To get a smooth finish in hard plastics like acrylic with your CNC router, use a low helix spiral end mill. The modified cutter angle produces a better finish in plastics than standard spiral bits. Downcut bits pull chips downward, producing a clean top edge in your parts. Consequently, the chips accumulate in the cut path and should be removed to prevent bit breakage. Low helix bits require slower feed speeds than normal "high helix" bits.