Near-cryogenic machining of polymethyl methacrylate for micromilling tool development

Micromilling tools with a diameter of 22 micrometers were developed to machine polymethyl methacrylate (PMMA) for micro systems applications. Due to the small diameter of the tool, and therefore its slow cutting speed, the specific cutting energy of PMMA in the near-brittle state was needed so the micromilling feed could be estimated. To determine the specific cutting energy in the near-brittle state, PMMA was cooled with liquid nitrogen and machined with diamond tools under normal machining conditions. Cutting forces and surface finish were measured from room temperature down to -53 degreesC. It was found that as the temperature of the PMMA was reduced, the specific cutting energy increased linearly to approximately that of oxygen-free high conductivity copper. It was also found that the surface finish became rougher as the temperature decreased. Using these results, micromilling tools were fabricated using focused ion beam machining. The tools were used to micromachine PMMA electroforming molds with high precision, small features, and excellent surface finish. Using the feed indicated from the cryogenic machining and other tests, the micromilling tools did not break after extended periods of machining.