Best Drill Bits for Stainless Steel: HSS, Cobalt, or Carbide?

Stainless steel has a reputation for eating drill bits, and that reputation is earned — but mostly by machinists using the wrong tooling or the wrong technique. The material itself isn't mystically difficult. It just punishes two specific mistakes: wrong drill selection and wrong feeds and speeds. Get those right and stainless drills cleanly.

Here's the definitive answer on drill material selection for stainless, and why the answer is usually cobalt HSS rather than carbide.

Why Stainless Is Hard on Drills

Austenitic stainless steels — 304, 316, and the other 300-series grades that make up the majority of what shops encounter — have two properties that make them difficult to drill:

Work hardening: 304 and 316 work-harden rapidly at the cutting zone. When the drill rubs rather than cuts — whether from dull geometry, inadequate feed, or wrong speeds — it hardens the material immediately ahead of the cutting edge. The next pass of the drill encounters a harder surface than the one it just left. This cascades quickly: the drill dulls faster, cuts less effectively, and hardens the stainless more, until the bit is effectively rubbing hardened steel with a dull edge.

Low thermal conductivity: Stainless conducts heat about 1/4 as well as mild steel. Heat generated at the cutting zone stays at the cutting zone instead of dissipating into the workpiece. This concentrated heat accelerates drill wear and can cause the drill tip to soften if not managed with coolant.

Standard HSS: Usually Not Enough

Plain M2 HSS drills will cut stainless — briefly. In practice, tool life is short enough that most shops find standard HSS uneconomical for regular stainless work. The drills wear quickly, need frequent replacement or regrinding, and the added cost of managing more frequent tooling turnover usually exceeds the price premium of cobalt.

There are situations where standard HSS is acceptable in stainless: a single hole, slow speeds, a lot of lubrication, and the operator is paying close attention to any sign of rubbing. For one-off work, it can get the job done. For production, it's the wrong choice.

M42 Cobalt: The Right Answer for Most Shops

M42 cobalt HSS — 8% cobalt alloy — is the correct drill material for regular stainless work in most job shops. The cobalt raises red hardness: the ability to hold a cutting edge at elevated temperatures. At the cutting zone in stainless, where heat concentration is high, M42 maintains its edge where M2 would soften.

The practical difference in tool life is significant. Where an M2 drill might last a handful of holes in 304 stainless before needing regrinding, an M42 cobalt drill will last several times longer under the same conditions. The price premium — cobalt drills run roughly 2–3x the price of equivalent quality HSS — pays back quickly in reduced replacement cost and less downtime for tool changes.

What to Look for in Cobalt Drills

Not all cobalt drills are equal. Verified M42 grade (8% cobalt) from a reputable manufacturer — Precision Twist, Norseman, Chicago-Latrobe, Dormer, OSG — is what you want. There are import drills labeled "cobalt" that are M35 grade (5% cobalt) or are cobalt-coated rather than cobalt-alloyed. These are not the same. When in doubt, buy from a distributor who can confirm the grade.

Split-point geometry in cobalt drills is a meaningful advantage for stainless. The split point eliminates the chisel edge, dramatically reduces thrust, and allows the drill to start without a center punch (although center punching is still good practice for accurate location). The reduced thrust means less work hardening at the entry point.

Carbide: For Production Volume, Not Job Shop Use

Solid carbide drills exist for stainless and they produce excellent results — in the right application. The right application is high-volume CNC work with rigid fixturing, through-coolant capability, and stable, controlled conditions. In those conditions, carbide's wear resistance delivers dramatically longer tool life than cobalt and justifies its much higher cost.

In a job shop context — manual or semi-CNC, varied setups, different materials day to day — carbide drills are fragile and expensive. Carbide is brittle. Any lateral load from workpiece movement, any vibration from an unstable setup, any interrupted cut can snap a carbide drill. The cost of a snapped carbide drill is substantial, and extracting one from a stainless workpiece is a painful operation.

Unless you're running production CNC work with consistent setups, carbide in stainless is usually the wrong call. Cobalt HSS is tougher, more forgiving of imperfect conditions, and resharpeable — carbide is not economically resharpened in a job shop setting.

Feeds, Speeds, and Technique

Drill selection matters, but technique seals the result. For stainless with cobalt HSS:

• SFM: 30–50 SFM. Lower than mild steel. High SFM in stainless generates heat faster than even cobalt can handle.
• Feed: Aggressive relative to RPM. A common error is feeding too lightly — rubbing the surface rather than cutting. Feed firmly enough to produce a visible chip on every revolution.
• Cutting fluid: Straight cutting oil, not water-soluble coolant. Sulfurized cutting oil provides the lubrication at the cutting zone that stainless demands to prevent built-up edge.
• No hesitation: Keep the feed moving once you start cutting. Stopping the feed while the spindle turns rubs the cutting edge against the hardened zone. Drill until you break through, then retract.
• Peck drilling in deep holes: Retract regularly to clear chips, which also helps cool the bit. Don't let chips pack in the flutes in stainless — they won't clear themselves.

The Short Answer

For most shops drilling stainless: use M42 cobalt split-point drills at 30–50 SFM with sulfurized cutting oil and a firm, consistent feed. That combination handles 304, 316, and most other austenitic grades reliably. Carbide is for CNC production. Standard HSS is for everything except stainless.

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Cobalt drills are resharpeable, just like HSS. MachinistPost resharpens both HSS and cobalt drill bits by mail, restoring split-point geometry, lip relief, and chisel edge on a WinsloMatic. Mail us your worn stainless-work drills and get them back sharp.