What is a letter Y drill in decimal and mm?

0.4040 in, or 10.262 mm; nearest fractional drill: 13/32" (0.4063 in, +0.0022 in); nearest metric drill: 10.3 mm (0.4055 in, +0.0015 in).

What RPM for a letter Y drill in steel?

In low-carbon steel (1018) an HSS letter Y drill (0.4040 in) runs about 850 RPM at 90 SFM; a carbide drill about 2,840 RPM at 300 SFM. RPM = 3.82 × SFM ÷ D.

What feed for a letter Y drill?

About 0.0097 in/rev in mild steel, 0.0088 in/rev in aluminum and 0.0068 in/rev in 304 stainless. At 850 RPM that is 8.25 IPM in steel (IPM = RPM × IPR).

What RPM for a letter Y drill in aluminum?

2,360 RPM with HSS (250 SFM) or up to 5,670 RPM with carbide (600 SFM), feeding about 0.0088 in/rev.

What tap uses a letter Y drill?

7/16-28 (cut tap, 75% thread); 7/16-20 (cut tap, 50% thread).

Letter Y drill size

A letter Y drill is 0.4040 in (10.262 mm). It is the cut tap, 75% thread drill for 7/16-28.

Decimal inch: 0.4040
Millimeters: 10.262
Nearest 1/64: 13/32" (-0.0022)
Type: letter drill

Nearest equivalents in other systems

Drills closest to Letter Y
System	Drill	Inch	mm	Difference
Fractional	13/32"	0.4063	10.319	+0.0022 in
Metric	10.3 mm	0.4055	10.300	+0.0015 in

Speeds and feeds: in mild steel an HSS drill starts at about 850 RPM and 0.0097 in/rev (8.25 IPM); in 6061 aluminum about 2,360 RPM.

Letter Y drill speeds and feeds by material

Jobber-length twist drill, D = 0.4040 in. RPM = 3.82 × SFM ÷ D, rounded to 10. Feed per rev scales with diameter and material; IPM = RPM × IPR.

Letter Y drilling speeds
Material	HSS SFM	HSS RPM	Carbide SFM	Carbide RPM	Feed (in/rev)	HSS IPM	Carbide IPM
Aluminum (6061)	250	2,360	600	5,670	0.0088	20.8	49.9
Brass / free-machining	150	1,420	400	3,780	0.0067	9.51	25.3
Low-carbon steel (1018)	90	850	300	2,840	0.0097	8.25	27.6
Alloy steel (4140, annealed)	70	660	250	2,360	0.0107	7.06	25.3
Stainless 304	40	380	150	1,420	0.0068	2.58	9.66
Gray cast iron	80	760	250	2,360	0.0070	5.32	16.5
Titanium 6Al-4V	25	240	100	950	0.0044	1.06	4.18
Plastics (acetal)	300	2,840	500	4,730	0.0052	14.8	24.6

Metric: Vc (m/min) = SFM × 0.3048; feed (mm/rev) = in/rev × 25.4. In steel, 850 RPM × 0.246 mm/rev = 209 mm/min.

Worked example: letter Y drill in 1018 steel

HSS drill in low-carbon steel: 90 SFM.
RPM = 3.82 × 90 ÷ 0.4040 ≈ 850 RPM.
Feed for a 0.404 in drill in mild steel ≈ 0.0097 in/rev.
IPM = 850 × 0.0097 = 8.25 IPM.

Peck cycle by hole depth

Letter Y drill: depth and canned cycle
Depth	Cycle	Peck	Note
3×D = 1.212 in (30.8 mm)	G81 (no peck)	None	Up to 3×D a sharp drill clears chips on its own.
5×D = 2.020 in (51.3 mm)	G73 chip-break	1 × D	Chip-break pecks; full retract if chips pack (soft aluminum, stainless).
8×D = 3.232 in (82.1 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 Y drill

7/16-28 — cut tap, 75% thread
7/16-20 — cut tap, 50% thread · tap speeds and feeds

Nearest drills

10.3 mm: 0.4055 in / 10.300 mm (+0.0015 in)
13/32": 0.4063 in / 10.319 mm (+0.0022 in)
10.2 mm: 0.4016 in / 10.200 mm (-0.0024 in)
10.4 mm: 0.4094 in / 10.400 mm (+0.0054 in)

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

Common questions

What is a letter Y drill in decimal and mm?: 0.4040 in, or 10.262 mm; nearest fractional drill: 13/32" (0.4063 in, +0.0022 in); nearest metric drill: 10.3 mm (0.4055 in, +0.0015 in).
What RPM for a letter Y drill in steel?: In low-carbon steel (1018) an HSS letter Y drill (0.4040 in) runs about 850 RPM at 90 SFM; a carbide drill about 2,840 RPM at 300 SFM. RPM = 3.82 × SFM ÷ D.
What feed for a letter Y drill?: About 0.0097 in/rev in mild steel, 0.0088 in/rev in aluminum and 0.0068 in/rev in 304 stainless. At 850 RPM that is 8.25 IPM in steel (IPM = RPM × IPR).
What RPM for a letter Y drill in aluminum?: 2,360 RPM with HSS (250 SFM) or up to 5,670 RPM with carbide (600 SFM), feeding about 0.0088 in/rev.
What tap uses a letter Y drill?: 7/16-28 (cut tap, 75% thread); 7/16-20 (cut tap, 50% 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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