Fiducial-aided on-machine positioning method for precision manufacturing of optical freeform surfaces

Abstract

There are still significant challenges in the accurate positioning of optical freeform surfaces on the machine tool and the measurement instrument due to the high accuracy requirement and their complex shapes. This paper proposes a Fiducial-aided On-machine Positioning method (FAOPM) that combines on-machine measurement and off-machine measurement to precisely position optical freeform surfaces during the precision manufacturing cycle including rough machining, fine machining, measurement, and error compensation. The FAOPM makes use of fiducials which are firstly measured on a coordinate measuring machine with nanometric accuracy to generate a Fiducial-aided Computer Aided Design (FA-CAD) of the designed optical surface, then the developed on-machine measuring device obtains the accurate positions of the fiducials after remounting in the machining coordinate system. Finally the relative position of the workpiece is identified so that the associated cutting paths and compensation tool path can be easily generated. Several optical freeform surfaces were experimentally machined to prove the capability of the proposed method. The results indicate that the positions of the workpiece during the precision manufacturing and measurement cycle were precisely achieved and the form accuracy of the optical freeform surfaces was remarkable improved based on the FAOPM.

A Fiducial-Aided Reconfigurable Artefact for the Estimation of Volumetric Errors of Multi-Axis Ultra-Precision Machine Tools

“…The repeatability of measurement of the integrated probing system is evaluated by probing a standard sphere three times. The Cartesian repeatability ranges contributed by the pooled standard deviation are 0.36 µm, 0.41 µm and 0.25 µm [18], respectively. In addition, the best fitted standard deviations of the sphere center coordinates are 0.18 µm, 0.21 µm and 0.24 µm, respectively.…”

A Fiducial-Aided Reconfigurable Artefact for the Estimation of Volumetric Errors of Multi-axis Ultra-Precision Machine Tools

“…The repeatability of measurement of the integrated probing system is evaluated by probing a standard sphere three times. The Cartesian repeatability ranges donated by the pooled standard deviation are 0.36 μm, 0.41 μm and 0.25 μm [18] respectively. In addition, the best fitted standard deviations of the sphere centres coordinates are 0.18 μm, 0.21 μm and 0.24 μm, respectively.…”

“…The fiducial is used to obtain the probing information containing both errors resulting from the integrated on-machine measurement system and the machine tool itself. The relationship between the tip of the probe and tip of tool has been established in the previous work of the authors [18] . As mentioned earlier, the repeatability of measurement of the probing system is relatively low at 0.24 μm.…”

“…As shown in Fig. 6, the fiducial centre distances in a five fiducials artefact were compared with the distances in the fiducial aided CAD (calibrated fiducials and the CAD of the designed surface [18]) by using the cloud point fitting algorithm. The tested positions of the fiducials are determined and transferred into a common frame using mathematic model.…”

Any-degrees-of-freedom (anyDOF) registration for the characterization of freeform surfaces

“…Such unwanted registration error can be avoided using a priori knowledge of the surface, i.e. utilizing pre-existing highly accurate reference features on the surface such as flat or spherical features [14] to assist the registration process. In such cases, the registration process is performed in two steps.…”