0.3750 in, +0.0070 in); nearest metric drill: 9.3 mm (0.3661 in, -0.0019 in).

What RPM for a letter U drill in steel?

In low-carbon steel (1018) an HSS letter U drill (0.3680 in) runs about 930 RPM at 90 SFM; a carbide drill about 3,110 RPM at 300 SFM. RPM = 3.82 × SFM ÷ D.

What feed for a letter U drill?

About 0.0093 in/rev in mild steel, 0.0084 in/rev in aluminum and 0.0061 in/rev in 304 stainless. At 930 RPM that is 8.65 IPM in steel (IPM = RPM × IPR).

What RPM for a letter U drill in aluminum?

2,590 RPM with HSS (250 SFM) or up to 6,230 RPM with carbide (600 SFM), feeding about 0.0084 in/rev.

Letter U drill size

A letter U drill is 0.3680 in (9.347 mm). It is the cut tap, 75% thread drill for 7/16-14.

Decimal inch: 0.3680
Millimeters: 9.347
Nearest 1/64: 3/8" (-0.0070)
Type: letter drill

Nearest equivalents in other systems

Drills closest to Letter U
System	Drill	Inch	mm	Difference
Fractional	3/8"	0.3750	9.525	+0.0070 in
Metric	9.3 mm	0.3661	9.300	-0.0019 in

Speeds and feeds: in mild steel an HSS drill starts at about 930 RPM and 0.0093 in/rev (8.65 IPM); in 6061 aluminum about 2,590 RPM.

Letter U drill speeds and feeds by material

Jobber-length twist drill, D = 0.3680 in. RPM = 3.82 × SFM ÷ D, rounded to 10. Feed per rev scales with diameter and material; IPM = RPM × IPR.

Letter U drilling speeds
Material	HSS SFM	HSS RPM	Carbide SFM	Carbide RPM	Feed (in/rev)	HSS IPM	Carbide IPM
Aluminum (6061)	250	2,590	600	6,230	0.0084	21.8	52.3
Brass / free-machining	150	1,560	400	4,150	0.0062	9.67	25.7
Low-carbon steel (1018)	90	930	300	3,110	0.0093	8.65	28.9
Alloy steel (4140, annealed)	70	730	250	2,590	0.0103	7.52	26.7
Stainless 304	40	420	150	1,560	0.0061	2.56	9.52
Gray cast iron	80	830	250	2,590	0.0064	5.31	16.6
Titanium 6Al-4V	25	260	100	1,040	0.0042	1.09	4.37
Plastics (acetal)	300	3,110	500	5,190	0.0049	15.2	25.4

Metric: Vc (m/min) = SFM × 0.3048; feed (mm/rev) = in/rev × 25.4. In steel, 930 RPM × 0.236 mm/rev = 220 mm/min.

Worked example: letter U drill in 1018 steel

HSS drill in low-carbon steel: 90 SFM.
RPM = 3.82 × 90 ÷ 0.3680 ≈ 930 RPM.
Feed for a 0.368 in drill in mild steel ≈ 0.0093 in/rev.
IPM = 930 × 0.0093 = 8.65 IPM.

Peck cycle by hole depth

Letter U drill: depth and canned cycle
Depth	Cycle	Peck	Note
3×D = 1.104 in (28 mm)	G81 (no peck)	None	Up to 3×D a sharp drill clears chips on its own.
5×D = 1.840 in (46.7 mm)	G73 chip-break	1 × D	Chip-break pecks; full retract if chips pack (soft aluminum, stainless).
8×D = 2.944 in (74.8 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 U drill

7/16-14 — cut tap, 75% thread · tap speeds and feeds

Nearest drills

9.3 mm: 0.3661 in / 9.300 mm (-0.0019 in)
9.4 mm: 0.3701 in / 9.400 mm (+0.0021 in)
9.2 mm: 0.3622 in / 9.200 mm (-0.0058 in)
9.5 mm: 0.3740 in / 9.500 mm (+0.0060 in)

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

Common questions

What is a letter U drill in decimal and mm?: 0.3680 in, or 9.347 mm; nearest fractional drill: 3/8" (0.3750 in, +0.0070 in); nearest metric drill: 9.3 mm (0.3661 in, -0.0019 in).
What RPM for a letter U drill in steel?: In low-carbon steel (1018) an HSS letter U drill (0.3680 in) runs about 930 RPM at 90 SFM; a carbide drill about 3,110 RPM at 300 SFM. RPM = 3.82 × SFM ÷ D.
What feed for a letter U drill?: About 0.0093 in/rev in mild steel, 0.0084 in/rev in aluminum and 0.0061 in/rev in 304 stainless. At 930 RPM that is 8.65 IPM in steel (IPM = RPM × IPR).
What RPM for a letter U drill in aluminum?: 2,590 RPM with HSS (250 SFM) or up to 6,230 RPM with carbide (600 SFM), feeding about 0.0084 in/rev.
What tap uses a letter U drill?: 7/16-14 (cut tap, 75% 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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