Early people used the width of a finger, the length of a foot, the distance covered in a stride, the length of a furrow plowed by a horse and other common, easily recognizable distances as the basis for linear measurements. Although improvements were made in the way length was measured--through the establishment of standards, such as the distance from the king's elbow to the tip of his middle finger officially equaling one yard--it wasn't until the Industrial Revolution that the importance of precision measurement was recognized as a key to successful manufacturing. That's when art became science.
The practical need to accurately measure part dimensions served as the catalyst in developing a host of precision measuring instruments, including calipers and micrometers. Of all the tools used to measure dimensions, the coordinate measuring machine is perhaps the ultimate in precision measurement tools.
Coordinate measuring machines collect detailed dimensional data by moving a sensing device, called a probe, along workpiece surfaces. First developed in the 1950s, CMMs--then called universal measuring machines--were manually operated. An operator physically moved the machine's probe from point to point on the workpiece, and the dimensional information registered on a Vernier scale. The operator recorded the data and manually performed any necessary calculations. A major breakthrough in technology occurred in the early 1960s with the introduction of the first electronic CMM, equipped with metal scales and an encoder that read the scales and automatically displayed data on a digital readout.
The advent of relatively inexpensive computers for commercial and industrial use in the early 1970s revolutionized the CMM's use in measuring and inspection operations. The computer receives the measurement data gathered by the sensor and, guided by the software program, manipulates it into usable information for the user. The computer also automatically aligns parts for measurement and compensates for errors induced by the structure of the machine.
In the mid-1970s, CMM manufacturers introduced direct computer control to coordinate metrology. In DCC applications, the computer operates the machine, eliminating any operator influence on the quality of the recorded data.
Making measurement more meaningful
The philosophical evolution of coordinate metrology mirrors the physical evolution of the CMM. During the past three decades, coordinate metrology has come of age, moving from a means of quality control to a method of process control.
The value of coordinate metrology as a process control tool has never been questioned. Pioneering manufacturers knew that the closer parts came to matching their ideal dimensions, the better they fit together and performed. Those manufacturers understood that the dimensional information gathered during the part's measurement could be used as a check on the condition of the process used to make it. For example, if a hole being drilled in a part drifted off center, there had to be a reason. Accurate dimensional measurement could point to an answer.
CMMs' flexibility to accurately measure objects of widely varying sizes and geometric configurations and their ability to provide the relationship between features of a workpiece makes them valuable process control tools. This flexibility, and the ability to perform coordinate measuring operations quickly when compared with surface plate techniques or fixed gages, means that measurement results can be used to cost-effectively refine manufacturing process applications and analyze process trends.
Traditionally, using coordinate metrology for process control normally requires measurement routines to be performed offline by specially trained technicians removed from manufacturing operations. Even though the dimensional data gathered during measurement and inspection operations is valuable and, after analysis, can be used to correct process variations that cause out-of-tolerance conditions, process corrections are often made after parts are scrapped or reworked to meet specifications. The answer is to move the measurement and inspection operation as close to the manufacturing process as possible.
Fitting CMMs to the shop environment
CMM manufacturers addressed the conflict between precision measurement capability and the need to integrate coordinate metrology into manufacturing operations by improving CMM design. To reach this objective, they focused on three areas: temperature compensation, high-speed data-gathering sensor technology and software improvements. Advances in all three areas continue to result in improved CMMs that can withstand the rigors of the factory floor while maintaining the sensitivity to perform precision measurement at speeds that meet throughput objectives.
Temperature and its effect on the measuring machine and the workpiece remains the biggest obstacle to designing CMMs for the shop floor. Air-conditioned enclosures protected early production CMMs from thermal gradients. These enclosures held constant the temperature of the machine and workpiece. While this solution worked, it created some difficulties in workpiece loading and unloading, plus the workpiece had to "soak" at the enclosure temperature before accurate measurements could be taken. While accuracy remained high, throughput dropped.
CMM designers have begun to compensate for thermal expansion and thermal distortion errors with a mix of hardware and software solutions that include placing a web of sensors in critical points in the machine structure. The sensors read the temperature on the CMM's structure, and a powerful algorithm extrapolates expansion and distortion values from the data. Through these values, the software compensates for each measured point so that the influence of temperature variations is virtually canceled over a wide range. The result allows dimensional inspection to be performed on the shop floor with an accuracy comparable to that of a lab.
Obviously, varying degrees of harshness exist in shop floor environments. In environments filled with oil, dirt and chips, CMMs must be built like the machine tools they support. Therefore, in addition to temperature compensation to assure accuracy and repeatability, these CMMs must also have completely sealed roller bearing packs and linear bearing packs, rather than air bearings. In operations that are less hostile, thermal compensation itself can assure top performance. Some production environments rely on the natural temperature-compensating effects of aluminum machine construction for reliable measurement results.
Improving sensor performance
Another trend concerns data-gathering sensor development. The touch-trigger probe has been standard on CMMs for the past 30 years. This data-gathering device is extremely reliable and repeatable; however, it's not fast enough for applications that require massive amounts of data, such as form evaluation.
The trend in sensor development is directed toward refinement of noncontact sensors. These sensors combine the elements of optics, video and laser technologies into devices that can rapidly scan complex shapes and surfaces, and accurately gather dimensional data. Some of these new sensors can gather up to 20,000 data points per second with extreme accuracy. Combined with these sensors are powerful mathematical engines that quickly analyze the billions of bits of dimensional data.
As versatile as these sensors are, they have drawbacks as well. In some applications, where the workpiece surface finish is smooth and shiny, reflections from noncontact sensors can create inaccurate data recording. Consequently, development efforts continue in the area of contact sensor technology to improve the speed at which these data-gathering devices can accurately function.
Software liberates metrology from the lab
Today's CMM software has been refined so that no computer programming knowledge is required to run even the most sophisticated programs. Virtually all new CMM software consists of off-the-shelf, menu-driven programs with comprehensive help screens. The result is that the software for any particular routine can be customized to fit individual applications using plain English, rather than a specific programming language tied to a complicated operating system.
A key software design element is the trend toward simplified operator interfaces, which presents only those interactions required for the application currently in use. This capability allows users to select the measurement operation most appropriate to the task, maximizing the use of measurement equipment. In most instances, users merely "point and click" to access powerful routines and accompanying data analysis.
Bridging design and manufacturing
The next generation of metrology software will provide an information conduit to link the design and manufacturing functions through the common language of metrology. This new software establishes a bridge between design and manufacturing by means of a common implementation of design tolerances, and between manufacturing and design by providing online and on-demand process information to design engineers.
For example, new software seamlessly interfaces with computer-aided design and computer-aided manufacturing systems and offline inspection systems so that CAD models don't have to be translated when downloaded to the inspection system.
The new software also features an open architecture, object-oriented approach to development. This will allow users to easily add their own, in-house developed inspection routines and analysis packages to the operating system. Third-party developers can also create metrology packages that can be incorporated into the basic software system.
New CMM software also makes it possible to accurately analyze the hundreds of thousands of data points that some CMMs collect during scanning, which is simply a means of automatically collecting data points to accurately define the shape of a workpiece. Massive amounts of dimensional data are critical for determining the form and fit of component parts and often provide a more meaningful analysis of process status than the amount of data that can be gathered efficiently using point-to-point techniques.
Adding value to measuring systems
As CMMs become more common in manufacturing operations, the need for CMM manufacturers to provide users with a host of services that enhance and add value to their measuring systems becomes more important as well. A developing trend is the new approach to service that CMM manufacturers such as Brown & Sharpe are employing. The availability of aftermarket services such as the repair and rebuilding of CMMs, software upgrades, machine calibration, contract inspection and programming indicate that coordinate metrology has reached an important stage in its development.
CMMs were once thought to be expensive specialty tools whose place was in an environmentally controlled clean room. Now, they are considered practical additions to the complete machine tool inventory of nearly every major manufacturer, and they have moved to their rightful place as process control instruments on the factory floor. In effect, CMMs are becoming more like machine tools, and that is a trend that is likely to continue for some time.
About the author
David H. Genest is Brown & Sharpe's marketing and corporate communications director. E-mail him at email@example.com .
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