Stainless is difficult for three specific reasons: it work-hardens, it has poor thermal conductivity (heat goes into the tool, not the chip), and austenitic grades are gummy at low speed. Everything below follows from those three facts.
Real cutting data for ISO M
Aggregated from the published data of every stainless-capable insert we stock — 1695 insert records:
| Parameter | Full published range | Typical (median) |
|---|---|---|
| Cutting speed vc | 60 – 250 m/min | 85 – 200 m/min |
| Feed fn | 0.01 – 1.20 mm/rev | 0.10 – 0.30 mm/rev |
| Depth of cut ap | 0.05 – 10.30 mm | 0.5 – 3.1 mm |
Duplex (2205) and super-duplex sit at the lower half of that speed range. Martensitic stainless (410, 420) machines more like a P-group steel and can run faster.
Grades that work
Our most-stocked stainless grades: AP200U, AC200M, AC100M, AP200M, AP301M, AP251U, AP403M, AP351U, AP100S, AP100U.
These are mostly PVD-coated or fine-grain substrates, because stainless needs a sharp, tough edge rather than a thick thermal-barrier coating. Using a heavy CVD steel grade on 304 is the classic mistake — the coating is too thick, the edge is rounded, and it rubs instead of cutting.
The four rules
1. Never cut in the work-hardened layer
Every pass leaves a hardened skin. If the next pass is shallower than that skin, you are machining hardened material and wear accelerates dramatically. Keep ap above roughly 0.2 mm at all times, and vary ap between roughing passes so the notch does not form at the same line.
2. Keep the tool moving — never dwell
A stationary or feeding-slowly tool work-hardens the surface instantly. Avoid pausing in the cut, avoid finishing with zero feed (spring passes), and programme lead-in and lead-out arcs.
3. Use positive geometry and a sharp edge
For boring and finishing thin-walled stainless parts, positive inserts (CCMT, DCMT, VBMT) cut with far less force and much less chatter. Reserve negative inserts for roughing on rigid setups.
4. Run at a decent speed
Counter-intuitively, going slower makes it worse: below roughly 60 m/min austenitic stainless forms a built-up edge, which then tears away pieces of the cutting edge. If you see BUE, raise vc rather than lowering it.
Chipbreakers
Stainless chips are tough and stringy, so you need an aggressive, sharp breaker aimed at low-to-medium feed. The geometries most often specified for M in our catalogue: PC2, MM4, MC3, PB1, MC4, KC2. If chips come off as long ribbons, you are below the breaker’s feed window — raise feed or move to a lighter geometry.
Coolant
Use it, and use plenty. Stainless generates heat at the edge and the workpiece holds it. Emulsion at 6–10% delivered at high pressure to the rake face gives noticeably better tool life than dry. High-pressure coolant also helps break the chips, which matters more in stainless than in any other group.
Troubleshooting
| Symptom | Cause | Fix |
|---|---|---|
| Notch wear at depth-of-cut line | Work-hardened layer | Vary ap between passes; tougher PVD grade |
| Built-up edge, torn surface | Speed too low | Raise vc above 80 m/min; sharper polished grade |
| Long stringy chips | Feed below breaker window | Raise fn; lighter chipbreaker |
| Poor finish, chatter | Force too high / weak setup | Positive geometry, smaller nose radius, lower ap |
Where to start
- General 304/316 turning: CNMG 120408 with an M grade, vc 120–160, fn 0.15–0.25, ap 1–3
- Profiling and undercuts: DNMG 150608
- Boring and finishing: CCMT 09T304, positive geometry
Related
- ISO 513 material groups explained
- Carbide grades explained
- Chipbreaker selection guide
- Cutting parameters calculator
Standards and further reading
The material group definitions used throughout this site follow ISO 513. The manufacturer links below are independent technical references.
For a worked example for 304 stainless, see this article.