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Stainless steel is harder on drill bits than ordinary mild steel. It generates heat quickly, work-hardens if the bit rubs instead of cutting, and can destroy a cheap bit in seconds. Mild steel is more forgiving, but it still rewards a sharp bit, controlled speed, and cutting fluid. The best set depends on whether you are making occasional holes in thin sheet or drilling repeated holes through thick steel.
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What to Look for in a Steel Drill Bit Set
For most cordless-drill owners, the useful baseline is a set of high-speed steel (HSS) twist bits from about 1/16 inch to 1/2 inch. HSS is suitable for mild steel and occasional stainless work, provided you use the correct speed and lubrication.
For regular stainless-steel drilling, choose cobalt-alloy HSS bits. Look for M35 cobalt, which contains about 5% cobalt, or M42, which contains about 8%. M42 generally holds its edge longer under heat, but it is more brittle and costs more. Cobalt bits are not magic: overheating them still ruins the temper. They should be used with firm pressure and cutting oil, not allowed to spin freely and polish the hole.
A split-point tip, commonly 135 degrees, is useful on metal because it reduces walking and starts more cleanly than a traditional 118-degree point. It does not eliminate the need for a pilot hole on larger sizes. A 1/4-inch hex shank is convenient in an impact driver, but ordinary round-shank bits are usually the better choice for a drill chuck. Impact-rated bits are worthwhile only when you specifically need to drill with an impact driver.
Best Bit Types by Job
| Bit type | Best use | Main limitation |
|---|---|---|
| Standard HSS | Mild steel, aluminum, plastic, occasional stainless | Edge dulls quickly if overheated on stainless |
| Cobalt HSS | Repeated holes in stainless and hard steel | More expensive and less tolerant of side loading |
| Titanium-coated HSS | General-purpose mild steel and thin sheet | Coating does not make the underlying bit suitable for severe heat |
| Step bit | Clean holes in thin sheet metal | Not intended for thick plate or deep holes |
| Carbide-tipped or solid carbide | Very hard materials and production work | Brittle, expensive, and usually unnecessary for a cordless drill |
For a general workshop, a cobalt drill bit set is the safest upgrade for stainless. If nearly all your work is mild steel, a quality HSS set is usually better value. Titanium-coated sets can work well, but “titanium” describes a surface coating, not the entire cutting material. Once the coating wears through at the tip, performance depends on the HSS underneath.
How to Drill Stainless Steel
Start by marking the hole with a center punch. Clamp the work securely; stainless sheet can grab a bit and spin or bend if it is loose. Put the drill in low gear and turn hammer mode off. Stainless commonly needs slower speeds than users expect. As a rough starting point, use about 300 to 600 rpm for a 1/4-inch hole and about 100 to 250 rpm for a 1/2-inch hole. Exact speed depends on the alloy, thickness, and bit quality.
Apply cutting oil and use enough pressure to produce a continuous chip. The bit should cut, not squeal or produce only fine dust-like particles. If you stop with the bit rubbing against the surface, the area can work-harden. The next attempt may then be drilling into a much harder patch, causing rapid edge failure.
For holes larger than roughly 1/4 inch, drill a pilot hole first, often 1/8 inch or 3/16 inch. Do not make the pilot hole excessively large; the larger bit needs enough material at its cutting edges to bite. Withdraw the bit periodically from deep holes to clear chips and add oil. Never touch the bit immediately afterward—stainless chips and the tool can become extremely hot.
How to Drill Mild Steel
Mild steel permits higher speeds and is less likely to work-harden, but heat remains the main cause of dulling. For a 1/4-inch HSS bit, roughly 800 to 1,200 rpm is a practical cordless-drill starting range. For a 1/2-inch bit, reduce that to about 300 to 600 rpm. Use cutting oil for holes deeper than thin sheet, especially when drilling structural steel or plate.
Thin sheet metal often needs a different approach. A step bit makes a round, clean hole with less grabbing and is especially useful for electrical boxes, brackets, and vehicle panels. Use moderate speed and light oil. A step bit is a poor choice for 1/4-inch-thick plate because each step is designed for relatively shallow cutting and can wear quickly.
A metal step drill bit set is worth adding if you regularly drill sheet metal. It does not replace a conventional twist-bit set; it complements it.
Choosing Bits for Your Drill
A cordless drill with a two-speed gearbox is preferable to a single-speed model. Use low gear for large bits and stainless, and high gear for small holes in mild steel when the bit manufacturer allows it. Keep the clutch fully engaged or set high enough to avoid slipping while drilling metal. A weak drill may stall on larger holes, but forcing it with excessive pressure can twist a bit or damage the chuck.
Most drill bits should be used in a standard drill, not hammer mode. Hammer action chips masonry but can fracture metal-cutting edges and enlarge the hole unpredictably. An impact driver can run hex-shank metal bits, but its pulsing torque makes it easier to break small bits. Use it for occasional sheet-metal holes, not precision drilling or repeated stainless work. A hex-shank metal bit set for an impact driver is convenient, but a drill remains the better tool when accuracy matters.
What to Buy
Buy a standard HSS set if you drill mild steel a few times a year and can replace individual bits when they dull. Choose a cobalt set if stainless is a regular material, if the holes are larger than 1/4 inch, or if downtime matters more than the initial price. Add a step-bit set for thin sheet. Avoid giant bargain sets with dozens of obscure sizes if the included bits are poorly sharpened; a smaller set with common sizes from 1/16 through 1/2 inch is more useful.
Keep bits dry, wipe off cutting oil after use, and replace any bit with chipped cutting edges. Sharpening can extend the life of larger HSS bits, but a badly overheated cobalt bit is often not worth rescuing. The cheapest option is fine when the material is mild steel, the hole count is low, and you control heat. Stainless steel demands better technique first, and better bits second.