The disk brake caliper is mechanical from the git-go. These are small calipers for bicycles, ATV's, mini bikes, etc. I purchased several varieties and the one featured on the 4th axis happens to be for what is called a 'pocket bike', which is a ridiculously tiny motorcycle. They all have similar internal cam arrangements.
I found the quality of the calipers to be unacceptable, so I had to extensively modify it. So much so that in the future, I plan to use the brake pads, but fabricate the caliper from scratch.
The disk is cut from 2024 of 7075 aluminium sheet. I must emphasize that this is a consideration for mass and these disks are not intended, nor capable of acting as brakes to slow a load. The disk on the milling machine spindle is another 'pocket bike' item. It simply needed a small hole for the sensors and have its mounting hub removed. It is the smallest pre made disk I could find at 5.5" OD
The servo motor will *try* to maintain one count on an encoder, but that is not going to happen if the load is too great and the amps too little. I have 35amps available and I can force the motor off center by hand with less than a 12" lever. The normal dithering is tiny with my 1800 line encoder (7200 with quadrature), but 'tiny' is relative. Cutting the teeth on the GT3 timing pulley was done with a tiny 4 flute ball end mil ll at 28,000 RPM. If you have a chip load of .0002, even a .001 oscillation is significant.
My 'anticipated' issue is off center drilling of fairly large holes in stainless or tool steel. The force needed for that would be greater than it takes me to move the spindle off hold. This is a heavy unit intended for 24/7 production, so it needs to be able to handle tough jobs.