Tooling Surface Polish Impact on First Article Inspection Accuracy
Uncalibrated optical scans on mirror-polished plastic introduce up to fifteen micrometers of false elevations, ruining First Article Inspection accuracy.
CMM Force Deflection
Meaning
Mechanical bending of touch-trigger or scanning probes during coordinate measurement introduces localized positional error when contacting compliant polymer walls. Contact touch probes exert a pre-travel threshold force that induces cmm force deflection in thin plastic ribs or unsupported flexible sidewalls. The resulting elastic strain displaces the contact point before the signal registers, creating an artificial under-measurement of wall thickness or pocket depth.
The dimensional shift remains confined to tactile inspection methods and vanishes when using non-contact optical scanning systems.
Probe Displacement
Stylus length and shaft diameter determine the physical magnitude of elastic bending during tactile inspection routines. Long ruby-tipped styli mounted on small-diameter carbide shanks yield pronounced cmm force deflection when driving against stiff semicrystalline resins like polyoxymethylene. The measured coordinate shifts away from the true surface vector because shaft flexure precedes electrical switch triggering.
Lowering the contact force or selecting stiffer silicon nitride styli reduces the systemic offset. High-speed scanning across flexible automotive interior trims amplifies the deviation through continuous drag.
Compliance Threshold
Part stiffness governs whether the probe shank bends or the plastic surface deforms under touch. Low-modulus thermoplastic elastomers yield readily under standard touch forces, causing localized surface indentation alongside cmm force deflection. Distinguishing between wall flexing and stylus bending requires evaluating the flexural modulus of the polymer against the trigger threshold of the CMM head.
High-modulus unfilled polycarbonate resists surface indentation, placing the entire displacement strain onto the probe assembly.
Defect Offset
False out-of-tolerance readings force quality teams to alter mold core dimensions unnecessarily. Uncorrected cmm force deflection registers as material sink or wall thinning on production inspection reports. Moulders who modify steel tooling based on uncalibrated tactile data introduce permanent dimensional errors into hard cavities.