Where the boundaries are
The hardness of the workpiece decides the process, because it decides what the cutting edge has to survive.
| Hardness | Turning | Milling | Alternative |
|---|---|---|---|
| Up to 45 HRC | Coated carbide, tougher grade | AlTiN or AlCrN carbide | Carbide is usually sufficient |
| 45 to 55 HRC | CBN or coated carbide | AlCrN hard milling carbide | Hard milling competes with grinding |
| 55 to 62 HRC | CBN or ceramic | AlCrN hard milling with rigid setup | Grinding frequently wins on cost |
| Above 62 HRC | Ceramic or CBN, limited | Not economic | Grinding or EDM |
Hard milling demands rigidity above all
A hard milling tool cuts a material nearly as hard as the tool itself, so the cutting forces are high relative to the edge strength available. Any vibration immediately becomes edge chipping rather than gradual wear.
The practical requirements are a short tool, a rigid holder such as shrink-fit, a stable workpiece setup, and a consistent tool engagement so the load does not fluctuate. Under those conditions hard milling works well; without them it destroys tools quickly and unpredictably.
CBN and ceramic turning
CBN is the standard choice for turning hardened steel where dimensional accuracy and surface finish both matter. It is far harder than carbide and retains its hardness at the temperatures involved, which is what allows a hardened surface to be turned rather than ground.
Ceramics suit continuous, high-speed turning of hard material where there is no interruption. They are more brittle than CBN, so an interrupted cut or a skin on the workpiece frequently rules them out.
Surface finish and the white layer
Hard turning can leave a thin thermally affected layer, sometimes called a white layer, where the surface has been heated above the tempering temperature and re-hardened. In fatigue-critical components that layer is a defect rather than a finish.
Keeping the cutting edge sharp, avoiding excessive speed, and ensuring coolant or a controlled dry condition all reduce the risk. Where the specification forbids a white layer, the requirement should be stated at quoting because it changes the parameters and frequently the process.
Frequently asked questions
At what hardness should I switch from carbide to CBN?
Around 45 HRC for turning. Below that a tougher coated carbide grade is usually more economical; above it, CBN lasts considerably longer and holds a better surface finish. For milling, AlCrN-coated carbide remains workable to about 60 HRC.
Is hard milling better than grinding?
It can be, because it removes a process step, avoids the coolant and sludge of grinding, and can produce a better surface. It requires much greater rigidity, so it wins where the setup is stiff and the geometry allows a short tool, and loses where the cavity is deep or the tool has to be slender.
Why do hard milling tools chip instead of wearing?
Because the workpiece is nearly as hard as the tool, so the cutting force is high relative to the edge strength. Any vibration converts directly into chipping. Rigidity at the holder and workpiece, and consistent engagement, are what prevent it.
What is a white layer and why does it matter?
A thin re-hardened surface layer produced when hard turning heats the surface above its tempering temperature. On fatigue-critical parts it is a defect because it can crack and initiate failure. Avoiding it needs a sharp edge and controlled parameters, and the requirement should be stated at quoting.
Can you supply tooling for hardened steel above 60 HRC?
Milling becomes uneconomic above roughly 60 HRC and grinding or EDM is the practical route. For turning, ceramic and CBN inserts remain available at higher hardness within limits. Tell us the hardness and the operation and we will say honestly which process applies.