Ductile-Regime Machining of Glass Using the Micromilling Process

Glass is a homogeneous material with amorphous crystal structure that is produced through the rapid cooling of its molten state below the glass transition temperature. Glass exhibits many excellent mechanical and physical properties, and it is widely used in automotive, communications, optics, electronics, architectural, and biomedical industries. For certain applications such as DNA microarrays, glass components with microfeatures are typically produced using a combination of photolithography and etching processes, which is generally time consuming and can involve hazardous chemicals. It would be ideal to fabricate some glass devices through mechanical micromachining for some rapid prototyping applications of glass-based devices, but the brittle nature of glass makes machining difficult. The machined surface is usually fractured and requires additional finishing processes that are costly and time consuming. Fortunately, it is found that the glass can be machined in a ductile regime under certain controlled cutting conditions. Machining in the ductile regime can produce continuous cutting chips. For micromilling to be used in the manufacturing of glass-based devices, further machining research is required to find optimum cutting configurations to produce high quality micro-scale features. It is known that the cutting regime transition from brittle to ductile cutting regimes is attributed to the effect of pressure and temperature in the cutting zone. The transition has also been correlated to the undeformed chip thickness. However, the mechanism behind ductile regime machining still cannot be fully explained. In this study, the effect of tilt angle on cutting regime transition has been studied in micromilling crown glass with a micro-ball end mill. Straight glass grooves were machined in a water bath by varying the tool tilt angle and feed rate, and the resulting surface was characterized using a scanning electron microscope and a profilometer to investigate the cutting regime transition. In characterizing the cutting regimes in glass micromilling, rubbing, ductile machining, and brittle machining regimes are hypothesized according to the undeformed chip thickness. In addition, mechanistic stress and temperature models are used in conjunction with experimental data to predict the stress and temperature information in glass micromilling in order to provide insight into why ductile machining happens. For the conditions investigated in this study, a 45¡ tool tilt angle was found to produce the highest ductile machining-related feed rate, 0.32 mm/min, and the best surface finish (less than 60 nm Ra) for feed rates less than 0.32 mm/min. The specific cutting energy relationship is determined based on the experimental force data and the effective undeformed chip thickness, which is derived based on the surface roughness measurements. The predicted stresses indicate that the 45¡ tilt angle easily leads to ductile cutting by increasing the glass fracture toughness while comparing with the performance under the other tilt angles (0¡, 15¡, 30¡, 60¡). The temperature rise is estimated negligible under the investigated micromilling conditions. This study offers a better understanding in optimizing the glass micromilling process, and it is expected that the occurrence of the glass ductile-brittle cutting regime transition will be elucidated based on the advances in glass material properties understanding and milling process modeling