A method of forming a part from a workpiece mountable on a porous spoil-board on the worktable of a CNC machine having means for applying a negative pressure on the underside of the workpiece, providing a pressure differential across the mounted workpiece, and a cutting tool displaceable along x, y and z axes, including programming the controller of the machine to generate a cutting pattern, computing the area of the part, comparing the computed area value of the part with a predetermined area value and positioning the cutting tool along the Z-axis to provide less than a through cut of the workpiece where the computed area value is less than the predetermined area value and providing a through cut when a computed area value exceeds the predetermined area value, mounting the workpiece on the worktable, actuating the negative pressure applying means and operating the machine to execute such program.