Drill Bit Geometry for Aluminum vs. Steel — What Changes and Why

A standard jobber drill designed for mild steel will drill aluminum — badly. It'll work, especially on soft alloys at low speeds, but it'll grab on breakthrough, build up edge material, produce a rough hole, and wear faster than it should. The geometry is wrong for the material.

Understanding what changes — and why — helps you spec the right bit and know what you're sacrificing when you use what you have.

POINT ANGLE: 118° VS. 135° AND BELOW

The standard included point angle for mild steel is 118°. This creates a moderate rake angle at the cutting lip and distributes the thrust load across a reasonable chisel edge length.

Aluminum benefits from a lower point angle — 90° to 100° is common for production aluminum work. The flatter point creates a more aggressive rake angle at the lip, which shears the material rather than pushing through it. This reduces built-up edge (BUE) and helps the chip form and evacuate cleanly instead of welding to the face.

Running a 118° bit on aluminum isn't catastrophic. Running a 90° aluminum-specific bit on steel produces a fragile, weak cutting edge prone to chipping under the higher forces steel requires. The geometry is optimized in opposite directions.

HELIX ANGLE: CHIP EVACUATION VS. RIGIDITY

Standard jobber drills have a helix angle around 28–30°. This is a compromise that works reasonably well on most ferrous materials.

Aluminum-specific drills often use a high helix — 40° or higher. The steeper helix acts more aggressively to pull chips out of the hole. This matters because aluminum chips are gummy and long-curling. They pack into the flutes and generate heat through friction if not cleared promptly. A high-helix drill physically lifts chips out faster.

On steel, a high helix reduces edge rigidity and is more prone to chatter under the harder material. The trade-off goes the other way.

RELIEF ANGLE AND RAKE

Aluminum drills use a higher rake angle (more positive) to shear material rather than plow through it. The softer, more ductile material responds well to a shearing cut. Steel, which is harder and more resistant, requires a slightly lower rake for edge strength — too high a rake on steel chips the edge under load.

WHAT THIS MEANS IN PRACTICE

For a one-material shop drilling steel all day: standard 118° jobber drills are correct. Resharpening returns them to that geometry.

For a shop that drills both materials on the same machine: keep separate sets. Color-code them, use a different flute length, or mark the shanks. Mixing geometry between materials is the fastest way to get bad holes and short tool life.

For a shop that drills aluminum primarily: invest in proper aluminum drills. The productivity gain and tool life improvement over running steel geometry on aluminum is significant enough to pay for the bit cost difference quickly.

When sending bits in for resharpening: tell us what you drill. MachinistPost grinds to the geometry your application requires, not just a generic "factory spec." An aluminum drill comes back as an aluminum drill. A steel drill comes back ready for steel.

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