What is a letter X drill in decimal and mm?

0.3970 in, or 10.084 mm; nearest fractional drill: 25/64" (0.3906 in, -0.0064 in); nearest metric drill: 10.1 mm (0.3976 in, +0.0006 in).

What RPM for a letter X drill in steel?

In low-carbon steel (1018) an HSS letter X drill (0.3970 in) runs about 870 RPM at 90 SFM; a carbide drill about 2,890 RPM at 300 SFM. RPM = 3.82 × SFM ÷ D.

What feed for a letter X drill?

About 0.0097 in/rev in mild steel, 0.0088 in/rev in aluminum and 0.0066 in/rev in 304 stainless. At 870 RPM that is 8.44 IPM in steel (IPM = RPM × IPR).

What RPM for a letter X drill in aluminum?

2,410 RPM with HSS (250 SFM) or up to 5,770 RPM with carbide (600 SFM), feeding about 0.0088 in/rev.

What size is a letter X drill?

0.3970 in / 10.08 mm. The nearest other drills are 10.1 mm (0.3976 in), 10.0 mm (0.3937 in), 10.2 mm (0.4016 in).

Letter X drill size

A letter X drill is 0.3970 in (10.084 mm).

Decimal inch: 0.3970
Millimeters: 10.084
Nearest 1/64: 25/64" (+0.0064)
Type: letter drill

Nearest equivalents in other systems

Drills closest to Letter X
System	Drill	Inch	mm	Difference
Fractional	25/64"	0.3906	9.922	-0.0064 in
Metric	10.1 mm	0.3976	10.100	+0.0006 in

Speeds and feeds: in mild steel an HSS drill starts at about 870 RPM and 0.0097 in/rev (8.44 IPM); in 6061 aluminum about 2,410 RPM.

Letter X drill speeds and feeds by material

Jobber-length twist drill, D = 0.3970 in. RPM = 3.82 × SFM ÷ D, rounded to 10. Feed per rev scales with diameter and material; IPM = RPM × IPR.

Letter X drilling speeds
Material	HSS SFM	HSS RPM	Carbide SFM	Carbide RPM	Feed (in/rev)	HSS IPM	Carbide IPM
Aluminum (6061)	250	2,410	600	5,770	0.0088	21.2	50.8
Brass / free-machining	150	1,440	400	3,850	0.0066	9.50	25.4
Low-carbon steel (1018)	90	870	300	2,890	0.0097	8.44	28.0
Alloy steel (4140, annealed)	70	670	250	2,410	0.0107	7.17	25.8
Stainless 304	40	380	150	1,440	0.0066	2.51	9.50
Gray cast iron	80	770	250	2,410	0.0069	5.31	16.6
Titanium 6Al-4V	25	240	100	960	0.0044	1.06	4.22
Plastics (acetal)	300	2,890	500	4,810	0.0052	15.0	25.0

Metric: Vc (m/min) = SFM × 0.3048; feed (mm/rev) = in/rev × 25.4. In steel, 870 RPM × 0.246 mm/rev = 214 mm/min.

Worked example: letter X drill in 1018 steel

HSS drill in low-carbon steel: 90 SFM.
RPM = 3.82 × 90 ÷ 0.3970 ≈ 870 RPM.
Feed for a 0.397 in drill in mild steel ≈ 0.0097 in/rev.
IPM = 870 × 0.0097 = 8.44 IPM.

Peck cycle by hole depth

Letter X drill: depth and canned cycle
Depth	Cycle	Peck	Note
3×D = 1.191 in (30.3 mm)	G81 (no peck)	None	Up to 3×D a sharp drill clears chips on its own.
5×D = 1.985 in (50.4 mm)	G73 chip-break	1 × D	Chip-break pecks; full retract if chips pack (soft aluminum, stainless).
8×D = 3.176 in (80.7 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 X drill

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

Nearest drills

10.1 mm: 0.3976 in / 10.100 mm (+0.0006 in)
10.0 mm: 0.3937 in / 10.000 mm (-0.0033 in)
10.2 mm: 0.4016 in / 10.200 mm (+0.0046 in)
25/64": 0.3906 in / 9.922 mm (-0.0064 in)

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

Common questions

What is a letter X drill in decimal and mm?: 0.3970 in, or 10.084 mm; nearest fractional drill: 25/64" (0.3906 in, -0.0064 in); nearest metric drill: 10.1 mm (0.3976 in, +0.0006 in).
What RPM for a letter X drill in steel?: In low-carbon steel (1018) an HSS letter X drill (0.3970 in) runs about 870 RPM at 90 SFM; a carbide drill about 2,890 RPM at 300 SFM. RPM = 3.82 × SFM ÷ D.
What feed for a letter X drill?: About 0.0097 in/rev in mild steel, 0.0088 in/rev in aluminum and 0.0066 in/rev in 304 stainless. At 870 RPM that is 8.44 IPM in steel (IPM = RPM × IPR).
What RPM for a letter X drill in aluminum?: 2,410 RPM with HSS (250 SFM) or up to 5,770 RPM with carbide (600 SFM), feeding about 0.0088 in/rev.
What size is a letter X drill?: 0.3970 in / 10.08 mm. The nearest other drills are 10.1 mm (0.3976 in), 10.0 mm (0.3937 in), 10.2 mm (0.4016 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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