What is a 51/64" drill in decimal and mm?

0.7969 in, or 20.241 mm; nearest number / letter drill: Letter Z (0.4130 in, -0.3839 in); nearest metric drill: 20.25 mm (0.7972 in, +0.0004 in).

What RPM for a 51/64" drill in steel?

In low-carbon steel (1018) an HSS 51/64" drill (0.7969 in) runs about 430 RPM at 90 SFM; a carbide drill about 1,440 RPM at 300 SFM. RPM = 3.82 × SFM ÷ D.

What feed for a 51/64" drill?

About 0.0117 in/rev in mild steel, 0.0114 in/rev in aluminum and 0.0097 in/rev in 304 stainless. At 430 RPM that is 5.03 IPM in steel (IPM = RPM × IPR).

What RPM for a 51/64" drill in aluminum?

1,200 RPM with HSS (250 SFM) or up to 2,880 RPM with carbide (600 SFM), feeding about 0.0114 in/rev.

What tap uses a 51/64" drill?

13/16-20 (form (roll) tap, 65% thread); 7/8-9 (cut tap, 50% thread).

51/64" drill size

A 51/64" fractional drill is 0.7969 in (20.241 mm). It is the form (roll) tap, 65% thread drill for 13/16-20.

Decimal inch: 0.7969
Millimeters: 20.241
Nearest 1/64: 51/64" (+0.0000)
Type: fractional drill

Nearest equivalents in other systems

Drills closest to 51/64"
System	Drill	Inch	mm	Difference
Number / letter	Letter Z	0.4130	10.490	-0.3839 in
Metric	20.25 mm	0.7972	20.250	+0.0004 in

Speeds and feeds: in mild steel an HSS drill starts at about 430 RPM and 0.0117 in/rev (5.03 IPM); in 6061 aluminum about 1,200 RPM.

51/64" drill speeds and feeds by material

Jobber-length twist drill, D = 0.7969 in. RPM = 3.82 × SFM ÷ D, rounded to 10. Feed per rev scales with diameter and material; IPM = RPM × IPR.

51/64" drilling speeds
Material	HSS SFM	HSS RPM	Carbide SFM	Carbide RPM	Feed (in/rev)	HSS IPM	Carbide IPM
Aluminum (6061)	250	1,200	600	2,880	0.0114	13.7	32.8
Brass / free-machining	150	720	400	1,920	0.0092	6.62	17.7
Low-carbon steel (1018)	90	430	300	1,440	0.0117	5.03	16.9
Alloy steel (4140, annealed)	70	340	250	1,200	0.0122	4.15	14.6
Stainless 304	40	190	150	720	0.0097	1.84	6.98
Gray cast iron	80	380	250	1,200	0.0102	3.88	12.2
Titanium 6Al-4V	25	120	100	480	0.0056	0.67	2.69
Plastics (acetal)	300	1,440	500	2,400	0.0072	10.4	17.3

Metric: Vc (m/min) = SFM × 0.3048; feed (mm/rev) = in/rev × 25.4. In steel, 430 RPM × 0.297 mm/rev = 128 mm/min.

Worked example: 51/64" drill in 1018 steel

HSS drill in low-carbon steel: 90 SFM.
RPM = 3.82 × 90 ÷ 0.7969 ≈ 430 RPM.
Feed for a 0.797 in drill in mild steel ≈ 0.0117 in/rev.
IPM = 430 × 0.0117 = 5.03 IPM.

Peck cycle by hole depth

51/64" drill: depth and canned cycle
Depth	Cycle	Peck	Note
3×D = 2.391 in (60.7 mm)	G81 (no peck)	None	Up to 3×D a sharp drill clears chips on its own.
5×D = 3.984 in (101.2 mm)	G73 chip-break	1 × D	Chip-break pecks; full retract if chips pack (soft aluminum, stainless).
8×D = 6.375 in (161.9 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 51/64" drill

13/16-20 — form (roll) tap, 65% thread
7/8-9 — cut tap, 50% thread · tap speeds and feeds

Nearest drills

20.25 mm: 0.7972 in / 20.250 mm (+0.0004 in)
20.0 mm: 0.7874 in / 20.000 mm (-0.0095 in)
20.5 mm: 0.8071 in / 20.500 mm (+0.0102 in)
25/32": 0.7813 in / 19.844 mm (-0.0156 in)

Drill speed calculator, prefilled for 51/64"Drill finder: sizes near 51/64"Fractional drill chart Decimal equivalent chart

Common questions

What is a 51/64" drill in decimal and mm?: 0.7969 in, or 20.241 mm; nearest number / letter drill: Letter Z (0.4130 in, -0.3839 in); nearest metric drill: 20.25 mm (0.7972 in, +0.0004 in).
What RPM for a 51/64" drill in steel?: In low-carbon steel (1018) an HSS 51/64" drill (0.7969 in) runs about 430 RPM at 90 SFM; a carbide drill about 1,440 RPM at 300 SFM. RPM = 3.82 × SFM ÷ D.
What feed for a 51/64" drill?: About 0.0117 in/rev in mild steel, 0.0114 in/rev in aluminum and 0.0097 in/rev in 304 stainless. At 430 RPM that is 5.03 IPM in steel (IPM = RPM × IPR).
What RPM for a 51/64" drill in aluminum?: 1,200 RPM with HSS (250 SFM) or up to 2,880 RPM with carbide (600 SFM), feeding about 0.0114 in/rev.
What tap uses a 51/64" drill?: 13/16-20 (form (roll) tap, 65% thread); 7/8-9 (cut tap, 50% 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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