3-D Microstructure Creation Using Elliptical Vibration-Assisted Machining (EVAM)

In elliptical vibration-assisted machining (EVAM), a diamond cutting tool is made to move in a micrometer-scale elliptical path at high frequency. This oscillation is superimposed on the normal feed motion of the tool. Compared to diamond turning and spindle-type micromachining processes, EVAM has several advantages for making millimeter-scale and smaller structures. These include smaller machining forces, reduced vibration, elimination of runout error, and extended diamond life when cutting ferrous or brittle materials. To date EVAM has been used mostly to make binary and low aspect ratio / low relief features with vertical dimensions of only a few micrometers. The current research explored EVAM's ability to make 3-D microstructures with geometry typical of micro-optical, micro-fluidic, and MEMS devices.;The Ultramill EVAM tool previously developed at North Carolina State University was employed in this work. A new hydrostatic oil-bearing Y-axis was installed on the Nanoform diamond turning machine which improved surface finish by increasing stiffness in the depth of cut direction. To suppress thermal upsets which could cause form error, the Ultramill's original gravity-driven cooling system was replaced with a closed-circulation arrangement. At the small feed velocities used for microstructure fabrication, surface roughness after the cooling system conversion was found to be caused principally by coolant pump pulsations which partially offset the improvement gained from the stiffer Y-axis.;Existing 2-D kinematic and theoretical surface roughness relationships for EVAM were expanded to the general case of 3-D surfaces. Commercial CNC motion planning software could not calculate the complicated cutter compensation required by EVAM's elliptical tool path, so an innovative method was developed for motion program generation, based on surface morphology methods used in image processing and contact probe microscopy.;A variety of 3-D parts were raster-cut with the Ultramill. Round-nosed diamond tools were used to make features with convex, concave, tilted planar and sculpted 3-D geometry. Positive features had heights up to 20 mum, and negative features depths as great as 130 mum. Height-to-width aspect ratios of 0.15 were achieved, compared to 0.01 for most previous parts. EVAM's ability to make functional components was demonstrated by fabrication of millimeter-scale reflecting optical surfaces. These included concave spherical elements with a form error of 62 nm RMS. A complex off-axis segment of an ellipsoidal surface was also made, as might be used in a free-space fiber optic beam splitter. This part had estimated centerline form error of ∼480 nm RMS limiting its functional performance. A significant cause of this form error was found to be axis squareness error, rather than being assignable to the EVAM process.;Sharp-nose and dead-sharp diamond tools were used to machine channels and pockets with steep straight sidewalls. Such tool geometries have not been previously discussed in the EVAM literature. Tetrahedron arrays from 5 mum to 80 mum tall were also cut with a dead-sharp tool, in copper and stainless steel. Severe burr appeared in stainless steel parts, unlike when a round-nosed tool was used to machine the same material. Tool cutting edge wear was also observed after machining stainless steel for a 2 m raster distance with the sharp nose tool, while negligible wear was noted at this distance for a round-nosed tool.