What is a letter Q drill in decimal and mm?

0.3320 in, or 8.433 mm; nearest fractional drill: 21/64" (0.3281 in, -0.0039 in); nearest metric drill: 8.4 mm (0.3307 in, -0.0013 in).

What RPM for a letter Q drill in steel?

In low-carbon steel (1018) an HSS letter Q drill (0.3320 in) runs about 1,040 RPM at 90 SFM; a carbide drill about 3,450 RPM at 300 SFM. RPM = 3.82 × SFM ÷ D.

What feed for a letter Q drill?

About 0.0085 in/rev in mild steel, 0.0076 in/rev in aluminum and 0.0056 in/rev in 304 stainless. At 1,040 RPM that is 8.84 IPM in steel (IPM = RPM × IPR).

What RPM for a letter Q drill in aluminum?

2,880 RPM with HSS (250 SFM) or up to 6,900 RPM with carbide (600 SFM), feeding about 0.0076 in/rev.

What tap uses a letter Q drill?

3/8-24 (cut tap, 75% thread); 3/8-16 (cut tap, 50% thread); 5/16-18 (Helicoil / STI insert (5/16-18×1D)).

Letter Q drill size

A letter Q drill is 0.3320 in (8.433 mm). It is the cut tap, 75% thread drill for 3/8-24.

Decimal inch: 0.3320
Millimeters: 8.433
Nearest 1/64: 21/64" (+0.0039)
Type: letter drill

Nearest equivalents in other systems

Drills closest to Letter Q
System	Drill	Inch	mm	Difference
Fractional	21/64"	0.3281	8.334	-0.0039 in
Metric	8.4 mm	0.3307	8.400	-0.0013 in

Speeds and feeds: in mild steel an HSS drill starts at about 1,040 RPM and 0.0085 in/rev (8.84 IPM); in 6061 aluminum about 2,880 RPM.

Letter Q drill speeds and feeds by material

Jobber-length twist drill, D = 0.3320 in. RPM = 3.82 × SFM ÷ D, rounded to 10. Feed per rev scales with diameter and material; IPM = RPM × IPR.

Letter Q drilling speeds
Material	HSS SFM	HSS RPM	Carbide SFM	Carbide RPM	Feed (in/rev)	HSS IPM	Carbide IPM
Aluminum (6061)	250	2,880	600	6,900	0.0076	21.9	52.4
Brass / free-machining	150	1,730	400	4,600	0.0059	10.2	27.1
Low-carbon steel (1018)	90	1,040	300	3,450	0.0085	8.84	29.3
Alloy steel (4140, annealed)	70	810	250	2,880	0.0091	7.37	26.2
Stainless 304	40	460	150	1,730	0.0056	2.58	9.69
Gray cast iron	80	920	250	2,880	0.0058	5.34	16.7
Titanium 6Al-4V	25	290	100	1,150	0.0038	1.10	4.37
Plastics (acetal)	300	3,450	500	5,750	0.0047	16.2	27.0

Metric: Vc (m/min) = SFM × 0.3048; feed (mm/rev) = in/rev × 25.4. In steel, 1,040 RPM × 0.216 mm/rev = 225 mm/min.

Worked example: letter Q drill in 1018 steel

HSS drill in low-carbon steel: 90 SFM.
RPM = 3.82 × 90 ÷ 0.3320 ≈ 1,040 RPM.
Feed for a 0.332 in drill in mild steel ≈ 0.0085 in/rev.
IPM = 1,040 × 0.0085 = 8.84 IPM.

Peck cycle by hole depth

Letter Q drill: depth and canned cycle
Depth	Cycle	Peck	Note
3×D = 0.996 in (25.3 mm)	G81 (no peck)	None	Up to 3×D a sharp drill clears chips on its own.
5×D = 1.660 in (42.2 mm)	G73 chip-break	1 × D	Chip-break pecks; full retract if chips pack (soft aluminum, stainless).
8×D = 2.656 in (67.5 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 Q drill

3/8-24 — cut tap, 75% thread · tap speeds and feeds
3/8-16 — cut tap, 50% thread · tap speeds and feeds
5/16-18 — Helicoil / STI insert (5/16-18×1D) · tap speeds and feeds

Nearest drills

8.4 mm: 0.3307 in / 8.400 mm (-0.0013 in)
8.5 mm: 0.3347 in / 8.500 mm (+0.0027 in)
21/64": 0.3281 in / 8.334 mm (-0.0039 in)
8.3 mm: 0.3268 in / 8.300 mm (-0.0052 in)

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

Common questions

What is a letter Q drill in decimal and mm?: 0.3320 in, or 8.433 mm; nearest fractional drill: 21/64" (0.3281 in, -0.0039 in); nearest metric drill: 8.4 mm (0.3307 in, -0.0013 in).
What RPM for a letter Q drill in steel?: In low-carbon steel (1018) an HSS letter Q drill (0.3320 in) runs about 1,040 RPM at 90 SFM; a carbide drill about 3,450 RPM at 300 SFM. RPM = 3.82 × SFM ÷ D.
What feed for a letter Q drill?: About 0.0085 in/rev in mild steel, 0.0076 in/rev in aluminum and 0.0056 in/rev in 304 stainless. At 1,040 RPM that is 8.84 IPM in steel (IPM = RPM × IPR).
What RPM for a letter Q drill in aluminum?: 2,880 RPM with HSS (250 SFM) or up to 6,900 RPM with carbide (600 SFM), feeding about 0.0076 in/rev.
What tap uses a letter Q drill?: 3/8-24 (cut tap, 75% thread); 3/8-16 (cut tap, 50% thread); 5/16-18 (Helicoil / STI insert (5/16-18×1D)).

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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