Quality Scan: Choose the Right CMM Probe/Sensor
Builders of CMM machines and probe and sensor manufacturers provide a wide array of interchangeable contact and noncontact measuring technology that focuses on specific measuring applications. It's important to select the technology that meets your measurement and inspection requirements.
Most parts measured on CMMs are 2-D and 3-D, and use contact touch-trigger measuring probe heads. These heads are differentiated by the level of indexing capability they employ. Because the probe system's cost can be as much as 40% of a CMM's price, selecting the unit that's appropriate for your range of parts and production requirements is paramount.
A mechanical touch probe head that is manually indexed by an operator is cost-effective. Such a manual-indexing probe can cost as much as $5000. Motorized indexing allows Aand Brotational angles of the head to be automatically indexed. Head index resolution can vary from 7.5 to 2.5°—depending on the probe manufacturer—and these probe heads can cost up to $20,000.
Automated nonmotorized indexing is relatively new. Programmed CNC movements of the CMM perform the indexing using a probe head designed with tabs and levers actuated by contact with a table-mounted index tool. Indexing isn't as fast as the motorized designs, but automated indexing can be 70% less costly than a motorized unit, and still provide the necessary function.
A continuous nonindexing indexing probe head has stepless wrist movements, and is used on larger CMMs that can accommodate a probe of this size and weight.This type of probe can use long extension bars to measure difficult-to-reach internal part features. It can achieve any angle, and does not require calibration for each angle used.These systems can cost up to $75,000.
Touch-trigger probes have been around almost as long as the CMM. The original touch probe design is mechanical, and employs a kinematic system of three precision balls mounted on six rollers.The most common design used today, it provides submicron repeatability, but suffers from lobing. Other probe designs include strain gages and piezo crystal designs that eliminate lobing, and can carry longer styli with little metrology degradation.
Contact scanning probes can quickly generate a lot of data. Over the past decade, scanners have been nonindexable and difficult to use, and required numerous tool changes and associated tool racks. New, lower-cost designs use lasers and optics to measure probe deflections, rather than internally mounted transducers with known stacked probe-axis errors. These new designs are better suited for modularity, fit on an indexing head for improved flexibility, and permit scanning as well as touch-trigger measurements.They also allow fast stylus changes and six-way sensing, and can carry styli as long as 400 mm. Submicron inspection accuracies can be obtained.
Probe systems are now available with repeatable stylus interchangeability, which increases the CMM's system flexibility for long and short styli configurations, as well as sphere, disk, or cylindrical tip designs required for different features.These automated changes can be built into the programming to reduce operator intervention. Modular stylus racks are available for mounting on the CMM table.
In contact probing, data collection begins at the stylus. In the absence of lubrication, the stylus form inevitably changes, causing measurement errors. For optimum stylus life, material-specific recommendations are: ruby stylus (for steel), silicon nitride (aluminum), or zirconia (cast iron).
The concept of measuring parts without touching them has been around as long as the CMM. Broader application of laser and optical scanning is a byproduct of size and weight reductions that have given these probes the ability to be carried by an indexing head, and automatically changed by the CMM.The laser line scanner basically produces a topographical map of a part, and is one of today's best reverse-engineering tools. An assembly of part geometrical data can be scanned and reconstructed into a CAD model. It's also possible to extract individual data features by using geometric filtering algorithms. In some circumstances, this type of scanner can be used for inspection of production parts.
Optical vision systems are specifically designed for use with small parts where a high level of part accuracy is not required. The CCD cameras in these systems are designed for rapid, single-shot analysis. New sensors combine laser and optical scanning into one device that uses lasers to measure the alignment of a feature,while the optics position the actual feature.
Take your pick. Today's CMM measuring tools have the appropriate level of sophistication, accuracy, and production capability necessary to handle any measurement and inspection application.
This article was first published in the January 2008 edition of Manufacturing Engineering magazine.