What is a letter T drill in decimal and mm?

0.3580 in, or 9.093 mm; nearest fractional drill: 23/64" (0.3594 in, +0.0014 in); nearest metric drill: 9.1 mm (0.3583 in, +0.0003 in).

What RPM for a letter T drill in steel?

In low-carbon steel (1018) an HSS letter T drill (0.3580 in) runs about 960 RPM at 90 SFM; a carbide drill about 3,200 RPM at 300 SFM. RPM = 3.82 × SFM ÷ D.

What feed for a letter T drill?

About 0.0091 in/rev in mild steel, 0.0082 in/rev in aluminum and 0.0060 in/rev in 304 stainless. At 960 RPM that is 8.74 IPM in steel (IPM = RPM × IPR).

What RPM for a letter T drill in aluminum?

2,670 RPM with HSS (250 SFM) or up to 6,400 RPM with carbide (600 SFM), feeding about 0.0082 in/rev.

What tap uses a letter T drill?

3/8-24 (form (roll) tap, 65% thread); 3/8-32 (cut tap, 50% thread).

Letter T drill size

A letter T drill is 0.3580 in (9.093 mm). It is the form (roll) tap, 65% thread drill for 3/8-24.

Decimal inch: 0.3580
Millimeters: 9.093
Nearest 1/64: 23/64" (-0.0014)
Type: letter drill

Nearest equivalents in other systems

Drills closest to Letter T
System	Drill	Inch	mm	Difference
Fractional	23/64"	0.3594	9.128	+0.0014 in
Metric	9.1 mm	0.3583	9.100	+0.0003 in

Speeds and feeds: in mild steel an HSS drill starts at about 960 RPM and 0.0091 in/rev (8.74 IPM); in 6061 aluminum about 2,670 RPM.

Letter T drill speeds and feeds by material

Jobber-length twist drill, D = 0.3580 in. RPM = 3.82 × SFM ÷ D, rounded to 10. Feed per rev scales with diameter and material; IPM = RPM × IPR.

Letter T drilling speeds
Material	HSS SFM	HSS RPM	Carbide SFM	Carbide RPM	Feed (in/rev)	HSS IPM	Carbide IPM
Aluminum (6061)	250	2,670	600	6,400	0.0082	21.9	52.5
Brass / free-machining	150	1,600	400	4,270	0.0061	9.76	26.1
Low-carbon steel (1018)	90	960	300	3,200	0.0091	8.74	29.1
Alloy steel (4140, annealed)	70	750	250	2,670	0.0100	7.50	26.7
Stainless 304	40	430	150	1,600	0.0060	2.58	9.60
Gray cast iron	80	850	250	2,670	0.0062	5.27	16.6
Titanium 6Al-4V	25	270	100	1,070	0.0041	1.11	4.39
Plastics (acetal)	300	3,200	500	5,330	0.0049	15.7	26.1

Metric: Vc (m/min) = SFM × 0.3048; feed (mm/rev) = in/rev × 25.4. In steel, 960 RPM × 0.231 mm/rev = 222 mm/min.

Worked example: letter T drill in 1018 steel

HSS drill in low-carbon steel: 90 SFM.
RPM = 3.82 × 90 ÷ 0.3580 ≈ 960 RPM.
Feed for a 0.358 in drill in mild steel ≈ 0.0091 in/rev.
IPM = 960 × 0.0091 = 8.74 IPM.

Peck cycle by hole depth

Letter T drill: depth and canned cycle
Depth	Cycle	Peck	Note
3×D = 1.074 in (27.3 mm)	G81 (no peck)	None	Up to 3×D a sharp drill clears chips on its own.
5×D = 1.790 in (45.5 mm)	G73 chip-break	1 × D	Chip-break pecks; full retract if chips pack (soft aluminum, stainless).
8×D = 2.864 in (72.7 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 T drill

3/8-24 — form (roll) tap, 65% thread · tap speeds and feeds
3/8-32 — cut tap, 50% thread

Nearest drills

9.1 mm: 0.3583 in / 9.100 mm (+0.0003 in)
23/64": 0.3594 in / 9.128 mm (+0.0014 in)
9.0 mm: 0.3543 in / 9.000 mm (-0.0037 in)
9.2 mm: 0.3622 in / 9.200 mm (+0.0042 in)

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

Common questions

What is a letter T drill in decimal and mm?: 0.3580 in, or 9.093 mm; nearest fractional drill: 23/64" (0.3594 in, +0.0014 in); nearest metric drill: 9.1 mm (0.3583 in, +0.0003 in).
What RPM for a letter T drill in steel?: In low-carbon steel (1018) an HSS letter T drill (0.3580 in) runs about 960 RPM at 90 SFM; a carbide drill about 3,200 RPM at 300 SFM. RPM = 3.82 × SFM ÷ D.
What feed for a letter T drill?: About 0.0091 in/rev in mild steel, 0.0082 in/rev in aluminum and 0.0060 in/rev in 304 stainless. At 960 RPM that is 8.74 IPM in steel (IPM = RPM × IPR).
What RPM for a letter T drill in aluminum?: 2,670 RPM with HSS (250 SFM) or up to 6,400 RPM with carbide (600 SFM), feeding about 0.0082 in/rev.
What tap uses a letter T drill?: 3/8-24 (form (roll) tap, 65% thread); 3/8-32 (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.

Free for machinists & programmers

Put your skills on a free talent profile

Machines, controls, certs and saved calculations in one profile shops can find.

Create free profile

For your shop

Bring shift handoffs to your shop

Stop losing job status between shifts. Every station's notes, counts and issues, handed off cleanly.

See shift handoffs