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.
Tactile CMM
Meaning
High-precision dimensional metrology utilizes a physical touch-trigger probe to determine the coordinates of points on a component surface. In manufacturing facilities, a tactile CMM verifies that injection-moulded plastic parts comply with their design drawings and dimensional tolerances. This equipment provides highly accurate and repeatable measurements for geometric features like hole diameters and planar flatnesses.
It remains a standard tool for first-article inspection and quality auditing in industrial moulding operations.
Physical Probing
Mechanical measurement processes involve bringing a ruby-tipped stylus into contact with the surface of the part, triggering a signal that records the coordinates. Because the probe must physically touch the surface, a tactile CMM requires a stable and rigid setup to prevent the part from moving during the measurement sequence. This physical interaction is highly effective for rigid plastic parts and machined tooling cavities where the surface does not deform under the light touch of the probe.
It delivers precise point coordinates that are unaffected by surface color or reflectivity, which can often disrupt optical scanners.
Polymer Verification
Moulded plastics are susceptible to post-moulding shrinkage, warp, and sink, making precise dimensional verification essential for part assembly. Applying a tactile CMM to evaluate these parts requires careful consideration of the probe force, as excessive force can deflect thin-walled plastic features and lead to false measurements. Quality engineers program the machine to touch the part with minimal pressure, ensuring that the deflection remains within acceptable measurement limits.
This method is particularly valuable for measuring internal features and deep cavities that are hidden from optical line-of-sight systems.
System Maintenance
Achieving sub-micron accuracy over the lifespan of the equipment requires controlled laboratory conditions and regular calibration of the stylus. To ensure the tactile CMM remains reliable, the measurement room must be kept at a constant temperature of twenty degrees Celsius, as polymer parts have high thermal expansion coefficients and will change size with temperature fluctuations. In addition, the probe tips must be kept clean and replaced when they show signs of wear or chipping.
Regular calibration against a certified reference sphere verifies that the machine account for probe tip wear and maintains its specified accuracy during production runs.