Optimum removal in ion-beam figuring

In an ion-beam figuring (IBF) process, the desired material removal, which is specified to the contour algorithm to calculate dwell time, decides the calculated process time and the resulted residual surface figure error. Usually, an IBF process with more removal consumes a longer process time, although the resulted residual error is smaller, whereas an IBF process with less removal needs only a shorter process time, but the resulted residual error is greater. Therefore, in order to balance the process time and the residual error, an optimum removal should be determined. In this study, the characteristic relationship between the process time and the residual error on different specified removals is investigated first. The investigation shows that for smaller removals, the residual error decreases rapidly while the process time increases slowly, and for larger removals, the process time increases rapidly without much decrease in residual error. This characteristic makes the figure-prediction curve (process times vs. rediual errors (RMS)) often in the shape of the letter “L”. Therefore, the optimum removal can be determined at the corner of the curve. Finaly, experiments are performed to verify the proposed method.