Abrasive assisted brush deburring of micromilled features with application to a novel surgical device
Mathai, George K.
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Burrs severely inhibit the performance and aesthetics in machined parts besides posing a safety risk to the user and manufacturer. Abrasive assisted brushing presents a fast and effective method for deburring these parts but is difficult to control. The dependence of deburring rate on the workpiece material, abrasive grit size, type and rotational speed of the brush is studied. It is found that deburring rate is proportional to initial burr height indicating fracture of the burr at the root. Deburring rate increases with spindle speed and is higher for diamond than SiC. The formation of burrs in micromilling of a thin nickel-titanium alloy (nitinol or NiTi) foil used in implantable biomedical device applications is analyzed as a function of micromilling process parameters such as spindle speed, feed, tool wear, backing material and adhesive used to attach the foil to the backing material. All factors except spindle speed are found to affect burr size. If initial penetration is sufficient to cause the foil to fail in tension, the foil tears with the crack starting closer to the upmilling side and thereby resulting in larger downmilling burrs. If penetration is insufficient, the foil plastically deforms until it tears typically in the middle of the cutter tooth path. A kinematic model that captures this behavior is used to predict burr widths and is verified through experiments. The thesis also presents an investigation of the abrasive impregnated brush deburring process for thin NiTi foils. Models based on Hertzian indentation and fracture mechanics are proposed to predict the rates of indentation and deburring during brushing and are validated using experiments. The predictions of the models are within the experimental variation. Burrs can be removed with this process within 12 minutes for a 6 mm long groove with no more than a micron change in foil thickness. Knowledge of burr formation and deburring is applied to a novel micromilled thin shape memory based NiTi foil device used for the surgical correction of Age-related Macular Degeneration (AMD), a leading cause of blindness in the western world in those over age 50. Burrs on the surface of the structure are used successfully to mechanically constrain and translocate an autograft to replace the diseased RPE-Bruch's membrane under the macula. The shape memory device is analyzed using experiments and simulations.