Below about a millimetre of tool diameter the usual rules stop transferring. Runout becomes the process, chip load stops scaling, and the tool bends before the part does. Here is what changes, and what it means for a part you are about to have cut.
On a 6 mm end mill a few microns of spindle runout is noise. On a 0.2 mm cutter it is a large fraction of the tool diameter, so one flute takes the whole chip and the other takes none. The tool does not wear, it breaks. This is why micro milling is a spindle-and-holder problem long before it is a feeds-and-speeds problem.
Every cutting edge has a radius, and below a certain chip thickness the tool stops cutting and starts ploughing — rubbing material aside, generating heat, work-hardening the surface it is about to cut again. Scaling a conventional chip load down with the cutter diameter walks straight into that. The feed per tooth has a floor set by the edge, not by the diameter.
Stiffness falls with the cube of length over diameter, so a slender cutter reaching into a deep cavity deflects away from the wall and leaves it thick at the bottom. The usual fix — a lighter cut — runs into the chip-load floor above. What actually works is shortening the reach, which is a decision about how the cavity is split, made long before anyone writes a toolpath.
Cutting cores and cavities directly in fully hardened steel removes a heat-treat cycle and the distortion that comes with it, and it holds a finish that would otherwise need polishing. It does not replace EDM everywhere — a sharp internal corner smaller than the smallest practical cutter is still an electrode. Knowing which features fall on which side of that line is most of what a quote depends on.
A feature you cannot measure repeatably is a feature you cannot hold, and contact probes deflect thin walls they are trying to measure. Optical and scanning methods have their own limits. Any conversation about micron-level work that does not also cover how the result will be verified is an incomplete one.
Got a feature you are not sure is cuttable? Show Marve the part or send it to the team, and we will tell you which side of the hard-mill / EDM line it falls on — and whether the dimension you need can be measured repeatably once it exists.