27/64" drill size

    A 27/64" fractional drill is 0.4219 in (10.716 mm). It is the cut tap, 75% thread drill for 1/2-13.

    Decimal inch
    0.4219
    Millimeters
    10.716
    Nearest 1/64
    27/64" (+0.0000)
    Type
    fractional drill

    Nearest equivalents in other systems

    Drills closest to 27/64"
    SystemDrillInchmmDifference
    Number / letterLetter Z0.413010.490-0.0089 in
    Metric10.7 mm0.421310.700-0.0006 in

    Speeds and feeds: in mild steel an HSS drill starts at about 810 RPM and 0.0099 in/rev (8.02 IPM); in 6061 aluminum about 2,260 RPM.

    27/64" drill speeds and feeds by material

    Jobber-length twist drill, D = 0.4219 in. RPM = 3.82 × SFM ÷ D, rounded to 10. Feed per rev scales with diameter and material; IPM = RPM × IPR.

    27/64" drilling speeds
    MaterialHSS SFMHSS RPMCarbide SFMCarbide RPMFeed (in/rev)HSS IPMCarbide IPM
    Aluminum (6061)2502,2606005,4300.009120.649.4
    Brass / free-machining1501,3604003,6200.00709.5225.3
    Low-carbon steel (1018)908103002,7200.00998.0226.9
    Alloy steel (4140, annealed)706302502,2600.01096.8724.6
    Stainless 304403601501,3600.00712.569.66
    Gray cast iron807202502,2600.00735.2616.5
    Titanium 6Al-4V252301009100.00451.034.10
    Plastics (acetal)3002,7205004,5300.005414.724.5

    Metric: Vc (m/min) = SFM × 0.3048; feed (mm/rev) = in/rev × 25.4. In steel, 810 RPM × 0.251 mm/rev = 204 mm/min.

    Worked example: 27/64" drill in 1018 steel

    1. HSS drill in low-carbon steel: 90 SFM.
    2. RPM = 3.82 × 90 ÷ 0.4219 ≈ 810 RPM.
    3. Feed for a 0.422 in drill in mild steel ≈ 0.0099 in/rev.
    4. IPM = 810 × 0.0099 = 8.02 IPM.

    Peck cycle by hole depth

    27/64" drill: depth and canned cycle
    DepthCyclePeckNote
    3×D = 1.266 in (32.1 mm)G81 (no peck)NoneUp to 3×D a sharp drill clears chips on its own.
    5×D = 2.109 in (53.6 mm)G73 chip-break1 × DChip-break pecks; full retract if chips pack (soft aluminum, stainless).
    8×D = 3.375 in (85.7 mm)G83 full retract0.5-1 × D, shrinking with depthRetract to clear chips; reduce feed ~10-20% past 5×D.

    Taps that use a 27/64" drill

    Nearest drills

    • 10.7 mm: 0.4213 in / 10.700 mm (-0.0006 in)
    • 10.8 mm: 0.4252 in / 10.800 mm (+0.0033 in)
    • 10.6 mm: 0.4173 in / 10.600 mm (-0.0046 in)
    • 10.9 mm: 0.4291 in / 10.900 mm (+0.0073 in)
    Drill speed calculator, prefilled for 27/64"Drill finder: sizes near 27/64"Fractional drill chartDecimal equivalent chart

    Common questions

    What is a 27/64" drill in decimal and mm?
    0.4219 in, or 10.716 mm; nearest number / letter drill: Letter Z (0.4130 in, -0.0089 in); nearest metric drill: 10.7 mm (0.4213 in, -0.0006 in).
    What RPM for a 27/64" drill in steel?
    In low-carbon steel (1018) an HSS 27/64" drill (0.4219 in) runs about 810 RPM at 90 SFM; a carbide drill about 2,720 RPM at 300 SFM. RPM = 3.82 × SFM ÷ D.
    What feed for a 27/64" drill?
    About 0.0099 in/rev in mild steel, 0.0091 in/rev in aluminum and 0.0071 in/rev in 304 stainless. At 810 RPM that is 8.02 IPM in steel (IPM = RPM × IPR).
    What RPM for a 27/64" drill in aluminum?
    2,260 RPM with HSS (250 SFM) or up to 5,430 RPM with carbide (600 SFM), feeding about 0.0091 in/rev.
    What tap uses a 27/64" drill?
    1/2-13 (cut tap, 75% thread); 7/16-28 (form (roll) tap, 65% thread); 3/8-24 (Helicoil / STI insert (3/8-24×1D)).

    Nearby drill sizes

    Sources

    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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