By Deighton BrunsonPresidentBrunson Instrument Co.Kansas City, MO
I believe it was Aristotle who said, “The whole is greater than the sum of its parts.” But in metrology, sometimes the least significant part causes trouble. Take the laser tracking system for instance. This highly engineered solution single-handedly enables accurate portable shop-floor measurement, alignment, and assembly of aircraft and other large manufactured objects. The operative words here are portable and accurate. The mobile laser tracker uses a laser beam, targets and target holders to precisely measure surfaces and critical features in mid- to long-range radial volumes. Despite our favorite philosopher’s belief in the holistic “whole,” at times the slightest item plays a crucial role in the quality process.Laser tracker operators use optical, retro-reflective targets to collect 3D coordinate data by reflecting the laser beam back to the tracker. Targets are placed directly on an object or in a target holder, enabling the operator to acquire a series of points that are important to the part inspection plan. These retro-reflective targets come in many flavors, and resemble your favorite childhood steely marble. These tiny works of art are meticulously constructed with trihedral mirrored surfaces using various materials such as glass, stainless steel and aluminum, depending on the manufacturer and the application of the target.On the other hand, a target holder is not only a little masterpiece of craftsmanship itself, but one of the most overlooked components of the system. Laser trackers and target holders are used by metrologists and manufacturers who build, inspect, or maintain airplanes, satellites, ships, particle accelerators, autos and more. Available in both inch and metric collections, target holders make it easier to measure standard geometric features on an object’s surface such as holes, edges, corners, surfaces, diameters and hidden points. Manufacturers also use custom-designed target holders to measure all types of nonstandard geometric features. Target holders may be the least expensive component in a laser tracking system, but they can play a major role in the precision of measurements. In a factory, there can be many laser tracker operators working around the quality control clock to inspect parts and assemblies. Flat nest, pin nest, vector, and custom target holders are in constant use. There are various factors impacting their quality such as unseen pin and seat wear, improper storage, and poor handling by operators. After prolonged usage, they become worn and out of spec, which in turn, adversely impacts the overall accuracy of the 3D data acquired by the laser tracker. So how can uncertainty be lessened in measurements using target holders? The answer lies in certification and serialization. First, each holder should be marked with a part number and serial number for improved component management and identification. This gives a lifelong record of your target holder inventory. Second, every holder must be verified using NIST-traceable standards. Finally, the serial number, with its associated NIST-traceable measurements of critical target holder dimensions, should be documented in a certificate with both print and online access. When you receive new certified and serialized target holders, each component should come with a certificate documenting all data points. These certified dimensional values can be used during measurement processes in lieu of the more general manufacturer’s tolerance range. If you are an ISO-certified manufacturer, or a company utilizing quality management systems, these certificates should be mandatory and available online, which puts a quality audit trail directly at your fingertips. If you are a Quality manager set on reducing measurement uncertainty in a laser tracking system, routine recertification of the target holder inventory is in order. If your holders don’t have a historical record or have been in use for a long while, recertification will authenticate their working state. This process measures and certifies each holder, and uses NIST-traceable standards to recertify holders. Each component will be inspected for factors like pin and seat wear and other critical features. Check to see if your certification or recertification service provider has an ISO 17025 lab accreditation for added confidence in their processes and measurements. Calibration, certification and serialization are all the tools you need to ensure the sum of all parts delivers measurements you can believe in. And when it comes to your laser tracking system, the least of its parts should not be overlooked in your day-to-day quest for quality excellence. ME
This article was first published in the May 2014 edition of Manufacturing Engineering magazine. Click here for PDF.