Drilling Into Metal: Choosing the Right Bit, Speed, and Technique

A bit that cuts clean through mild steel can burn up in seconds on stainless. SC Fastening Systems breaks down what actually changes when the material does.

Drilling into metal isn’t one job — it’s several, depending on what kind of metal is on the other side of the bit. The same drill bit that breezes through mild steel will overheat, dull, or snap outright on stainless or hardened steel. Getting consistent, clean holes in metal comes down to three things: the bit, the speed, and how you manage heat.

Match the Bit to the Metal

Not every drill bit is rated for every metal. Pushing the wrong bit into a hard alloy is the fastest way to burn up a bit — and your patience:

• High-speed steel (HSS) — The standard choice for mild steel, aluminum, and general fabrication work. Affordable and reliable for everyday jobs, but it loses its edge faster on harder alloys.
• Cobalt-blended HSS — Cobalt isn’t a coating, it’s blended into the steel itself, and it raises the bit’s heat tolerance significantly. This is the bit to reach for on stainless steel and other tough, work-hardening alloys where a standard HSS bit would overheat and dull almost immediately.
• Titanium nitride (TiN) coated — A thin, hard coating over HSS that reduces friction and extends life for high-speed drilling in hardened materials, though the coating wears through with resharpening.
• Carbide-tipped — Reserved for the hardest materials — hardened steel, cast iron, and abrasive metals — where even cobalt struggles. Carbide holds an edge far longer but is more brittle and less forgiving of side-load or bit wobble.

As a rule: the harder or more heat-sensitive the alloy, the more heat tolerance you need built into the bit. Stainless steel in particular work-hardens where you drill — hesitate or slow down mid-hole, and the surface actually gets harder, making the rest of the hole tougher to finish.

Slow Down for Harder Metals

Drill speed (RPM) needs to drop as material hardness goes up — running a hard alloy at the same RPM you’d use on mild steel is one of the most common causes of premature bit failure:

• Aluminum and mild steel — Tolerate higher RPMs and cut relatively easily, though aluminum benefits from a sharp bit and steady feed to avoid grabbing.
• Stainless steel — Requires a slower, steadier RPM with firm, continuous pressure. Backing off mid-hole is what triggers work-hardening.
• Hardened steel and cast iron — Needs the slowest speeds and the most consistent feed pressure of all, paired with a carbide or cobalt bit built for the load.

As a general guide: larger diameter bits and harder materials both call for lower RPM. If you’re hearing a high-pitched squeal or seeing discoloration on the bit or the workpiece, that’s the metal telling you to slow down before you lose the bit.

Manage Heat, Don’t Fight It

Heat is the enemy of both the bit and the hole quality. A few habits make a real difference:

• Use cutting fluid or lubricant on metal — especially stainless, aluminum, and anything thicker than light-gauge sheet. It reduces friction, clears chips, and extends bit life considerably.
• Center punch first. A punched starting point keeps the bit from wandering on a hard, smooth surface, which otherwise forces you to apply extra side pressure that accelerates wear.
• Peck drill on deep or thick holes — retract periodically to clear chips and let the bit cool, rather than pushing straight through in one pass.
• Apply steady, consistent pressure. Too light and the bit rubs instead of cutting, generating heat without progress; too heavy and you risk snapping the bit, especially in carbide.

If a bit comes out blued or discolored, it’s already lost hardness at the tip — that bit will drill slower and duller from that point forward, even if it still technically cuts.

A Quick Reference by Metal Type

• Mild/carbon steel: HSS bit, moderate speed, light cutting oil.
• Stainless steel: Cobalt bit, slower speed, steady pressure, cutting fluid.
• Aluminum: HSS bit, higher speed, light lubricant to prevent chip welding.
• Hardened steel/cast iron: Carbide-tipped bit, slowest speed, firm consistent feed.