I needed to cut down a few tool holders, the first one was quite hard but a drill would make some impression so I drilled a number of divots all to the same depth (as near as I could manage) then used an angle grinder to take it down to the bottom of the divots. I was suprised how 'true' the sureface was doing it that way.
Then I got smart and realised the tool holders could be held in the lathe tool post, but on their side, then I chopped the next couple of holders with a fly cutter I made, but it was really hard on the HSS tools in the fly cutter.