it is wholly possible to disassemble the front brake disc( rotor) and get the steel disc surface ground, i did it on my 1973 XS
at the same we set the disc up on the CNC mill and drilled the 8.5mm holes
zoom in the picture you will see the disc work
the advantage to getting the disc absolutely flat, is much better braking, we took around 0.006'' of each side ( 6/1000s of an inch) the linit is much more than that, in fact most modern discs are only 5mm thick