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What works for thick aluminum
For drilling thick aluminum stock, a sharp high-speed steel (HSS) twist bit is usually the sensible starting point. Choose a standard 118-degree point for general work or a 135-degree split point if you want easier starting and less walking. Cobalt HSS bits cost more and can last longer under sustained cutting, but they are not automatically better for an occasional hole in aluminum. Carbide bits are generally unnecessary for ordinary drilling in this material.
Buy bits with the shank your drill can hold securely. A round shank works in a keyed or keyless chuck; a hex shank can be handy for quick changes, but check that the bit’s diameter and length suit the chuck and the job. For a set of common sizes, compare HSS split-point drill bit sets. For repeat work in larger diameters, buying individual bits can make more sense than replacing a whole set when one wears out.
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Bit choices compared
| Bit type | Best use | Trade-off |
|---|---|---|
| HSS, 118-degree point | Occasional holes and general shop work | Low cost; may walk more on smooth stock |
| HSS, 135-degree split point | More accurate starts, especially without a pilot hole | Often costs a little more; still needs proper speed and lubrication |
| Cobalt HSS | Frequent drilling or tougher conditions where bit life matters | Higher price; not a substitute for clearing chips or preventing heat |
| Step bit | Thin sheet and enlarging existing holes | Not the right choice for deep holes through thick stock |
| Carbide | Specialized production setups | Brittle and generally a poor match for handheld drilling |
A step bit is useful for sheet metal, but its short cutting stages do not provide the reach or geometry needed for a deep hole through thick aluminum. For a one-off hole in stock around 1/8 inch thick, an ordinary sharp HSS bit is usually fine. As diameter and hole depth increase, chip control, workholding, and speed become more important than paying for the most expensive bit on the shelf.
Speed, setup, and lubrication
Aluminum cuts readily, but it can grab a bit and carry chips up its flutes. Clamp the work securely; do not hold a loose piece in your hand. Use a drill press for repeat holes or larger diameters when available. A cordless drill can handle many jobs, but use its low gear for larger bits and keep a firm two-handed grip. A hammer drill’s hammer mode should be off: impact action does not help cut metal and can damage the bit or workpiece.
As a practical starting point, use about 2,000–3,000 rpm for a 1/8-inch HSS bit, 800–1,500 rpm for a 1/4-inch bit, and roughly 300–700 rpm for a 1/2-inch bit. These are ranges, not universal settings; alloy, bit sharpness, drill stability, and the hole depth all matter. Slow down if the bit squeals, smears metal, or heats rapidly. A cordless drill that only runs fast may be difficult to control with a large bit, even if its chuck accepts it.
Use a small amount of aluminum-compatible cutting fluid or light cutting oil. Lubricant helps reduce heat and the tendency of aluminum to stick to the cutting edge. Reapply when needed, especially in a deep hole. Do not use a lubricant that is unsuitable for the workpiece or your shop’s safety requirements. Wear eye protection, and keep hands and loose clothing away from the rotating bit.
Drill the hole without binding
Mark the location and make a shallow center mark with a punch. This helps keep a conventional point from skating. With a sharp split-point bit, a center mark is still useful on a slippery surface, but avoid making a deep crater that can interfere with the point. If alignment is critical, use a drill press or a guide rather than relying on a handheld drill alone.
For holes larger than about 3/8 inch, drill a smaller pilot hole first—often 1/8 to 3/16 inch—then step up to the final diameter. A pilot hole reduces the load on a large bit and helps the cutting edges engage cleanly. Do not make the pilot so large that the final bit loses support at its center. For a deep hole, drill in short passes and withdraw the bit periodically to clear chips. Packed chips can overheat the bit, score the hole, or jam the drill.
Ease pressure as the bit is about to break through the far side. Sudden breakthrough is a common cause of grabbing and ragged edges. If the exit finish matters, clamp a scrap backing piece tightly against the underside. Deburr both sides afterward with a deburring tool or a larger bit turned gently by hand.
When to spend more—and what failure looks like
For a few holes, a budget HSS bit is adequate if it is sharp, the stock is clamped, and you use sensible speed and lubrication. Consider a cobalt HSS bit when you drill often, need repeated holes, or want longer edge life under careful use. Cobalt bits still need proper technique; they do not stay sharp through careless high-speed drilling.
A bit that produces a polished, smeared hole instead of chips is likely rubbing rather than cutting. Reduce speed, check that the edge is sharp, and add lubricant. A bit that squeals or discolors may be running too hot. A bit that wanders usually needs a better start, firmer workholding, or a split-point geometry. If the drill twists in your hands, binds, or stalls, stop and reset rather than forcing it. Large bits can catch suddenly, so a side handle is worthwhile when your drill supports one.
Before buying, check the maximum bit diameter recommended for your drill, particularly if you plan to use large bits in a cordless model. For occasional work, a modest HSS set plus a correctly sized individual bit is often the better value than a premium assortment. Spend first on sharp edges, secure clamping, and a drill that can run slowly; those choices prevent more ruined holes than exotic bit materials.