Weapons for Flexible Inspection
Greater productivity is the battle cry in the automotive industry. Every piece of machinery must advance the mission of making products faster, better and cheaper. For this reason, a growing number of automotive manufacturers are deploying a new breed of coordinate measuring machine (CMM), putting these highly flexible machines next to batteries of machining centers. With machining-center-like productivity, the machines wield an arsenal of sensors, fetching the appropriate tool for each inspection task automatically and completing its mission efficiently.
In power-train manufacturing, for example, multisensor CMMs do more than use trigger probes to register workpieces and find crucial datum points. They use either scanning probes or laser scanners to measure the forms of holes and other features quickly. In the case of engine blocks, some manufacturers have discovered that their CMMs can detect and scan all features on a block in one to two hours with equipment such as the three-beam laser XC50 Cross Scanner from Metris USA Inc. (Rochester Hills, MI). The process would take days and weeks with scanning probes and eight hours with older laser scanners.
CMMs measuring engine and transmission components also can fetch a thermocouple and measure the temperature of the workpiece automatically. Software then can "correct subsequent dimensional measurements to provide a reading as it would be at 68 degrees F," says Kevin Legacy, manager, applications engineering, Carl Zeiss IMT Corp. (Brighton, MI). Another use for temperature readings is to identify hot parts and to delay measurements until they have cooled to a reasonable gaging temperature.
Legacy says that temperature probes have become common features on the turnkey packages that Zeiss has been developing for power-train machining. One reason for the demand is that machining processes and other heat sources can introduce temperature fluctuations that can skew results for high-precision features. Moreover, the automakers are specifying aluminum for more engine and transmission components to trim weight from their products. Aluminum has a high coefficient of expansion, allowing it to react noticeably to even small changes in temperature.
Choosing the probe
All in all, builders identify five classes of probes for their new breed of CMM: digital-trigger and analog-scanning contact probes and optical, laser and temperature noncontact probes. Having any mixture of them ready in a changing rack allows manufacturers in the automotive and other industries to choose the strongest measuring technology for the inspection task at hand. For example, trigger probes might be slow, but they can measure tolerances accurately and precisely in the micron range. At the other end of the spectrum, laser probes can measure thousands of points per minute, but perform best only when tolerances are between 40 to 60 microns.
Another production department where automakers and their suppliers are finding use for multisensor CMMs is in the body shop. Here, they often will begin inspections with trigger probes to locate workpieces and find specific points. Then they will switch to noncontact scanners to measure the alignment of body components and the gaps between panels. Mixing and matching technology in this way for flush-and-gap checks can make the job go 20 to 30 times faster than using only touch probes to determine any deviation between surfaces.
Such is the case with Zeiss' new Eagle Eye, a combination of a noncontact optical sensor from Perceptron Inc. (Plymouth, MI) and a three-axis wrist designed and made by Zeiss. The probe flashes a line of light along a joint, such as where a closed door meets an adjacent body panel. Because the CMM can emulate a six-axis robot in the way it manipulates the sensor, it can keep the light source perpendicular to the measured surface at all times, getting around a fundamental limitation of optical sensors. "Any amount that you're off [from perpendicular] can equate to an error of measurement," explains Legacy.
Not only does the Eagle Eye eliminate this problem, but it also allows the machine to measure hundreds of points while it is in motion. "If we had to do it with a touch probe, we would have to stop the machine, take a point, move out, move over, take another point and so on," says Legacy. "It just takes so much time." By being able to move in six axes, on the other hand, the Eagle Eye can do a job in seconds that normally would take minutes.
To speed changeovers, Zeiss' engineers have designed a plate on the end of the wrist to hold a variety of probes. Consequently, the wrist always stays on the machine, but can accept touch probes as well as scanners.
Stiffer, flexible machines
Despite the proliferation of sensors for CMMs, manufacturers simply cannot buy a collection of sensors and expect to have their inspectors plug them into any CMM in the shop or quality lab. Rather, they must have machines designed and built to accommodate the needs of a variety of sensors. Builders have designed such machines to have stiffer structures, responsive axis drives, standard physical connections and sophisticated software.
These machines need a stiffer superstructure because they, unlike their predecessors, tend to perform much more scanning and, therefore, are more likely to move briskly in three dimensions. Consequently, inertia can cause errors if the machine is not designed to withstand the forces. "The dynamics of the machine is not nearly as important for touch-trigger probes as it is for scanning probes," notes Jay Elepano, special pro-jects coordinator, M3 Solutions-Americas, Mitutoyo America Corp. (Aurora, IL). "With touch-trigger probes, you hit it and back away."
The volumetric-error maps for machines designed for this activity are insufficient for describing the errors caused by forces generated by rapid changes in motion. Although many creative ways exist for adding dynamic-error compensation, most builders are taking a more proactive approach. They have gone back to the drawing boards to design new machines that can change acceleration quickly without overshooting their planned paths of travel and that do not flex when subjected to the loads generated during maneuvers.
The resulting designs borrow heavily from the drives and structures that make high-speed machining centers both responsive and rigid. Using finite-element analysis, for example, engineers have redistributed the mass of moving members. In Mitutoyo's case, they designed the company's new multisensor CMM with a moving Y-axis rather than the moving bridge to add dynamic stiffness. "Our equipment is migrating [to the shop floor] to sit right next to machine tools, so we have to think and build like a machine-tool builder," says Elepano.
CMM builders, however, have been more progressive than machine tool builders at experimenting with materials over the past decade. Jeff Walker at LK Metrology Systems Inc. (Brighton, MI) reports that builders have developed a variety of ceramics that are not only stiff but also thermally stable. Building machines from such materials provides both the rigidity to avoid deflection and the dimensional stability to keep thermal expansion to a minimum.
The ability to wield any of several sensors requires more than responsive drives and a stiff superstructure. It also needs user-friendly hardware and software that operators can use whenever they need them. Consequently, builders seem to be striving for a measure of openness in their architectures. Until recently, CMMs tended to be closed systems, much like microcomputers before International Busi-ness Machines Corp. (White Plains, NY) revolutionized the computing industry with its Personal Computer. Although CMM technology remains closed in many ways, a door seems to have opened.
One way has been through the emergence of the PH10 probe from Renishaw Inc. (Hoffman Estates, IL) as a kind of standard interface for connecting sensors to CMMs. Because of the probe's popularity among users, CMM builders and scanner manufacturers are developing products to fit the platform, elevating it to enjoy status as the apparent "Windows" of multiprobe systems. "The Renishaw design gives us the ability to hot swap, if you will, from one probe to the other," says Elepano at Mitutoyo. "It allows people to use a variety of sensors without having to invest a heck of a lot more development dollars."
Nevertheless, machine builders and probe manufactures continue to develop and produce their own designs to accommodate the specific needs of each application. Mitutoyo, for example, has responded to the need for rigidity for high-speed scanning by designing its scanning probe to mount directly to the machine's quill. Its machine, however, can come with probe racks so users can deploy strategies from both Mitutoyo and Renishaw.
Other evidence pointing toward more openness is found in new software developments. These developments are important because every sensor that comes onto the machine works differently and requires a new set of operating strategies and tactics. Consequently, CMM builders are developing software to make those differences transparent to users. "If we put another probe on the machine, our customers don't want to have to learn another software package on top of the one for touch-trigger probing," says Walker at LK Metrology.
His company's approach is to construct the software so operators simply specify a probe and describe the features to be measured, using protocols from the dimensional measuring interface standard (DMIS). For inspecting a hole, "the operator just types in the command to inspect a hole, for example, entering a nominal value and tolerance," says Walker. If the selected sensor is a touch-trigger probe, the software uses the dimensional data to tell the machine to go to the center of the hole and to move in-and-out radially to touch the surface a specified number of times. For an analog contact probe, the software would direct the machine to put the stylus onto the hole's surface and to follow a path along its circumference.
For specialty probes, such as lasers and optical units, CMM builders are working with third-party vendors, sharing details about controllers. The fruit of this collaboration is the embedding of software into their controllers for making the programming requirements of these probes transparent to the user. "Our probe essentially shakes hands with any of the major CMM controllers," reports Jim Clark at Metris. "It's almost a plug-and-play application."
Now that more users are adding a variety of scanners to their CMMs, data analysis is becoming a hot issue among CMM builders. CMMs using these probes and scanners "can generate huge amounts of data, but the question is can the software accept it and produce something meaningful from it," says Walker at LK. "If you were to scan a 1-inch diameter hole, you really would be looking for location, size and form. All you want is three pieces of information, so our software has to distill the thousands of points down to that." Consequently, LK and other builders are working with Metris and other probe and scanner manufacturers to integrate such software into their machines' controllers-and build even more powerful weapons of flexible inspection.
Sidebar: Tech tips
• Multisensor CMMs allow the most applicable sensor to be used for each operation.
• Five classes of probes have been identified for the new breed of CMM: digital-trigger and analog-scanning contact probes and optical, laser and temperature noncontact probes.
• Until recently, CMMs tended to be closed systems, but the ability to wield any of several sensors requires a measure of openness in the systems.