Designing a Metal milling bushless Spindle

[totts]

Hi, yes it is a bit vage! My cnc isnt highly robust so roughing cuts are usually a 6mm slot drill 0.5 deep at around 6000rpm, usually at 200mm/m. I do a lot of engraving so its usually running flat out, but finding i need higher rpm, so currently finding other solutions. There are calculations that can be used to find the apropriate speeds and feeds, but machine stability limits these.
I currently have a er20 set up as this will cope upto 13mm shanks which is ideal for 1/2" shanks.

Hope this helps.