For clean mortises, start at both ends

Whether you make a mortise with a drill press or mortising machine, drill both ends of the mortise first. Then drill evenly spaced holes between them, as shown above, left. Leaving the ends until last could cause the bit to deflect into the mortise, resulting in ends that are not straight up and down. If you drill the mortise, pare the sides smooth with a ³⁄₄ " or 1" chisel, shown above, right. Square the corners and ends with a ¹⁄₄ " chisel.
—from the WOOD shop