did you put a groove in the ER collet shaft for the circlip or not?
no.... my thoughts where that the tight fit (bearings n housing) would hold it all in place, iv had no problems so far using the double row a/c... i did have an issue with one my C8 spindle bearings (cheap skate bearings/bad fit on the shaft) resonating the inner race around the shaft at certain speeds so i used a bit of epoxy on that and its been ok so far.... now i think about it i guess the bellville washers become redundant once the glue has set and when the bearings wear i might get some slop ... it may need some further thought

What RPM do you use the flycutter at?
hi adam... iv not measured rpm's for a while however im pretty sure i skim acetal at 1500rpm and aluminium at less (maybe 1000rpm) thats with a 30mm radius fly cutter

with aluminium i have to be pretty careful (light cuts) or i get resonance patterns but im pretty certain thats my gantry sides and not my spindle

i should really do a test to see how low i can take the rpm and still cut with the fly cutter but iv never seen the need as iv allways got by at those speeds