Mid-size sedans represent the biggest car market in the United States for very logical reasons. More power and comfort than cheaper compacts. Performance similar to a luxury car, albeit without the bells and whistles. But perhaps the real success of this market segment relates to price. While most can't afford a Jaguar, a new-car buyer can usually come up with the few extra bucks every month to leave the Escort on the lot and drive home a Taurus.
This same principle has led Carl Zeiss IMT Corporation to develop a new mid-level scanning coordinate measuring machine (CMM). The Contura is designed for suppliers that need CMM capability on the production floor, but get scared off by the $250,000 price tag of a machine like their high-end Prismo. According to Felix Hoben, Zeiss senior product manager, these smaller companies will typically purchase touch-trigger CMMs and make do with their limitations. It's his aim, however, to sell these potential customers on the increased functionality of scanning. Then he'll sell them a Contura—for only about $150,000.
The Contura is designed to be something of a "Prismo-lite." It lacks some of the robustness of Zeiss's more expensive model, but can still deliver plenty of functionality. The bridge-type CMM has a stationary machine table and uses a VASTXT active scanning probe head that relies on a patented electromagnetic force generation system to take measurements. Compared to passive, spring-deflected probes that need controller algorithms to compensate for stylus bend, the Zeiss machine is said to provide better accuracy. The active scanning probe also gives the Contura the capability of using a broader selection of probes, up to 300 mm in length or down to a tip diameter of only 0.5 mm.
VAST technology first appeared in 1995 on a more expensive Zeiss CMM. This interface simplifies data collection by automatically computing scanning speed and data point density based on geometry and tolerance limits. Used in conjunction with Zeiss' Windows NT-based measuring software, Calypso, the package is designed to eliminate operator error and simplify use. As Hoben explains, "VAST makes a scanning CMM as easy to use as a touch-trigger CMM." VAST supports several different scanning scenarios: single-point measurement, form measurement (to a precision of 3.4 µm), known contour measurement, free-form scanning for digitizing surfaces, and scanning optimized for grooves and slots.
While the VASTXT head in the Contura has similar performance to the VAST head in the Prismo (though the XT head is smaller), Zeiss has trimmed cost from the rest of the machine. The operating range of the Contura is only 65-71° F. Its scanning range is also limited to 28 × 40 × 24 in. Maximum workpiece weight weighs in at 1606 lb. A PC-based controller is integrated into the machine base and samples data at 200 Hz.
But what are the advantages of moving from a touch-trigger CMM to a scanning machine? Hoben points to a few interesting scenarios. First is the matter of accuracy. Taking a traditional four-point measurement with a touch-trigger machine leaves far more room for error than a multi-point scan with a Contura. And according to Hoben, cycle times for scanning several thousand points with the Contura and scanning only four with a touch trigger machine are a dead heat. He continues to explain that if quality control can be improved with a scanning CMM, then successful self-inspection can win new business. Furthermore, a scanning CMM can also verify form, meaning less handling of parts and no need to invest in a separate machine for applications that require a form test. And keeping with an agile philosophy, a scanning CMM can be re-programmed as production requirements change, unlike fixed gaging. Of course, a business case must be made for any investment. However, the flexibility of this scanning CMM, if fully taken advantage of, could certainly give a smaller operation the ability to do more.