Meaning
Dimensional measurement deviations arising during coordinate metrology represent the limits of physical contact sensors. In coordinate measuring machines, cmm probing error accounts for the difference between the recorded contact point and the true coordinate of the part surface. This deviation arises from the physical interaction of the stylus tip with the polymer material.
Uncertainty Source
Elastic deformation of the thermoplastic surface under the force of the stylus introduces localized deflection during inspection. This deflection increases cmm probing error when measuring soft or thin-walled plastic components. The triggering force of the probe must be balanced against the flexural modulus of the resin to prevent false readings.
Defect Contribution
Measurement inaccuracy during quality audits propagates through statistical process control limits. When cmm probing error remains uncorrected, it masks actual tool wear in injection moulding operations. This issue leads to the acceptance of out-of-specification parts or the unnecessary adjustment of stable moulding parameters.
Molders often misinterpret these sensor variations as part shrinkage.
Mitigation Strategy
Calibration of the stylus using a certified reference sphere of known diameter establishes the baseline compensation factor. In plastic part inspection, reducing cmm probing error requires the selection of larger stylus spheres to distribute the contact force over a wider area. Low-force trigger modules and slower approach speeds further minimize the local surface indentation.
Correcting for these factors ensures that the reported dimensions represent the actual physical geometry of the molded specimen rather than a mechanical artifact of the measurement process.
