Best Drill Bits for Hardened Steel and Stainless

For stainless and many hard-steel jobs, a quality cobalt-alloy HSS bit is the practical first choice; truly hardened steel may require solid carbide, a rigid machine and application-specific cutting data. Before buying a bit, identify the alloy and hardness. “Steel” can mean soft low-carbon sheet, work-hardening stainless, a heat-treated bolt or hardened tool steel, and no single bit/setup is ideal for all four.

Quick selection guide for steel

Why cobalt works well for stainless steel

Cobalt is part of the high-speed-steel alloy, commonly sold as M35 or M42, rather than a surface coating. Its value is hot hardness: the cutting edge can retain useful hardness at temperatures that punish ordinary HSS. Bosch describes its cobalt drill-bit sets as intended for hard, abrasive materials and high drilling temperatures in stainless steel.

That does not make cobalt unbreakable. Harder bits tolerate less bending. Use the shortest practical bit, keep the tool square to the work, minimize chuck runout and do not pry sideways to enlarge a hole.

When carbide is appropriate

Solid carbide may cut materials that defeat cobalt, but it expects a rigid setup and controlled feed. A loose handheld drill, flexible workpiece or worn chuck can chip carbide immediately. If the work is safety-critical, expensive or of unknown hardness, a machine shop may be cheaper than broken tooling and a damaged part.

Carbide-tipped masonry bits are not interchangeable with precision solid-carbide metal drills. Buy tooling whose manufacturer explicitly lists the steel grade or hardness range you need to cut.

Point angle and starting on hard steel

A 135-degree split point can reduce walking and starting thrust. DeWalt’s official cobalt-bit specification lists a 135-degree split point for accurate starts in stainless and hard metal. Curved fasteners and damaged bolts are harder: flattening or spotting the start may require appropriate machine tooling and skill.

Technique for stainless and hard steel

1. Confirm the material and hardness. A file test can provide a clue, but it is not a certified hardness measurement.
2. Use rigid workholding. Clamp the work and support thin sections so neither the work nor drill can grab and rotate.
3. Choose RPM from the tooling maker’s chart. Diameter, alloy, bit material and cooling all affect surface-speed limits.
4. Apply a steady feed. The edge should cut chips rather than polish the surface. Do not compensate for a dull bit with extreme force.
5. Use suitable cutting fluid. Follow the bit, material and fluid manufacturer’s guidance and ventilation requirements.
6. Peck deep holes when appropriate. Withdraw under control to clear chips, stopping the machine before manual cleanup.
7. Stop if the edge is damaged. A chipped or blue, overheated bit is unlikely to improve by pushing harder.

Drilling a hardened bolt: important cautions

A broken bolt is an awkward, high-risk target because its surface may be curved, off-center and harder than the surrounding part. Protect threads and adjacent components. Start accurately, use a rigid guide when possible, and remember that a broken extractor or carbide drill can be harder to remove than the original bolt. If the assembly is valuable or affects vehicle, structural or machine safety, use a qualified machinist.

Safety around drill presses and metal chips

Wear suitable eye protection, secure the work with a vise or fixture, and keep hands away from rotating tooling. OSHA’s drill-press training guidance says workpieces should be clamped and warns against gloves, jewelry and loose clothing near rotating machinery. Stop and isolate the machine before adjustment or cleanup; use a brush or tool for sharp swarf, not bare hands.

Common reasons a bit will not cut steel

• the bit is dull, chipped or rotating backward;
• speed is too high and the edge has overheated;
• feed is too light, causing rubbing instead of cutting;
• the work surface has hardened after earlier rubbing;
• the claimed “cobalt” bit is only a colored coating or unsuitable geometry;
• the steel is harder than expected and needs different tooling or professional machining;
• runout, vibration or poor workholding prevents the edge from cutting consistently.

For ordinary aluminum, sheet and mild-steel projects, use the broader best drill bits for metal guide. It covers HSS, coatings, step bits and general selection without treating every steel job as hardened material.