Quick Guide to Precision Measuring Instruments

PG69Quick Guide to MeasurementExample of circle measurement by CMMQuantication of CMM uncertainty elements by experimentMajor contributions that cause measurement uncertainty of the CMMThe Virtual CMM software enables estimation of complicated measurement uncertainty of a CMM. The software simulates a CMM on a PC based on its machine characteristics and performs virtual (simulated) measurements. The simulated measurements are performed according to the part program created by the operator. The machine characteristics are evaluated from experimental values based on geometrical characteristics of the actual machine, probing characteristics, and tempera-ture environment, etc. The measurement uncertainty of the CMM can be easily estimated by using the Virtual CMM software package.ISO15530 Part 4 (ISO/TS 15530-4(2008)) denes how to verify the validity of task-specic measurement uncertainty using computer simulations.Virtual CMM conforms to this specication.Note: Virtual CMM is a software package originally developed by PTB (Physikalisch-Technische Bundesanstalt).Relevant parts of ISO15530: Geometrical Product Specications (GPS) -- Coordinate measuring machines (CMM): Technique for determining the uncertainty of measurement --Part 3: Use of calibrated workpieces or measurement standardsPart 4: Evaluating task-specic measurement uncertainty using simulation [Technical Specication]Measurement Uncertainty of CMMMeasurement uncertainty of the CMM and the Virtual CMM softwareUncertainty of circle proleUncertainty of center positionPositioning of measuring pointsUncertainty of measurementCMMProbeEnvironmentWorkpieceEnvironmentData processingProbeMeasurement taskCMMMeasurement uncertainty is an indication used for evaluating reliability of measurement results.In ISO 14253-1:2013, it is proposed to consider the uncertainty when evaluating the measurement result in reference to the specication. However, it is not easy to estimate the uncertainty of the measurement performed by a CMM.To estimate the uncertainty of the measurement, it is necessary to quantify each source of the uncertainty, and determine how it propagates to the measurement result. The CMM is capable of having all types of settings that determine how the measurement should be performed, such as measurement point distribution, or datum denition, according to the drawing instruction or operator's intention. This feature makes it harder to detect the source of uncertainty inuencing the result. Taking the circle measurement as an example, just a difference of one measurement point and its distribution causes the necessity of recalcu-lation of the uncertainty.Also, there are many sources of uncertainty to be considered with the CMM and their interactions are complicated. Because of the above, it is almost impossible to generalize on how to estimate measurement uncertainty of the CMM.

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