Why aluminium is tricky to drill

Steel is harder than aluminium but in many ways easier to drill: the higher resistance slows the drill bit naturally and chips break cleanly. Aluminium is so soft that the bit can dig in aggressively, the material can weld itself to the cutting edge (built-up edge), and swarf wraps around the bit rather than breaking off. The result is chatter, oversized holes, and broken bits.

Three factors fix most of these problems: speed, feed rate, and lubrication.

Drill bit choice

Use HSS (high-speed steel) drill bits with a 118° point angle — the standard angle works for aluminium. Bits designed for aluminium often have a polished flute to reduce built-up edge and a slightly different helix angle (32–40°) for better chip evacuation. If you drill aluminium regularly, dedicated aluminium bits are worth the investment.

Keep bits sharp. A dull bit generates heat instead of cutting, which accelerates the welding effect and leaves a rough hole.

Speed

Aluminium is cut at high surface speed — much higher than for steel. As a starting point:

  • 3mm bit: 3,000–4,000 RPM
  • 6mm bit: 1,500–2,000 RPM
  • 10mm bit: 800–1,200 RPM

These are guidelines. If the bit is chattering, try increasing speed. If the aluminium is welding to the bit, slow down and add lubrication. With a hand drill (which has lower RPM than a drill press), use the highest speed setting available.

Lubrication

Lubrication dramatically improves the result — it reduces heat, prevents built-up edge, and helps swarf clear the flutes. For occasional work, WD-40 applied at the entry point works adequately. For regular use, a purpose-made cutting oil or a product like Collos gives better results and extends bit life.

Collos cutting lubricant — used for drilling and threading aluminium
A cutting lubricant like Collos reduces heat and prevents built-up edge when drilling aluminium.

Technique

  • Centre punch first: A sharp centre punch mark stops the bit from wandering at the start of the cut. This is the single most effective step for accurate hole placement.
  • Start slow: Begin with light pressure until the bit is engaged and not skidding, then increase feed rate.
  • Consistent feed pressure: Too little pressure causes rubbing (heat, no cutting); too much causes dig-in and chatter. The right pressure feels natural — the bit is cutting, not grinding.
  • Clear swarf regularly: Retract the bit partially every few seconds on deeper holes to clear chips from the flutes. Swarf packed into the hole re-cuts the walls and overheats the bit.
  • Clamp the workpiece: Never hold aluminium sheet by hand while drilling. Thin sheet in particular can rotate with the bit with little warning. Use a vice or clamps — always.

Large holes

For holes larger than about 10–12mm, drill a pilot hole first (4–6mm) and then step up in increments. A large bit starting in solid material has a much higher tendency to grab. Stepping up through intermediate sizes gives each bit a thinner annulus to cut, reducing grab and heat.

For holes larger than 20mm in thin sheet, a step drill or hole saw with appropriate lubricant is often the cleanest approach. Step drills leave a very clean edge in sheet aluminium if run at appropriate speed with lubrication.

Deburring: After drilling, always deburr both faces with a countersink bit, deburring tool, or even the edge of a larger drill bit rotated by hand. Aluminium edges are sharp and can damage wiring insulation in observatory enclosures — deburring takes 10 seconds and prevents problems.