As someone who started drilling steel with whatever bit came with my drill kit, burned through several sets the hard way, and eventually learned what the material science actually means for practical results, I learned everything there is to know about drill bits for steel. Today, I will share it all with you.
Material Composition of Drill Bits
High-Speed Steel (HSS) is the baseline for metal drilling and handles mild steel and structural steel adequately when you use correct technique. HSS is formulated to retain its hardness at temperatures that would soften standard carbon steel — that heat resistance is the whole point, because drilling steel generates significant heat at the cutting edge. Where HSS starts to struggle is on hardened steels and stainless, where the heat generated exceeds what the alloy can handle without losing its temper at the tip.
Cobalt bits — M35 (5% cobalt) and M42 (8% cobalt) — are the right step up for stainless steel and hardened alloys. The cobalt in the alloy raises the red hardness, meaning the bit keeps its cutting edge at the higher temperatures that stainless generates. The trade-off is brittleness: cobalt bits are harder and will snap under lateral loads that a standard HSS bit would flex through. Use a drill press when you can, and let the bit cut rather than forcing it.
Carbide bits are the industrial tier — maximum hardness, maximum heat resistance, suitable for the hardest alloys and high-speed production environments. They require rigid setups and controlled cutting conditions that make them impractical for most shop metalwork. For furniture shop steel hardware work, cobalt gets you where you need to go without the handling demands of carbide.
Coatings: What They Do and Don’t Do
Black oxide is a surface treatment that provides mild corrosion resistance and a small reduction in friction — a modest improvement over bare HSS at minimal cost. Titanium nitride (TiN) coating is a meaningful upgrade: harder surface, lower friction coefficient, noticeably longer service life than uncoated HSS on most steels. The limitation is that TiN is a surface treatment on an HSS substrate — once it wears through from resharpening, you’re back to HSS performance. The coating doesn’t change the hot hardness of the underlying alloy.
Types of Drill Bits for Steel
Standard twist bits with a 118-degree point angle are the general-purpose choice for mild steel. The 135-degree split-point design cuts its own starting indentation and reduces walking on smooth metal — a meaningful advantage when you’re drilling without a drill press and center punching isn’t perfectly centered. For sheet metal, step drill bits (the Irwin Unibit is the standard) let you drill multiple hole sizes without changing bits and produce clean holes without the burring that standard twist bits can leave in thin material. For holes larger than 1/2 inch in thick steel, a pilot bit and stepping up through intermediate sizes is more reliable than trying to plunge a large bit directly.
Drilling Technique for Steel
Center punch before drilling — the punch dimple gives the bit a starting seat and prevents it from skating across the work surface. Apply cutting fluid: Tap Magic for steel and stainless, regular cutting oil for softer metals. The cutting fluid cools the bit and lubricates chip evacuation; it’s the single biggest factor in bit longevity after bit quality itself. I’m apparently someone who skipped cutting fluid “for quick holes” and then replaced bits more often than I should have, and Tap Magic works for me while dry drilling never extended bit life the way I needed.
Speed matters: slower RPM for harder steels and larger diameters, faster for soft metals and small diameters. The rule is slower than you think. When the bit is cutting correctly you’ll hear a steady chip-cutting sound; squealing or smoking means too much speed or not enough cutting fluid. Let the bit cut at its own pace rather than forcing it — pressure that goes beyond what the cutting edge can clear causes heat buildup and dulling faster than anything else. Clear chips frequently on deep holes to prevent packing.
Safety
Eye protection is non-negotiable — steel chips travel fast and have sharp edges. Clamp the workpiece solidly; an unsecured steel part that grabs the bit and spins can cause serious injury. Gloves are appropriate for handling sharp-edged steel pieces but some metalworkers avoid gloves when drilling because a glove caught in a spinning bit creates its own injury risk — make your own judgment based on the specific task. Keep the work area clean of chips that can cause slipping.
Care and Storage
Wipe bits clean after use and store in a drill index — the individual slots protect cutting edges from contact damage. If a bit dulls, it can be resharpened on a bench grinder with practice; a properly sharpened HSS bit outperforms a half-dulled new bit. For cobalt bits, sharpening requires care to avoid overheating the tip during grinding, which defeats the purpose. A light oil coat on stored bits prevents rust in humid shop environments. Inspect bits before use for chipped or missing carbide, cracked shanks, or significant wear — drilling with a damaged bit is a safety risk and produces poor results.
David Chen is a professional woodworker and furniture maker with over 15 years of experience in fine joinery and custom cabinetry. He trained under master craftsmen in traditional Japanese and European woodworking techniques and operates a small workshop in the Pacific Northwest. David holds certifications from the Furniture Society and regularly teaches woodworking classes at local community colleges. His work has been featured in Fine Woodworking Magazine and Popular Woodworking.
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