304 and 316 stainless steel have a hardness of around 150–200 HBW — not particularly hard. A 36 mild steel comes in around 120 HBW. On paper, the difference shouldn't be dramatic. In practice, stainless is significantly harder to drill than mild steel, and the reason is work hardening.
Austenitic stainless (304, 316, 321) work-hardens rapidly under cold-working. When a drill rubs instead of cuts — when feed pressure is too light, when the drill dulls and starts skating over the surface instead of cutting through it — the material beneath the drill tip cold-works instantly and becomes significantly harder than the surrounding metal. The next pass of the cutting edge hits that work-hardened zone and wears faster, which causes more rubbing, which causes more work hardening. It's a failure cascade.
The fix isn't slower speeds. The fix is aggressive feed combined with correct SFM — keeping the cutting edge actively engaged and removing material before the work-hardened layer builds up.
For HSS cobalt drilling 304 or 316 stainless, the correct surface feet per minute range is 25–40 SFM. This is slow — significantly slower than the 80–100 SFM you would use on mild steel. The reason is heat: stainless has low thermal conductivity (about 16 W/m·K for 316, compared to 50 W/m·K for mild steel), so heat generated at the cutting edge dissipates slowly. Too high an SFM and the edge burns. Too low, combined with light feed, and you invite work hardening.
For a 1/2" cobalt drill in 304 stainless, 30 SFM translates to approximately 229 RPM. For a 1/4" drill at the same SFM: 458 RPM. These are slow by comparison to general machining speeds, and they feel slow on a drill press — but the combination of low SFM and aggressive feed is what produces clean holes in stainless.
Feed rate matters as much as SFM. In stainless, a firm, consistent feed — enough that you feel the drill cutting, not skating — is essential. If your drill sounds like it's singing or whining rather than cutting with a steady chip sound, increase feed pressure. The material will work-harden and destroy the edge if the drill isn't continuously removing material.
A 135° split-point geometry is the correct choice for stainless steel drilling. The split point eliminates the dead center of the chisel edge — the portion of a standard 118° drill that doesn't actually cut but instead extrudes material outward. Eliminating the chisel walk reduces the thrust force required to enter the material and reduces the tendency to work-harden at drill entry.
Standard 118° drills can drill stainless, but they require significantly more thrust to engage, which translates to more operator pressure on a manual drill press or higher thrust forces in CNC — both of which increase the risk of work hardening at the drill tip. The 135° split point is worth the investment for any shop that drills stainless regularly.
Web thinning on resharpened drills is especially important for stainless work. As a drill is resharpened shorter, the web (the core of the drill at the tip) gets thicker proportionally. A thicker web means more chisel edge, which means more thrust and more work-hardening risk. Any cobalt drill used for stainless should be web-thinned during resharpening once it gets significantly shorter than its original length.
Coolant is more important in stainless drilling than in most other materials. The combination of low thermal conductivity and the work-hardening problem means heat management is critical. Water-soluble coolant applied at the cutting zone — not just flooded onto the outside of the workpiece — extends tool life substantially.
For manual drill press work in stainless, a cutting paste or tapping fluid applied directly to the drill tip before each hole is a practical alternative to flood coolant. Sulphurized cutting oil works well; paste-type products also work. The goal is to get lubricant into the cutting zone, not just to cool the outside.
In CNC applications, through-coolant drills designed for stainless are available and produce significantly better results in deep holes than flood coolant from outside — but for the typical job shop drilling 1/4" to 1/2" holes to moderate depth, flood coolant or manual oil application is sufficient.
Standard M2 HSS does not hold up in stainless drilling. The work-hardening problem combined with low thermal conductivity generates cutting temperatures that exceed what M2 can sustain at the edge. M35 cobalt (5% Co) is adequate for light stainless work; M42 cobalt (8% Co) is the correct grade for any shop doing regular stainless drilling. The higher cobalt content gives better red hardness — the ability to maintain edge hardness at elevated temperatures — which directly translates to more holes per edge in stainless.
A resharpened M42 cobalt drill returns to the same geometric specification as when new, which means you're not sacrificing the work-hardening resistance when you resharpen. A standard M2 drill reground by an inexpensive service shop may produce a geometrically correct angle but won't have the heat resistance you need in stainless — the metallurgy is in the steel, not the grind.
Before drilling stainless: confirm drill is M35 or M42 cobalt, 135° split point. Set SFM at 25–40 (calculate RPM from your drill diameter). Apply cutting oil or paste to the drill tip. Plan for aggressive, continuous feed — no pecking unless depth requires chip clearing.
During drilling: listen for the cutting sound. Smooth chip sound = correct feed. Singing or squealing = increase feed. Any hesitation or backing off will start work hardening. Chip color: light silver to straw is acceptable. Blue chips mean your SFM is too high or your feed is too light.
After drilling: check hole entry and exit for burrs. Stainless burrs can be significant — a deburring tool or file is standard procedure. Inspect the drill tip. In stainless, edge wear shows up as a bright, polished land behind the cutting edge rather than the dull, smeared appearance you see in mild steel wear.
Work hardening punishes a dull edge faster in stainless than almost any other material. Keep your M42 cobalt drills resharpened to spec and stop throwing away tooling that still has life in it.
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