A Constant Chip-load Turning Approach To Generating Freeform Optical Surfaces

In the freeform optical surface machining, the chip load deponds on the location of thetool location point and the tool contact point, which leads to the figure error non-uniformity.To solve this problem, in this paper we propose a novel turning approach to reduce the figureerror non-uniformity for the diamond turning of freeform optical surface.This approach has several characteristics: The workpiece surface which is to bemachined could be rotationally symmetric high-order non-spherical surface; In the cuttingprocess, tool location points are adjusted along the radial coordinate of workpiece to limit theinstantaneous chip load within the predetermined range; A diamond turning machine with twostraight axes and a fast tool servo(FTS) with two straight auxiliary axes can make the toolmove along the workpiece radial direction and axial direction respectively.In this paper we provide the involved key algorithms: the construction of the surfacewhich is to be machined and the surface which is desired; the formation of the machinedsurface topography during turning; the method in which the tool location points could besolved; the instantaneous chip load calculating method; and the self-adaptive toolpathgenerating method. Furthermore, we consider the FTS auxiliary axis as weak rigidity section.Then establish a coupling model governing the static and dynamic displacement between thediamond tool and the workpiece to be machined to explore the influencing of the generatedtoolpath on the figure errors of machined surface, The result demonstrates that thedependence of the chip load and figure errors on the coordinate of tool location points hasbeen dramatically reduced in the turning approach proposed in this paper compared to theexisting constant radial feeding method.The research in this paper could help to improve the figure accuracy in the freeformoptical surface machining and reduce the number of repeated error calibration cuttingprocesses.