Hard milling versus EDM
Hard milling has replaced EDM for many mould applications because it removes the electrode and the separate sinking operation. The trade-off is that the cutting forces and the heat are concentrated on a small, slender tool.
The deciding factor is usually the depth-to-diameter ratio of the cavity. Where the tool can remain stiff, hard milling is faster and less costly. Where the reach required makes the tool too slender to cut reliably, EDM remains the practical route.
| Situation | Better route | Reason |
|---|---|---|
| Shallow cavity, large radius | Hard milling | Tool stays stiff; no electrode needed |
| Deep narrow rib | EDM | Required reach makes a milling tool too slender |
| Sharp internal corner | EDM | A rotating cutter cannot produce a true sharp corner |
| Fine surface texture requirement | Hard milling then polish | Milling provides the base finish; EDM leaves a recast layer |
| Hardened steel above 60 HRC | EDM or grinding | Milling tool life becomes uneconomic |
Long reach without chatter
A long-reach cutter is a slender cantilever, and the milling equivalent of the boring bar problem applies. The countermeasures are a reduced radial engagement, a climb-milling path that keeps the tool loaded consistently, and a variable-pitch geometry that breaks the regularity of the cutting impulse.
Shrink-fit holders are particularly valuable here because they add no nut overhang and hold the shank along its whole length, which is the stiffest available connection between the spindle and a slender tool.
Choosing the ball nose geometry
The ball nose cutter is the workhorse of cavity finishing because it can follow a three-dimensional surface. Its limitation is that cutting speed falls to zero at the centre of the ball, so the material at the tool centre is rubbed rather than cut.
That rubbing is why ball nose tools wear at the tip first and why a finishing strategy that tilts the tool, or uses a smaller step-over, extends tool life. Where the geometry allows, a toroidal or radius cutter produces a better finish and a longer life than a true ball nose at the same step-over.
Coatings for hardened steel
AlCrN is the usual choice for hard milling because it retains hardness and oxidation resistance at the temperatures a hardened-steel cut generates. AlTiN is used where the operation runs dry or with minimum-quantity lubrication.
Coating alone does not make a hard milling tool work. The edge preparation, the substrate grade and the rigidity of the setup each contribute, and a well-prepared edge in a stable setup outperforms a harder coating in a flexible one.
Frequently asked questions
Can I machine hardened steel instead of using EDM?
Yes, up to roughly 60 HRC, provided the tool can stay stiff enough. Hard milling is faster and removes the electrode, but where the cavity is deep and narrow the required reach makes the tool too slender and EDM remains the practical answer.
What coating is best for hard milling?
AlCrN for coolant or high-temperature work, because it retains hardness and oxidation resistance at the temperatures involved. AlTiN suits dry cutting and minimum-quantity lubrication. The edge preparation matters as much as the coating.
How do I stop long-reach cutters chattering?
Reduce radial engagement, use a consistent climb-milling path so the load stays steady, choose a variable-pitch geometry, and use a shrink-fit holder to remove nut overhang. Stiffness at the holder is the largest single gain available.
Why does my ball nose cutter wear at the tip?
Because cutting speed falls to zero at the centre of the ball, so the material there is rubbed rather than cut. Tilting the tool relative to the surface, or reducing the step-over, moves the contact away from the dead centre and extends tool life.
Do you supply radius cutters as well as ball nose?
Yes. A toroidal or radius cutter frequently produces a better surface finish and a longer tool life than a true ball nose at the same step-over, because the contact geometry and the cutting speed at the edge are both more favourable.