The powder from the previous step is still far too coarse — think beach sand. A magnet needs powder hundreds of times finer. That is the job of the jet mill, a machine that grinds without ever touching the powder.
How it works, in one sentence
High-pressure nitrogen gas is blasted through nozzles in a round chamber. The gas accelerates the powder particles to more than the speed of sound, and the particles smash into each other until they are just a few microns across — far thinner than a human hair.
No grinding tools
Because the particles break themselves against each other, there are no balls or blades to wear out — and nothing to contaminate the powder. The expanding gas also keeps everything cool, which suits this heat-sensitive alloy.
Size matters
A classifier wheel at the top of the mill spins at a set speed and lets only the right-sized particles pass. Too coarse, and the grains will not align well later; too fine, and they are impossible to handle safely. Good production powder sits in a tight window around three to four microns.
The big enemy: oxygen
This is where the powder becomes dangerous. Every fresh particle surface wants to react with oxygen, and really fine powder can burst into flames on contact with air. That is why the mill runs inside a sealed nitrogen loop with oxygen sensors watching the gas, and why the powder stays under inert gas all the way to the pressing step.
A tiny additive helps
A small amount of organic lubricant is added during milling. It coats every particle, blocks oxygen, and helps the powder flow and align better later — then burns off cleanly during sintering.
The micron powder is now ready to be given a direction — that happens in the next step, field pressing.
Read on: field pressing → · how NdFeB is made →