Let suppose, for the sake of arguement I have a 60mm dia brass disc, approx 1mm thick. Around the periphery, approximately 3mm in from the edge, I wish to cut 400 slots, each 2mm radially and 0.23mm wide.

To index the disc it will be mounted on a backing sheet of some description and attached to a stepper which will be 1/2 stepped to give 400 indexable locations.

So what to cut with? PCB routing cutters can get close to that level but would need careful depth control to get a consistent width. I've seen MicroCarbide endmills at 0.2mm on the web, but how realistic is that?

Laser, spark erosion, what?