Drilling 304 Stainless Without Work Hardening It

Five to ten holes, then a dead drill. If you've done much drilling 304 stainless, that probably sounds familiar.

Why Do Drills Burn Out So Fast in 304 Stainless?

Because the drill is rubbing instead of cutting. Rubbing makes heat, heat work-hardens the bottom of the hole, and the next revolution has to cut through harder material with a duller edge.

That's the r/Machinists post in a nutshell. The shop was drilling 303, 304 and 316, 4–6 mm holes at 5–7×D deep, at 45 SFM and 0.003" per tooth, pecking 0.5×D with full retract. Cobalt and HSS both burned out after 5–10 holes. They had flood coolant but no through-spindle coolant.

One reply said to find the fewest pecks that get the job done, because every peck work-hardens the bottom of the hole. Another said the only production success he'd had was carbide, preferably with through coolant. A third said 45 SFM is too fast for HSS and to aim nearer 20.

What drilling 304 stainless needs

Regal Cutting Tools' stainless drilling guide describes the same mechanism. If speed is too high and feed too low, the drill rubs, makes heat and hardens the bottom and sides of the hole just before they're cut. Regal says not to let the tool dwell, to keep the drill at full feed when it exits and re-enters the hole, and to flood coolant throughout. It also suggests drilling deep holes in stages. Put that next to the pecking advice and the common thread is rubbing. At 5–7×D the chips still have to come out, so the aim is fewer, clean pecks at full feed. Regal also reads chip color: blue is the goal, while brown or black means too much heat.

So feed matters more than people expect. In a separate r/Machinists thread on 304, one machinist put it simply: you need to be pulling a chip, and if you're pushing hard without one, stop and get a new bit.

Tooling and coolant

Regal says cobalt drills are worth the cost in stainless because they keep their hardness under heat. For production runs, the reply above favored carbide. Coolant strength matters as well, so check whether a rich coolant mix is a problem. Tapping comes next, and stainless brings its own trouble there: see why 316 galls on a tap where 304 doesn't. Still choosing a grade? Read when 316 is worth it over 304.

Ordering 304 parts

Stainless 304 is on our Materials page. The CNC capability page lists what it checks automatically, starting with "Hole diameter, depth-to-diameter ratio and flat-bottom detection". If you have deep, small holes in stainless, ask us when you quote.