What is a 55/64" drill in decimal and mm?

0.8594 in, or 21.828 mm; nearest number / letter drill: Letter Z (0.4130 in, -0.4464 in); nearest metric drill: 21.75 mm (0.8563 in, -0.0031 in).

What RPM for a 55/64" drill in steel?

In low-carbon steel (1018) an HSS 55/64" drill (0.8594 in) runs about 400 RPM at 90 SFM; a carbide drill about 1,330 RPM at 300 SFM. RPM = 3.82 × SFM ÷ D.

What feed for a 55/64" drill?

About 0.0119 in/rev in mild steel, 0.0119 in/rev in aluminum and 0.0099 in/rev in 304 stainless. At 400 RPM that is 4.76 IPM in steel (IPM = RPM × IPR).

What RPM for a 55/64" drill in aluminum?

1,110 RPM with HSS (250 SFM) or up to 2,670 RPM with carbide (600 SFM), feeding about 0.0119 in/rev.

What tap uses a 55/64" drill?

7/8-20 (form (roll) tap, 65% thread).

55/64" drill size

A 55/64" fractional drill is 0.8594 in (21.828 mm). It is the form (roll) tap, 65% thread drill for 7/8-20.

Decimal inch: 0.8594
Millimeters: 21.828
Nearest 1/64: 55/64" (+0.0000)
Type: fractional drill

Nearest equivalents in other systems

Drills closest to 55/64"
System	Drill	Inch	mm	Difference
Number / letter	Letter Z	0.4130	10.490	-0.4464 in
Metric	21.75 mm	0.8563	21.750	-0.0031 in

Speeds and feeds: in mild steel an HSS drill starts at about 400 RPM and 0.0119 in/rev (4.76 IPM); in 6061 aluminum about 1,110 RPM.

55/64" drill speeds and feeds by material

Jobber-length twist drill, D = 0.8594 in. RPM = 3.82 × SFM ÷ D, rounded to 10. Feed per rev scales with diameter and material; IPM = RPM × IPR.

55/64" drilling speeds
Material	HSS SFM	HSS RPM	Carbide SFM	Carbide RPM	Feed (in/rev)	HSS IPM	Carbide IPM
Aluminum (6061)	250	1,110	600	2,670	0.0119	13.2	31.8
Brass / free-machining	150	670	400	1,780	0.0094	6.30	16.7
Low-carbon steel (1018)	90	400	300	1,330	0.0119	4.76	15.8
Alloy steel (4140, annealed)	70	310	250	1,110	0.0124	3.84	13.8
Stainless 304	40	180	150	670	0.0099	1.78	6.63
Gray cast iron	80	360	250	1,110	0.0104	3.74	11.5
Titanium 6Al-4V	25	110	100	440	0.0057	0.63	2.51
Plastics (acetal)	300	1,330	500	2,220	0.0074	9.84	16.4

Metric: Vc (m/min) = SFM × 0.3048; feed (mm/rev) = in/rev × 25.4. In steel, 400 RPM × 0.302 mm/rev = 121 mm/min.

Worked example: 55/64" drill in 1018 steel

HSS drill in low-carbon steel: 90 SFM.
RPM = 3.82 × 90 ÷ 0.8594 ≈ 400 RPM.
Feed for a 0.859 in drill in mild steel ≈ 0.0119 in/rev.
IPM = 400 × 0.0119 = 4.76 IPM.

Peck cycle by hole depth

55/64" drill: depth and canned cycle
Depth	Cycle	Peck	Note
3×D = 2.578 in (65.5 mm)	G81 (no peck)	None	Up to 3×D a sharp drill clears chips on its own.
5×D = 4.297 in (109.1 mm)	G73 chip-break	1 × D	Chip-break pecks; full retract if chips pack (soft aluminum, stainless).
8×D = 6.875 in (174.6 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 55/64" drill

7/8-20 — form (roll) tap, 65% thread

Nearest drills

21.75 mm: 0.8563 in / 21.750 mm (-0.0031 in)
22.0 mm: 0.8661 in / 22.000 mm (+0.0068 in)
21.5 mm: 0.8465 in / 21.500 mm (-0.0129 in)
27/32": 0.8438 in / 21.431 mm (-0.0156 in)

Drill speed calculator, prefilled for 55/64"Drill finder: sizes near 55/64"Fractional drill chart Decimal equivalent chart

Common questions

What is a 55/64" drill in decimal and mm?: 0.8594 in, or 21.828 mm; nearest number / letter drill: Letter Z (0.4130 in, -0.4464 in); nearest metric drill: 21.75 mm (0.8563 in, -0.0031 in).
What RPM for a 55/64" drill in steel?: In low-carbon steel (1018) an HSS 55/64" drill (0.8594 in) runs about 400 RPM at 90 SFM; a carbide drill about 1,330 RPM at 300 SFM. RPM = 3.82 × SFM ÷ D.
What feed for a 55/64" drill?: About 0.0119 in/rev in mild steel, 0.0119 in/rev in aluminum and 0.0099 in/rev in 304 stainless. At 400 RPM that is 4.76 IPM in steel (IPM = RPM × IPR).
What RPM for a 55/64" drill in aluminum?: 1,110 RPM with HSS (250 SFM) or up to 2,670 RPM with carbide (600 SFM), feeding about 0.0119 in/rev.
What tap uses a 55/64" drill?: 7/8-20 (form (roll) tap, 65% 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.

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