Aluminum is one of the most common materials machinists and fabricators drill, and one of the most deceptive. It's soft, it cuts fast, and then suddenly you're dealing with a clogged flute, a grabbed bit, or a galled hole that looks nothing like the clean bore you expected. The material itself isn't difficult — but it has specific behaviors that bite you if you don't account for them.
Aluminum is soft and gummy. Unlike steel, which shears cleanly under a cutting edge, aluminum wants to weld itself to the tool. When you drill aluminum with a standard geometry HSS bit at the wrong speed or with the wrong lubricant, two things happen:
Built-up edge (BUE): Aluminum particles weld to the cutting edge of the drill. This changes the effective geometry — your drill is no longer cutting with a sharp edge, it's tearing with a glob of aluminum on top. Holes get rough, the bit starts to squeal, and heat builds fast.
Flute packing: Aluminum chips are long and stringy. They don't break up into small segments the way steel chips do. At low RPM or without the right lubricant, they pack into the flutes instead of evacuating. A packed flute can't cool itself, can't eject chips, and the drill starts galling the sidewall of the hole.
Both problems get worse if you slow down when you think something's wrong — slow speed in aluminum is almost always the wrong move.
Aluminum wants high RPM. The general rule is 3–4x the RPM you'd run for mild steel. For a ½" HSS bit in 6061 aluminum on a drill press, you're looking at 1,500–2,500 RPM depending on your setup. For smaller bits — ¼" and under — you can push even higher if your machine allows.
Fast cutting speed reduces the contact time per revolution, which means less heat transfer into the tool and less tendency for the aluminum to weld to the cutting edge. This is counterintuitive if you're coming from drilling steel, where slowing down buys you control. In aluminum, speed is your friend.
Dry drilling aluminum is fine for a single hole at low material removal rates. Anything more than that, use a lubricant.
WD-40 is the most common shop solution for aluminum and it works well for handheld drilling and light machine work. It's not a cutting fluid — it's a displacing lubricant — but the slippery film it creates dramatically reduces built-up edge.
Kerosene or mineral spirits is what machining texts have recommended for decades, and it still works fine. Dedicated cutting oil (Tap Magic Aluminum is a common one) is the right choice for production work or whenever finish quality matters.
The lubricant does two things: it prevents the aluminum from welding to the cutting edge, and it helps chips evacuate the flute instead of packing in.
Standard 118° HSS bits drill aluminum, but they're not optimal. The geometry you want for aluminum has two features:
Polished flutes. Aluminum chips need to slide out easily. A rough flute surface has microscopic grooves that give the chips something to catch on. Polished or bright-finished flutes evacuate aluminum chips much better.
Higher helix angle. The flute helix controls how aggressively chips are pulled out of the hole. A higher helix angle (around 40°) is better for aluminum than the standard 28–30° you'll find on general-purpose bits. High-helix bits evacuate those long stringy chips faster, which keeps flutes clear.
For occasional aluminum work, a standard HSS bit with good lubrication and correct speed will do fine. For regular aluminum work, dedicated aluminum geometry bits are worth having.
Sheet: Use a split-point or brad-point bit that self-centers. The full-point geometry on a standard bit pushes the thin material around before it starts cutting. Center-punch your location. Use a backup board under the sheet to support it through the exit. Clamp everything that moves.
Plate and billet: Peck drilling helps on deeper holes — withdraw the drill periodically to break and clear the chip. Even with good flute geometry, long continuous cuts in aluminum pack up. Peck at depth increments of 2–3x the drill diameter for anything deeper than ¾".
Aluminum is an easy material once you understand what it wants. Give it speed, give it lubrication, and use a bit with the right geometry — and it drills cleaner than most steels.
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