Machining Splines

摘要

Splines and particularly NURBS are nowadays widely used for modeling parts in production engineering. Spline curves are suited especially for representing freeform features. However, for machining the traditional linear interpolation is still used. The reasons for this are the restricted functionality offered by common machine controls and the difficulties to determine the exact algorithms that are implemented in the control unit but hardly documented. The representation of the original curves with straight lines during the generation of NC programs causes loss of information and increases the size of the NC files compared to spline representation, which requires less data for processing. As a consequence, the calculating capacity of the control unit may be exceeded using linear interpolation. This leads to instability of process parameters like deviating velocities and thus a decreased surface quality. Within this paper, two scenarios for the machining of splines are discussed. Within the scope of the first one, the minimization of the 'airtime', I.e. non-productive time the tool spends traveling in the air, using spline interpolation is investigated. In this case, the inability to precisely anticipate the tool path is not critical and does not pose a threat for the machining process itself, since the deviations are negligible for this application. As a second scenario, the finishing process has been chosen. The restrictions imposed on the precision of the NC code in this situation are stricter. To achieve the required machining accuracy a method is discussed, which enables to provide controls with suitable input data.