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Models are presented to predict the shape and size of masked and unmasked holes machined in glass and polymethymethacrylate (PMMA) using abrasive jet micromachining (AJM). An existing AJM surface evolution model for brittle materials was modified by introducing a curvature-dependent smoothing (viscosity) term to the surface velocity function, greatly improving the prediction of hole shape in cases where the erosive power creates a sharp corner. The modified model predicts hole profiles that agree well with both experiments and a computer simulation. The profiles of holes machined in PMMA using a mask are also compared with the predictions of a recently developed theoretical model for the AJM of ductile materials. There is good agreement in both shape and depth with a maximum error of 13% up to an aspect ratio of 0.6 in PMMA.