Hi Ard - There is an old engineering saying that covers a lot of situations. Figure out how stiff it has to be, then make it 10x stiffer. So there are two ways to estimate the machine stiffness as I see it:
1) Look at successful machines for the job and make your machine the same stiffness. say 10-200N/um stiffest machine I found was 650N/um
2) Looking at FSwizard pick a 10mm slot mill - cutting force 12kg in aluminium. 14000rpm 650mm/min CL=0.026mm power = 0.87kW torque = 0.59Nm. So to cut a chip 0.026mm wide we need it to deflect <0.0026mm say. So 120N (12kgf) at 3um and 40N at 1um which probably would work. Do same in mild steel
rpm 2640 T=0.59Nm feed 60mm/min CL=0.0114mm so need ~0.0014mm to cut. tool load is still 12kgf so 120N to move 1um which is about what a new VMC is say around 150N/um... so have a play with that and see if it makes sense. Peter