Theory and Practice of Uncertainty Evaluation of Coordinate Measurements

Abstract

Long-lasting works on the ISO 15530 series can be a proof for complexity of uncertainty evaluation of coordinate measurements for different characteristics defined according to the rules of geometrical product specification (GPS). Even the best known procedures (using the calibrated workpiece and using computer simulation) have not been implemented widely. The authors have elaborated the methodology and the software (EMU-CMMUncertaintyTM) which makes possible evaluation of the measurement uncertainty with less effort. The software has been developed with following assumptions. Uncertainty budget includes influences of particular geometrical errors of the CMM, probing system errors as well as temperature errors. Applied algorithms use type B method according to the rules of GUM. Uncertainty is analysed separately for each characteristic (task-specific approach). Uncertainty of measurement for particular characteristic is calculated with the formula describing the characteristic as a function of differences of coordinates of characteristic points of the workpiece. The paper presents some new details concerning formulation of uncertainty budget. Among others the methodology is presented for estimation of uncertainty component arising from the fact that some characteristic points are not integral features (surface points) but derived features (axes points).