Assessment of non-rigid body, direction- andvelocity-dependent error motions and their cross-talk by two-dimensional digital scale measurements at multiple positions

Abstract

The conventional volumetric error compensation for a machine tool is based on the machine's kinematic model, which formulates the tool center point (TCP) position based on the assumption of rigid-body motion of each axis. Particularly in large-sized machine tools, error motions that do not satisfy this assumption may have a significant impact on the machine's overall volumetric accuracy. In addition to position-dependent quasi-static error motions included in the conventional kinematic model, this paper proposes a scheme to assess quasi-static cross-talk between axes and direction- and velocity-dependent error motions. This paper proposes test procedures to measure two dimensional (2D) contour error trajectories by using a 2D digital scale (cross grid encoder). Unlike many previous works, this paper proposes to perform the tests at multiple positions over the machine tool's entire workspace, and the error motions are assessed by comparing contour error profiles. An experimental case study on a large-sized bridge-type vertical machine tool shows that such influences can be significant error contributors. The proposed tests can assess error motions only at discrete points where a 2D digital scale is installed. It can be applied as accuracy inspection to ‘‘roughly’’ assess the machine's volumetric accuracy by using a 2D digital scale only, but it is generally not for the compensation.