Yep,

John S has the rub of it there. Cast is better then extruded any day of the week. Need to get chips out of the way fast (Mine is a vacuum dust shoe that pulls the chips out of the area). Most days I am cutting Perspex or an Acrylic based material and it really depends on what type and what your cutting with. To fast will ruin the finish, too slow and you get re-cutting and heat build up. 6mm single flue I would be running at about 3mm depth and with my spindle (another thing to consider) at around 3m/min maybe more (haven't done 6 mm in a while). Right now I am running a 2mm ball nose 2 flue at 1850mm/min with a max depth of 3.25 mm (cutting a 2.5D bas-relief).

Good luck

Michael