What is a 41/64" drill in decimal and mm?

0.6406 in, or 16.272 mm; nearest number / letter drill: Letter Z (0.4130 in, -0.2276 in); nearest metric drill: 16.25 mm (0.6398 in, -0.0009 in).

What RPM for a 41/64" drill in steel?

In low-carbon steel (1018) an HSS 41/64" drill (0.6406 in) runs about 540 RPM at 90 SFM; a carbide drill about 1,790 RPM at 300 SFM. RPM = 3.82 × SFM ÷ D.

What feed for a 41/64" drill?

About 0.0111 in/rev in mild steel, 0.0106 in/rev in aluminum and 0.0091 in/rev in 304 stainless. At 540 RPM that is 5.99 IPM in steel (IPM = RPM × IPR).

What RPM for a 41/64" drill in aluminum?

1,490 RPM with HSS (250 SFM) or up to 3,580 RPM with carbide (600 SFM), feeding about 0.0106 in/rev.

What tap uses a 41/64" drill?

11/16-24 (cut tap, 75% thread); 5/8-11 (Helicoil / STI insert (5/8-11×1D)).

41/64" drill size

A 41/64" fractional drill is 0.6406 in (16.272 mm). It is the cut tap, 75% thread drill for 11/16-24.

Decimal inch: 0.6406
Millimeters: 16.272
Nearest 1/64: 41/64" (+0.0000)
Type: fractional drill

Nearest equivalents in other systems

Drills closest to 41/64"
System	Drill	Inch	mm	Difference
Number / letter	Letter Z	0.4130	10.490	-0.2276 in
Metric	16.25 mm	0.6398	16.250	-0.0009 in

Speeds and feeds: in mild steel an HSS drill starts at about 540 RPM and 0.0111 in/rev (5.99 IPM); in 6061 aluminum about 1,490 RPM.

41/64" drill speeds and feeds by material

Jobber-length twist drill, D = 0.6406 in. RPM = 3.82 × SFM ÷ D, rounded to 10. Feed per rev scales with diameter and material; IPM = RPM × IPR.

41/64" drilling speeds
Material	HSS SFM	HSS RPM	Carbide SFM	Carbide RPM	Feed (in/rev)	HSS IPM	Carbide IPM
Aluminum (6061)	250	1,490	600	3,580	0.0106	15.8	38.0
Brass / free-machining	150	890	400	2,380	0.0087	7.74	20.7
Low-carbon steel (1018)	90	540	300	1,790	0.0111	5.99	19.9
Alloy steel (4140, annealed)	70	420	250	1,490	0.0118	4.96	17.6
Stainless 304	40	240	150	890	0.0091	2.18	8.10
Gray cast iron	80	480	250	1,490	0.0093	4.46	13.9
Titanium 6Al-4V	25	150	100	600	0.0053	0.80	3.18
Plastics (acetal)	300	1,790	500	2,980	0.0066	11.8	19.7

Metric: Vc (m/min) = SFM × 0.3048; feed (mm/rev) = in/rev × 25.4. In steel, 540 RPM × 0.282 mm/rev = 152 mm/min.

Worked example: 41/64" drill in 1018 steel

HSS drill in low-carbon steel: 90 SFM.
RPM = 3.82 × 90 ÷ 0.6406 ≈ 540 RPM.
Feed for a 0.641 in drill in mild steel ≈ 0.0111 in/rev.
IPM = 540 × 0.0111 = 5.99 IPM.

Peck cycle by hole depth

41/64" drill: depth and canned cycle
Depth	Cycle	Peck	Note
3×D = 1.922 in (48.8 mm)	G81 (no peck)	None	Up to 3×D a sharp drill clears chips on its own.
5×D = 3.203 in (81.4 mm)	G73 chip-break	1 × D	Chip-break pecks; full retract if chips pack (soft aluminum, stainless).
8×D = 5.125 in (130.2 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 41/64" drill

11/16-24 — cut tap, 75% thread
5/8-11 — Helicoil / STI insert (5/8-11×1D) · tap speeds and feeds

Nearest drills

16.25 mm: 0.6398 in / 16.250 mm (-0.0009 in)
16.5 mm: 0.6496 in / 16.500 mm (+0.0090 in)
16.0 mm: 0.6299 in / 16.000 mm (-0.0107 in)
5/8": 0.6250 in / 15.875 mm (-0.0156 in)

Drill speed calculator, prefilled for 41/64"Drill finder: sizes near 41/64"Fractional drill chart Decimal equivalent chart

Common questions

What is a 41/64" drill in decimal and mm?: 0.6406 in, or 16.272 mm; nearest number / letter drill: Letter Z (0.4130 in, -0.2276 in); nearest metric drill: 16.25 mm (0.6398 in, -0.0009 in).
What RPM for a 41/64" drill in steel?: In low-carbon steel (1018) an HSS 41/64" drill (0.6406 in) runs about 540 RPM at 90 SFM; a carbide drill about 1,790 RPM at 300 SFM. RPM = 3.82 × SFM ÷ D.
What feed for a 41/64" drill?: About 0.0111 in/rev in mild steel, 0.0106 in/rev in aluminum and 0.0091 in/rev in 304 stainless. At 540 RPM that is 5.99 IPM in steel (IPM = RPM × IPR).
What RPM for a 41/64" drill in aluminum?: 1,490 RPM with HSS (250 SFM) or up to 3,580 RPM with carbide (600 SFM), feeding about 0.0106 in/rev.
What tap uses a 41/64" drill?: 11/16-24 (cut tap, 75% thread); 5/8-11 (Helicoil / STI insert (5/8-11×1D)).

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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