Technical Foundation
What Is a Extra Long Drill Bit?
Extra long drill bits extend beyond standard jobber-length twist drills to reach hole depths a normal drill can't, without needing a separate extension. Aircraft extension length drills (per ANSI/ASME B94.11M) run roughly 2-3x the flute length of a jobber drill of the same diameter, while taper length drills split the difference with a longer shank but flutes closer to jobber proportions, trading rigidity for reach depending on the application.
Because flute length increases faster than diameter in these bits, chip evacuation and deflection become the limiting factors rather than cutting geometry alone — a long, thin drill wandering off-axis is the most common failure mode, not dulling. HSS suits general steel and aluminum drilling; cobalt (M35/M42) holds an edge in stainless and high-temp alloys at the higher heat these longer cuts generate; carbide-tipped versions are specified for high-volume production runs into hardened or abrasive material.
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| Spec | What It Means | Why It Matters |
|---|---|---|
| Length Class | Jobber, taper length, aircraft extension (ANSI B94.11M) | Determines maximum reachable hole depth for a given diameter |
| Material | HSS, cobalt (M35/M42), carbide-tipped | Cobalt and carbide hold hardness at the higher heat long-reach drilling generates |
| Shank Type | Straight, reduced, 3-flat | Reduced shank lets a large-diameter bit fit a smaller chuck without losing overall reach |
| Flute Style | Standard helix, slow spiral | Slow spiral improves chip evacuation in deep, narrow holes prone to packing |
| Point Angle | 118°, 135° split point | 135° split point resists walking on start, important since long bits deflect easily |
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Applications
Where Extra Long Drill Bits Get Used
Extra long bits are specified anywhere the hole depth exceeds what a standard jobber-length drill can reach in one pass.
Aerospace Structural Drilling
Reaching through deep wing spars and fuselage frames where aircraft-extension length is specified by the assembly drawing.
Machine Tool & Fixture Building
Drilling deep coolant or lubrication passages through thick machine bases and spindle housings.
Plumbing & Electrical Rough-In
Boring through multiple wall studs or floor joists in one continuous pass during rough-in work.
Mold & Die Making
Drilling deep ejector pin and waterline holes into thick tool steel blocks.
Woodworking & Cabinetry
Running wiring or hardware bores through thick stock and stacked panel assemblies.
Oil & Gas Equipment Fabrication
Deep-hole drilling in thick-walled flanges and manifold blocks during equipment fabrication.
FAQ
Extra Long Drill Bits Questions, Answered
What's the difference between taper length and aircraft extension drill bits?
Taper length drills have a longer shank than a jobber drill but flute length closer to standard proportions, giving more reach with less deflection risk. Aircraft extension length drills have much longer flutes relative to diameter, per ANSI/ASME B94.11M, giving maximum reach for the deepest holes but requiring slower feed rates to control deflection and wander.
Why does my long drill bit wander or drill an oversized hole?
Deflection increases sharply with unsupported flute length — a long, thin bit flexes under normal drilling forces and can bow off-axis, producing an oversized or angled hole. Reducing feed rate, starting with a shorter pilot hole, and using a bushing or guide at the workpiece surface all help control this.
Should I use cobalt or HSS for extra long drill bits?
HSS is adequate for general steel and aluminum drilling at moderate depths. Cobalt (M35/M42) holds hardness better at the higher heat that longer, slower-evacuating cuts generate, and is the standard choice for stainless steel, high-temp alloys, and any deep hole where heat buildup is a concern.
How do I keep chips from packing in a deep hole?
Peck drilling — periodically retracting the bit fully to clear chips — is standard practice for deep-hole work. A slow-spiral flute geometry also improves chip evacuation compared to a standard helix, and reducing feed rate lowers the chip volume generated per pass.
What overall lengths are available for extra long drill bits?
Aircraft extension lengths commonly run from about 6in up to 24in or more depending on diameter, following ANSI/ASME B94.11M length standards. Taper length drills are shorter, typically extending a jobber-equivalent diameter by a few inches. Confirm the overall length against your hole depth plus chuck clearance before ordering.
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