Study on the Uncertainties of Form Errors Evaluation Under the New GPS Framework

The uncertainty concept has been expanded from measurement uncertainty to a series of related uncertainties in the improved Geometrical Product Specifications and Verification (GPS) system. When using a Coordinate Measuring Machine (CMM), the uncertainty to the result mainly comes from the successive operations of separation, extraction, association and evaluation in the process of tolerance assessment. The uncertainties from CMM sampling, least squared association and evaluation methods have been discussed on emphasis. The uncertainty calculations for typical tolerance items have been demonstrated according to the error propagation theories and statistical principles. Some interesting conclusions have been drawn form the theories and examples of the flatness assessment: 1) the uncertainties to the result are from CMM sampling errors not the evaluation datum parameters, namely the association strategy; 2) If the uncertainties from evaluation parameters are ignored, the calculation of the uncertainties will be efficiently achived especially for the items with more parameters to describe its datum, for example, the cylindricity.