What is a letter H drill in decimal and mm?

0.2660 in, or 6.756 mm; nearest fractional drill: 17/64" (0.2656 in, -0.0004 in); nearest metric drill: 6.8 mm (0.2677 in, +0.0017 in).

What RPM for a letter H drill in steel?

In low-carbon steel (1018) an HSS letter H drill (0.2660 in) runs about 1,290 RPM at 90 SFM; a carbide drill about 4,310 RPM at 300 SFM. RPM = 3.82 × SFM ÷ D.

What feed for a letter H drill?

About 0.0069 in/rev in mild steel, 0.0063 in/rev in aluminum and 0.0045 in/rev in 304 stainless. At 1,290 RPM that is 8.90 IPM in steel (IPM = RPM × IPR).

What RPM for a letter H drill in aluminum?

3,590 RPM with HSS (250 SFM) or up to 8,620 RPM with carbide (600 SFM), feeding about 0.0063 in/rev.

What tap uses a letter H drill?

1/4-28 (Helicoil / STI insert (1/4-28×1D)).

Letter H drill size

A letter H drill is 0.2660 in (6.756 mm). It is the Helicoil / STI insert (1/4-28×1D) drill for 1/4-28.

Decimal inch: 0.2660
Millimeters: 6.756
Nearest 1/64: 17/64" (+0.0004)
Type: letter drill

Nearest equivalents in other systems

Drills closest to Letter H
System	Drill	Inch	mm	Difference
Fractional	17/64"	0.2656	6.747	-0.0004 in
Metric	6.8 mm	0.2677	6.800	+0.0017 in

Speeds and feeds: in mild steel an HSS drill starts at about 1,290 RPM and 0.0069 in/rev (8.90 IPM); in 6061 aluminum about 3,590 RPM.

Letter H drill speeds and feeds by material

Jobber-length twist drill, D = 0.2660 in. RPM = 3.82 × SFM ÷ D, rounded to 10. Feed per rev scales with diameter and material; IPM = RPM × IPR.

Letter H drilling speeds
Material	HSS SFM	HSS RPM	Carbide SFM	Carbide RPM	Feed (in/rev)	HSS IPM	Carbide IPM
Aluminum (6061)	250	3,590	600	8,620	0.0063	22.6	54.3
Brass / free-machining	150	2,150	400	5,740	0.0054	11.6	31.0
Low-carbon steel (1018)	90	1,290	300	4,310	0.0069	8.90	29.7
Alloy steel (4140, annealed)	70	1,010	250	3,590	0.0070	7.07	25.1
Stainless 304	40	570	150	2,150	0.0045	2.57	9.67
Gray cast iron	80	1,150	250	3,590	0.0048	5.52	17.2
Titanium 6Al-4V	25	360	100	1,440	0.0032	1.15	4.61
Plastics (acetal)	300	4,310	500	7,180	0.0041	17.7	29.4

Metric: Vc (m/min) = SFM × 0.3048; feed (mm/rev) = in/rev × 25.4. In steel, 1,290 RPM × 0.175 mm/rev = 226 mm/min.

Worked example: letter H drill in 1018 steel

HSS drill in low-carbon steel: 90 SFM.
RPM = 3.82 × 90 ÷ 0.2660 ≈ 1,290 RPM.
Feed for a 0.266 in drill in mild steel ≈ 0.0069 in/rev.
IPM = 1,290 × 0.0069 = 8.90 IPM.

Peck cycle by hole depth

Letter H drill: depth and canned cycle
Depth	Cycle	Peck	Note
3×D = 0.798 in (20.3 mm)	G81 (no peck)	None	Up to 3×D a sharp drill clears chips on its own.
5×D = 1.330 in (33.8 mm)	G73 chip-break	1 × D	Chip-break pecks; full retract if chips pack (soft aluminum, stainless).
8×D = 2.128 in (54 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 H drill

1/4-28 — Helicoil / STI insert (1/4-28×1D) · tap speeds and feeds

Nearest drills

17/64": 0.2656 in / 6.747 mm (-0.0004 in)
6.8 mm: 0.2677 in / 6.800 mm (+0.0017 in)
6.7 mm: 0.2638 in / 6.700 mm (-0.0022 in)
Letter G: 0.2610 in / 6.629 mm (-0.0050 in)

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

Common questions

What is a letter H drill in decimal and mm?: 0.2660 in, or 6.756 mm; nearest fractional drill: 17/64" (0.2656 in, -0.0004 in); nearest metric drill: 6.8 mm (0.2677 in, +0.0017 in).
What RPM for a letter H drill in steel?: In low-carbon steel (1018) an HSS letter H drill (0.2660 in) runs about 1,290 RPM at 90 SFM; a carbide drill about 4,310 RPM at 300 SFM. RPM = 3.82 × SFM ÷ D.
What feed for a letter H drill?: About 0.0069 in/rev in mild steel, 0.0063 in/rev in aluminum and 0.0045 in/rev in 304 stainless. At 1,290 RPM that is 8.90 IPM in steel (IPM = RPM × IPR).
What RPM for a letter H drill in aluminum?: 3,590 RPM with HSS (250 SFM) or up to 8,620 RPM with carbide (600 SFM), feeding about 0.0063 in/rev.
What tap uses a letter H drill?: 1/4-28 (Helicoil / STI insert (1/4-28×1D)).

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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