Surface Finish of Ball-End Milled Microchannels ICOMM 2013 No.80 D. Berestovskyi 1 and W.N.P. Hung 2 1 Texas A&M University, USA; d.berestovskiy@gmail.com 2 Texas A&M University, USA; hung@tamu.edu Key Words: micromilling, surface finish, biocompatible materials, minimum quantity lubrication, electrochemical polishing. ABSTRACT This study develops micromanufacturing techniques to fabricate extremely smooth surface finish, high aspect ratio, and complex microchannel patterns. Computer controlled micromilling on a high speed machine system with minimum quantity lubrication is used to remove most materials and define a channel pattern. Assessment of microchannel is performed with optical microscopy, scanning electron microscopy, atomic force microscopy, and white light interferometry. Meso-scale milling confirms the validity of theoretical surface finish of ball-end milling, but surface finish in micro-scale milling is measured to be few orders of magnitude higher. Build-up-edge is reduced with optimally coated tool and milling in minimum quantity lubrication. The surfaces of milled microchannels are then further enhanced by subsequent electrochemical polishing process. When applying to 304, 316L stainless steel alloys and NiTi alloy, this hybrid technique can repeatedly produce microchannels with average surface finish in the range of 100-300 nm. A. INTRODUCTION Manufacturing of medical devices requires utmost precision and state-of-the-art quality control. Fluidic microdevices require small channels to transport precise doses of medicine or an exact amount of liquid to prescribed locations. Fabrication of microchannels on biocompatible materials is still a challenge when high aspect ratio, anisotropic profile, submicron surface finish, and controlled contour of the channel surface are required. Currently, manufacturing techniques such as laser machining and chemical etching are used to produce similar type of microchannels. However, laser can only produce microchannels with a flat bottom surface and an isotropic chemical etching process cannot produce high aspect ratio channels. In addition, chemical etching requires utilization of unique chemical solution for each material. This makes the process complex and time consuming. Conventional micromilling and microdrilling have the potential to be the most cost effective and efficient material removal processes. Depending on a system, micromilling could provide reasonable surface roughness, dimensional and geometric accuracy, and higher productivity comparing to other micromachining techniques. Also, different materials such as metal alloys, polymers, ceramics, and composites can be machined using the same milling cutters and setups [1]. However, conventional micromachining has much more challenges and constrains comparing to conventional macro machining. The challenges associated with micromachining rise from size effect of miniaturized cutting tools, work pieces, and overall process. When dimensions of a microtool and depth of cut are on the same order of magnitude as the grain size of the machined material, anisotropy of grain’s mechanical properties and its crystallographic orientation influence the micro cutting process, which is not the case for the conventional macro cutting [2, 3]. For micromachining the shear process at the tool tip is more complicated and depends on the degree of size effect, which is the effect observed when the depth of cut is about the same or smaller than cutting edge radius. In this case the rake angle has a high negative value, which leads to the serious increase in the shear force on the tool, surface roughness, elastic-plastic deformation, and the plowing during micromachining [4]. The plastic deformation of the machined surface results in more difficult separation of the material because material is work-hardened by increased dislocation density. Due to the large rake angle, the rise in the force results in a faster wear, an increase in the tool deflection and vibrations, and a build-up edge (BUE) formation on the micro tools [4, 5]. Very often, failure of microtools occurs when chip thickness is smaller than cutting edge radius, which is about 3µm or larger for microtools [6]. Several researchers have studied the critical chip thickness. Vogler et al [7] utilized finite element analysis to find critical chip thickness for micromachining of steel. According to their study, the critical chip thickness can be estimated as 20-30% of the cutting edge radius during micromechanical machining of the pearlite and ferrite steel. However, Shimada et al [8] determined that critical chip thickness is about 5% of the tool edge radius during micromachining of aluminum and copper. For microfluidic applications, surface finish of a channel is as important as the geometry of the channel profile. In a study of surface finish of micromilled cold worked SLD11 steel using TiAlN coated WC ϕ900µm flat end mills, it was found that surface finish does not depend on feed as it does in conventional macromilling. The finish is not affected by axial depth of cut, but radial depth of cut has the most significant impact if there is any chattering [9]. Other authors [10] investigated the influence of the cutting fluid and cutting speed on the surface roughness during micro milling of Ti6Al4V with WC flat end mill. Surface finish rises with machined distance during dry cutting, but does not change Proceedings of the 8th International Conference on MicroManufacturing University of Victoria, Victoria, BC, Canada, March 25-28, 2013 22