Brayks, why it mill model and zirconia on the wet mode ?
regarding zirconia; you will find some controversy on this, mainly attributed to the quality of the mill and post machining drying requirements, but generally you will see longer tool life and better finishes with wet milling without vacuum systems and the machine damaging abrasive dust (regardless of vac system) that is the by product of dry milling. Cycle times can be a bit longer than dry milling but, production requirements permitting, with adequate drying, wet milling is a very good method of machining zirconia.
Regarding polyurethane machining:
Urethanes have much lower thermal conductivity than metals, so heat generated by cutting tools stays close to the tool (destroying the tool) and raises the urethane temperature quickly (above 400 deg F. melting can occur) Also, heat generated by machining causes the part to expand so when the part cools, it shrinks and can end up under sized. If excess heat is generated and not dissipated by coolant, the result will be gumming, poor finishes, and poor dimensional control.
When machining anything it is important to maintain the correct feed and speed such that any heat generated is in the chip and not the tool or work-piece. When done correctly you can actually grab the tool or workpiece and they will not be hot. We work with our customers (dental and non-dental) to help them understand this and how to effectively set the correct cutting parameters/machining strategies for just about any material, cutter and material removal volumes. Its pretty wild when you see a 1" end mill racing around a billet of stainless steel at a high feed rate, with no coolant, sparks and chips flying and when finished the part and tool are just warm to the touch.
These videos show hard materials (hard milling) programmed with Mastercam and Delcam PowerMILL (the CAM products we distribute in the non-dental side of our business). Not dental related I know, but still kinda of fun and interesting maybe?