3D Scanning Metrology: How Global and Local Alignment Affect Deviation Heat Maps

In 3D scanning metrology, inspection begins with two datasets: the nominal CAD model and the scan of the physical part. Metrology software aligns the scanned data with the nominal model, typically through an initial pre-alignment followed by a global best-fit alignment. This provides an overall view of surface deviations between the manufactured part and its design.

Interpreting the resulting heat map requires context. A part may be warped, bent, worn, twisted, or incorrectly assembled. Some areas may appear green, while others appear red or blue. These colours show deviations within the chosen alignment and colour scale; they should not, on their own, be treated as a definitive judgement of part quality.

Consider a screw manufactured to a specified design. Scanning the individual screw and comparing it with its nominal CAD model allows its geometry to be assessed against the required tolerances.

Now consider the same screw installed incorrectly in an assembly—for example, at an angle or with a damaged thread. It may appear acceptable to the naked eye, yet its position or condition may differ from the intended design.

When the scanned assembly is globally aligned with the nominal assembly model, the screw and its surrounding region may show substantial deviations. The heat map reflects the comparison with the correctly positioned screw in the CAD assembly, rather than the screw’s individual geometry alone.

The key question is what the inspection needs to evaluate: the screw’s geometry or its position within the assembly. A local alignment focused on the screw can produce a different deviation pattern by assessing that region independently of the overall assembly fit.

This distinction helps separate component geometry from assembly-related deviations. A screw may conform closely to its nominal shape while being incorrectly positioned in the assembly. Assembly conditions, including excessive tightening torque, may also introduce deformation. The alignment method should therefore support the specific inspection objective.

Global and Local Alignment: What Is the Difference?

Global and Local alignment support different inspection objectives. Global best-fit alignment evaluates the overall fit of a part or assembly, while local alignment focuses on selected geometry, such as a functional surface or mating region.

Global Alignment

• Definition: Global best-fit alignment fits the scanned mesh or point cloud to the nominal CAD model using the included geometry, often through an iterative closest point (ICP) or similar best-fit algorithm.

• Primary applications: Assessing overall shape, freeform surface deviations, warpage, and broader thermal or moulding-related distortion.

• Limitation: A low root mean square (RMS) deviation does not necessarily indicate that every feature meets its requirements. An overall best fit can distribute deviations across the part and obscure significant local or positional errors in critical features.

Local Alignment

• Definition: Local alignment determines the fit using a selected region or set of features, such as a plane, cylinder axis, datum pads, or reference point system (RPS).

• Primary applications: Examining selected functional interfaces, including bearing seats, bolt-hole patterns, and sealing surfaces, without allowing the overall part fit to dominate the comparison.

• Advantage and limitation: Focusing the alignment on relevant geometry reduces the influence of unrelated regions. However, a favourable local fit should not be mistaken for confirmation that the feature is correctly positioned within the complete part or assembly.

Key Comparison