Quality Scan: Software Can Provide Missing Data Link
By Chris Lawrie
Technical Developments Manager
Complex surfaces are everywhere in the mold and die arena and elsewhere in metalworking. Splined surfaces, for example, are found wherever fluids are moved--in airfoils, fans, impellers, and turbines. Fillets and blends (fillets join two surfaces, and blends three or more) are ubiquitous in tools for stamping, forming, forging, injection molding, and die-casting. Doubly curved surfaces are used throughout consumer goods from tool handles to cosmetics packaging.
A new type of inspection software can anchor clouds of points generated by scans of such surfaces to the real-world coordinate systems to which the scanned object must relate. This builds on existing capability that links geometric features in point clouds to computer-aided design (CAD) files of solid or surface models.
This capability is very important because it means surface inspections, such as in a new product or the tooling that forms it, can be automatically oriented to any coordinate scheme of whatever is being produced: vehicles, aircraft, boats, large industrial machinery, and so on.
The package's technical trick is merging two types of inspection data files: point-cloud files from the laser probe or scanner, and positional feedback from the encoders in the principal arm joints of a portable coordinate measuring machine (or the axis-travel feedback from a conventional CMM).
The software also:
- Instantaneously compares geometric features in the point cloud to the CAD file or solid model from which the inspected part was created, without inspection programming.
- Automatically deletes overlapping data points from multiple scans, eliminating a bugaboo of many low-end reverse engineering systems that also rely on scanning.
Called real-time laser probe surface inspection, this technology and the associated software provide a kind of missing link between surfaces to be inspected and the surfaces of mating parts.
Laser-probe scanning acts as a "digital paint roller" to literally cover the inspected surface with data points. The software, combined with fast computer hardware, allows users to gather millions of points in just a few minutes.
The obvious question is: Who needs millions of points? The answer is, anyone who is tasked with inspecting surfaces of a part or a forming tool. Touch-probing leaves the overwhelming percentage of such surfaces uninspected.
Inspecting a surface represented by millions of data points would pick up subtle differences within curved and flat surfaces, areas that are just a little off-specification or out of tolerance. These differences would be barely perceptible bulges or sags. Given a CAD file for comparison, these can be found automatically, without any programming preliminaries. Inspectors don't have to worry about whether or not a given set of curves is, for example, mathematically definable.
To ensure that these kinds of surface errors aren't overlooked, real-time laser probe surface inspection software generates contoured color "maps" that compare the surface data to the CAD file from which it was generated. The maps show the most minute deviations from specifications as well as the relative distance of any surface point to the CAD nominal. Some packages even use an intuitive color scheme to map errors: green for points in tolerance; red for points with a plus tolerance, or too much material; and blue for points with a minus tolerance, or too little material.
These color displays clearly show every area where something must be done to maintain tolerances and specifications. This is a vital process-control capability. It is also indispensable for what-if analyses. If the parameters of a contour map are loosened or tightened, changes in the out-of-tolerance points are immediately reflected in the color map.
The software is hardware-independent; it can be interfaced to many different laser probes and scanners, and to multiple brands of portable and fixed CMMs. Hardware independence enables users to retrofit and upgrade existing inspection systems, and helps protect their investments.
This article was first published in the September 2004 edition of Manufacturing Engineering magazine.