What is a 53/64" drill in decimal and mm?

0.8281 in, or 21.034 mm; nearest number / letter drill: Letter Z (0.4130 in, -0.4151 in); nearest metric drill: 21.0 mm (0.8268 in, -0.0014 in).

What RPM for a 53/64" drill in steel?

In low-carbon steel (1018) an HSS 53/64" drill (0.8281 in) runs about 420 RPM at 90 SFM; a carbide drill about 1,380 RPM at 300 SFM. RPM = 3.82 × SFM ÷ D.

What feed for a 53/64" drill?

About 0.0118 in/rev in mild steel, 0.0116 in/rev in aluminum and 0.0098 in/rev in 304 stainless. At 420 RPM that is 4.96 IPM in steel (IPM = RPM × IPR).

What RPM for a 53/64" drill in aluminum?

1,150 RPM with HSS (250 SFM) or up to 2,770 RPM with carbide (600 SFM), feeding about 0.0116 in/rev.

What tap uses a 53/64" drill?

7/8-20 (cut tap, 75% thread); 7/8-9 (form (roll) tap, 65% thread); 7/8-14 (cut tap, 50% thread); 3/4-16 (Helicoil / STI insert (3/4-16×1D)).

53/64" drill size

A 53/64" fractional drill is 0.8281 in (21.034 mm). It is the cut tap, 75% thread drill for 7/8-20.

Decimal inch: 0.8281
Millimeters: 21.034
Nearest 1/64: 53/64" (+0.0000)
Type: fractional drill

Nearest equivalents in other systems

Drills closest to 53/64"
System	Drill	Inch	mm	Difference
Number / letter	Letter Z	0.4130	10.490	-0.4151 in
Metric	21.0 mm	0.8268	21.000	-0.0014 in

Speeds and feeds: in mild steel an HSS drill starts at about 420 RPM and 0.0118 in/rev (4.96 IPM); in 6061 aluminum about 1,150 RPM.

53/64" drill speeds and feeds by material

Jobber-length twist drill, D = 0.8281 in. RPM = 3.82 × SFM ÷ D, rounded to 10. Feed per rev scales with diameter and material; IPM = RPM × IPR.

53/64" drilling speeds
Material	HSS SFM	HSS RPM	Carbide SFM	Carbide RPM	Feed (in/rev)	HSS IPM	Carbide IPM
Aluminum (6061)	250	1,150	600	2,770	0.0116	13.3	32.1
Brass / free-machining	150	690	400	1,840	0.0093	6.42	17.1
Low-carbon steel (1018)	90	420	300	1,380	0.0118	4.96	16.3
Alloy steel (4140, annealed)	70	320	250	1,150	0.0123	3.94	14.2
Stainless 304	40	180	150	690	0.0098	1.76	6.76
Gray cast iron	80	370	250	1,150	0.0103	3.81	11.8
Titanium 6Al-4V	25	120	100	460	0.0057	0.68	2.62
Plastics (acetal)	300	1,380	500	2,310	0.0073	10.1	16.9

Metric: Vc (m/min) = SFM × 0.3048; feed (mm/rev) = in/rev × 25.4. In steel, 420 RPM × 0.300 mm/rev = 126 mm/min.

Worked example: 53/64" drill in 1018 steel

HSS drill in low-carbon steel: 90 SFM.
RPM = 3.82 × 90 ÷ 0.8281 ≈ 420 RPM.
Feed for a 0.828 in drill in mild steel ≈ 0.0118 in/rev.
IPM = 420 × 0.0118 = 4.96 IPM.

Peck cycle by hole depth

53/64" drill: depth and canned cycle
Depth	Cycle	Peck	Note
3×D = 2.484 in (63.1 mm)	G81 (no peck)	None	Up to 3×D a sharp drill clears chips on its own.
5×D = 4.141 in (105.2 mm)	G73 chip-break	1 × D	Chip-break pecks; full retract if chips pack (soft aluminum, stainless).
8×D = 6.625 in (168.3 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 53/64" drill

7/8-20 — cut tap, 75% thread
7/8-9 — form (roll) tap, 65% thread · tap speeds and feeds
7/8-14 — cut tap, 50% thread · tap speeds and feeds
3/4-16 — Helicoil / STI insert (3/4-16×1D) · tap speeds and feeds

Nearest drills

21.0 mm: 0.8268 in / 21.000 mm (-0.0014 in)
21.25 mm: 0.8366 in / 21.250 mm (+0.0085 in)
20.75 mm: 0.8169 in / 20.750 mm (-0.0112 in)
13/16": 0.8125 in / 20.638 mm (-0.0156 in)

Drill speed calculator, prefilled for 53/64"Drill finder: sizes near 53/64"Fractional drill chart Decimal equivalent chart

Common questions

What is a 53/64" drill in decimal and mm?: 0.8281 in, or 21.034 mm; nearest number / letter drill: Letter Z (0.4130 in, -0.4151 in); nearest metric drill: 21.0 mm (0.8268 in, -0.0014 in).
What RPM for a 53/64" drill in steel?: In low-carbon steel (1018) an HSS 53/64" drill (0.8281 in) runs about 420 RPM at 90 SFM; a carbide drill about 1,380 RPM at 300 SFM. RPM = 3.82 × SFM ÷ D.
What feed for a 53/64" drill?: About 0.0118 in/rev in mild steel, 0.0116 in/rev in aluminum and 0.0098 in/rev in 304 stainless. At 420 RPM that is 4.96 IPM in steel (IPM = RPM × IPR).
What RPM for a 53/64" drill in aluminum?: 1,150 RPM with HSS (250 SFM) or up to 2,770 RPM with carbide (600 SFM), feeding about 0.0116 in/rev.
What tap uses a 53/64" drill?: 7/8-20 (cut tap, 75% thread); 7/8-9 (form (roll) tap, 65% thread); 7/8-14 (cut tap, 50% thread); 3/4-16 (Helicoil / STI insert (3/4-16×1D)).

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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