Tool force, chip formation and surface finish in diamond turning

Machining accuracy in diamond turning could be improved though tool force feedback. Tool forces are ideally suited for use in a control algorithm because they contain information on the instantaneous depth of cut, feed rate and condition of the tool. A tool force model that could form the basis of this new control technique has been developed. This model is similar to the shear plane models discussed in the literature in that chip formation is regarded as a process of continuous shear. By measuring the shear angle from micrographs of chip cross sections and by assuming that the shear and normal stresses in the primary deformation zone are related to the material hardness, equations for the forces due to chip formation and the friction between the chip and the tool have been written. Furthermore, the effects of elastic deformation of the workpiece (spring back) on chip formation and the measured forces, which can be significant in precision machining, have been included in the model. The equations for tool force were verified through diamond turning experiments with electroless nickel, electroplated copper, 6061-T6 and 5086 aluminum alloys. For machining with newly lapped as well as worn tools, the calculated machining forces differed by less than 50% from those measured. The accuracy of the model makes the on-line estimation of tool sharpness possible. A technique to determine tool edge geometry from the measured thrust (normal) force has been developed. The information contained in the measured forces can be exploited to reduce machining errors if the relationships between tool force, tool wear, chip formation and surface characteristics are known. To this end, an explanation for surface generation that relates tool sharpness and cutting conditions to surface features has been proposed. This model incorporates a minimum chip thickness criterion, as well as plastic and elastic deformation of the workpiece, to describe features measured along the lay of diamond turned surfaces. Surface features in the cutting direction have been quantitatively linked to the cyclic shear failure of the material that results in chip generation.