What is a 39/64" drill in decimal and mm?

0.6094 in, or 15.478 mm; nearest number / letter drill: Letter Z (0.4130 in, -0.1964 in); nearest metric drill: 15.5 mm (0.6102 in, +0.0009 in).

What RPM for a 39/64" drill in steel?

In low-carbon steel (1018) an HSS 39/64" drill (0.6094 in) runs about 560 RPM at 90 SFM; a carbide drill about 1,880 RPM at 300 SFM. RPM = 3.82 × SFM ÷ D.

What feed for a 39/64" drill?

About 0.0109 in/rev in mild steel, 0.0104 in/rev in aluminum and 0.0089 in/rev in 304 stainless. At 560 RPM that is 6.10 IPM in steel (IPM = RPM × IPR).

What RPM for a 39/64" drill in aluminum?

1,570 RPM with HSS (250 SFM) or up to 3,760 RPM with carbide (600 SFM), feeding about 0.0104 in/rev.

What tap uses a 39/64" drill?

5/8-24 (form (roll) tap, 65% thread); 9/16-18 (Helicoil / STI insert (9/16-18×1D)).

39/64" drill size

A 39/64" fractional drill is 0.6094 in (15.478 mm). It is the form (roll) tap, 65% thread drill for 5/8-24.

Decimal inch: 0.6094
Millimeters: 15.478
Nearest 1/64: 39/64" (+0.0000)
Type: fractional drill

Nearest equivalents in other systems

Drills closest to 39/64"
System	Drill	Inch	mm	Difference
Number / letter	Letter Z	0.4130	10.490	-0.1964 in
Metric	15.5 mm	0.6102	15.500	+0.0009 in

Speeds and feeds: in mild steel an HSS drill starts at about 560 RPM and 0.0109 in/rev (6.10 IPM); in 6061 aluminum about 1,570 RPM.

39/64" drill speeds and feeds by material

Jobber-length twist drill, D = 0.6094 in. RPM = 3.82 × SFM ÷ D, rounded to 10. Feed per rev scales with diameter and material; IPM = RPM × IPR.

39/64" drilling speeds
Material	HSS SFM	HSS RPM	Carbide SFM	Carbide RPM	Feed (in/rev)	HSS IPM	Carbide IPM
Aluminum (6061)	250	1,570	600	3,760	0.0104	16.3	39.1
Brass / free-machining	150	940	400	2,510	0.0086	8.08	21.6
Low-carbon steel (1018)	90	560	300	1,880	0.0109	6.10	20.5
Alloy steel (4140, annealed)	70	440	250	1,570	0.0117	5.15	18.4
Stainless 304	40	250	150	940	0.0089	2.23	8.37
Gray cast iron	80	500	250	1,570	0.0092	4.60	14.4
Titanium 6Al-4V	25	160	100	630	0.0052	0.83	3.28
Plastics (acetal)	300	1,880	500	3,130	0.0064	12.0	20.0

Metric: Vc (m/min) = SFM × 0.3048; feed (mm/rev) = in/rev × 25.4. In steel, 560 RPM × 0.277 mm/rev = 155 mm/min.

Worked example: 39/64" drill in 1018 steel

HSS drill in low-carbon steel: 90 SFM.
RPM = 3.82 × 90 ÷ 0.6094 ≈ 560 RPM.
Feed for a 0.609 in drill in mild steel ≈ 0.0109 in/rev.
IPM = 560 × 0.0109 = 6.10 IPM.

Peck cycle by hole depth

39/64" drill: depth and canned cycle
Depth	Cycle	Peck	Note
3×D = 1.828 in (46.4 mm)	G81 (no peck)	None	Up to 3×D a sharp drill clears chips on its own.
5×D = 3.047 in (77.4 mm)	G73 chip-break	1 × D	Chip-break pecks; full retract if chips pack (soft aluminum, stainless).
8×D = 4.875 in (123.8 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 39/64" drill

5/8-24 — form (roll) tap, 65% thread
9/16-18 — Helicoil / STI insert (9/16-18×1D) · tap speeds and feeds

Nearest drills

15.5 mm: 0.6102 in / 15.500 mm (+0.0009 in)
15.25 mm: 0.6004 in / 15.250 mm (-0.0090 in)
15.75 mm: 0.6201 in / 15.750 mm (+0.0107 in)
19/32": 0.5938 in / 15.081 mm (-0.0156 in)

Drill speed calculator, prefilled for 39/64"Drill finder: sizes near 39/64"Fractional drill chart Decimal equivalent chart

Common questions

What is a 39/64" drill in decimal and mm?: 0.6094 in, or 15.478 mm; nearest number / letter drill: Letter Z (0.4130 in, -0.1964 in); nearest metric drill: 15.5 mm (0.6102 in, +0.0009 in).
What RPM for a 39/64" drill in steel?: In low-carbon steel (1018) an HSS 39/64" drill (0.6094 in) runs about 560 RPM at 90 SFM; a carbide drill about 1,880 RPM at 300 SFM. RPM = 3.82 × SFM ÷ D.
What feed for a 39/64" drill?: About 0.0109 in/rev in mild steel, 0.0104 in/rev in aluminum and 0.0089 in/rev in 304 stainless. At 560 RPM that is 6.10 IPM in steel (IPM = RPM × IPR).
What RPM for a 39/64" drill in aluminum?: 1,570 RPM with HSS (250 SFM) or up to 3,760 RPM with carbide (600 SFM), feeding about 0.0104 in/rev.
What tap uses a 39/64" drill?: 5/8-24 (form (roll) tap, 65% thread); 9/16-18 (Helicoil / STI insert (9/16-18×1D)).

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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