The CNC mill (Tormach PCNC1100) is a hobby-level 4-axis machine with a maximum build volume of about 17″ (430mm) (X), 9″ (225mm) (Y), and 10″ (250mm) (Z). For these projects it has a precision of about 0.002″ (0.05mm) which is similar to that of the geometric data.
The STL file generated by ZBrush is imported into the CNC program (SprutCAM) that creates the tool-paths. Generally, there is a roughing operation using a 0.5″ (13mm) ball end-mill followed by 1-2 finishing operations using a 0.125″ (3mm) ball end-mill. The best milling strategies have been developed by much trial and error.
The tool-path TAP file is then transferred to the G-code mill controller software (Mach3) where tool offsets are set.
The urethane material (Reshape, NC Proof-board, Butter-board) is a high-density foam that is specially formulated for milling. It has cutting characteristics similar to wood without the difficulties of grain, water absorption, splitting or decay. The urethane material density that provides good surface quality and durability varies between 30-50 lb/ft3.
Small tooling marks (about 0.002″ (0.05mm)) left by milling are removed by hand using sandpaper. Hand carving is done to add fine details and remove material in locations the milling tool cannot access. The final surface has layers of lacquer, gesso, acrylic paint and varnish. The resulting sculptures are very durable even for outdoor use.