I have done both method, the on wheel skimming and machine shop (its using lathe machine, turning process they call it, milling still possible but not economical and complicated for this kinda job).. Actually both are based on the same principle, only they have design simplified jig for skimming purpose so that can do it on wheel, no need to cabut the disc.. easier what, makes the worksyop happy.. hehe..
what i can say, the machine shop can deliver better result but subject to the operator skill and the machine parameters la.. the on wheel skimming (while i did it) is not giving a very "clean" surface, lotsa scratch / grooves on the disc n the cutting bit have goyang2 abit, another thing is the operator also tend to cut too much, wont happen to a well maintained lathe machine and skilled operator..
in order to get better/ cleaner surface, need high speed turning and slow bit feeding rate, not easy to do for on wheel skimming right??
but still, no harm in both method, as long as it serve the purpose with acceptable quality, why we need to worry..