The Coordinate Measuring Machine has been around for a very long time with the first machines introduced to the market by DEA and Ferranti back in the early 1960’s. These early CMMs were manually operated with ‘hard probing’, eventually migrating to computer controlled CNC units with touch-trigger probes.
CMM touch-trigger probe pioneer Renishaw was founded in 1973 following the patenting of the touch-trigger probe by its joint founder while working at Rolls Royce. Today the CMM can be found globally in almost every precision manufacturing company and is the center piece of most quality control processes.
The CMM is a very flexible measuring device with its measurement limitation typically only constrained by physical size, probing configuration and accuracy grade. Over past decades CMMs have become faster, more accurate, and cheaper. Developments by CMM manufacturers have included making structures stiffer, lighter and thermally compensated for use outside of the traditional temperature controlled quality laboratory. Improvements in scanning probes, included the latest Renishaw 5 axis technology, has allowed CMMs to measure geometry of critical parts such as power-train components providing more measurement information than traditional gauging techniques and with considerably more flexibility.
The global installed base of coordinate measuring machines is estimated at around 150,000 units. The typical air bearing CMM structure rarely wears out and can be easily upgraded with new electronics and software extending life expectancy by another 10 years; in fact many 30 year old CMMs remain in daily use throughout the precision manufacturing sector with many ‘senior citizen’ CMM frames having been retrofitted multiple times.
CMMs have also been the source of much frustration for manufacturing engineers and managers as they all too frequently become bottle-necks in the manufacturing cycle; typically the CMM needs an expert programmer who invariably is also the CMM operator in most small and medium sized companies. The battle cry heard from many Plant Managers “we want to eliminate all CMMs” probably comes more out of frustration with CMM programming and measuring bottlenecks than with the functionality and quality of measured data delivered by the CMM.
There has been much speculation over the past decade associated with the demise of the CMM but in reality what is the alternative? The CMM is affordable, reliable, repeatable and accurate. Any replacement technology needs to be all of these … and more.
Taking the CMM out of the quality laboratory and closer to the point of production is nothing new; pioneers in flexible cell manufacturing such as GE were doing this back in the early 90’s. In those days the CMM was placed inside an air-conditioned enclosure with the production operator loading and unloading parts through an opening door. CMM enclosures still remain common place today.
Shop-hardened CMMs have emerged whereby the CMM air-bearings are replaced with linear guide-ways and structures manufactured from cast-iron or polymer concrete in an attempt to create a thermally inert device. Granite, with its natural ability to absorb humidity and expand, is not the the ideal material for shop-floor CMMs. Small footprints have also been demanded for shop-floor CMMs and interestingly the CMM industry has returned to the early cantilever design principle, pioneered by Ferranti, as its compact foot-print offers the most effective measuring volume.
Rise of Optical Scanners
The CMM has also proven to be a efficient carrier of non-tactile measuring probes, such as lasers scanners and optical sensors, with many suppliers now offering multi-sensor solutions complete with automatic probe changers. Line of sight with optical sensors is an issue and the traditional motorized indexing CMM probe head, used for carrying sensors, incurs both weight and index limitations.
With the current appetite for more digital data it’s not only tactile scanning that is seeing a huge growth opportunity. Optical scanners with their long stand-offs are seeing favor particularly for sheet-metal and plastic part inspection since they are capable of digitizing complete part geometries with much fewer sensor positions and in a faster measuring time.
Multi-Axis Industrial Robot
The multi-axis industrial robot has emerged as a competitor to the traditional Cartesian CMM since it offers a low cost, shop-hardened, robust and reliable mover for carrying the heavier optical scanners. Such solutions have already penetrated the automotive sheet-metal application and are eliminating the coordinate measuring machines in many pioneering companies. Seamless object orientated programming and measuring software, used in conjunction with the robot, has also reduced dramatically device programming complexity; accuracy trade-off appears to out-weigh the endured CMM frustrations whether actual or perceived.
Portable Articulated Arm
The manual 6 axis portable arm has also significantly eroded the manual CMM population over the past decade given its over whelming advantages of portability, flexibility and dexterity. Few CMM manufacturers today even offer a manual Cartesian CMM in their product portfolio such has been the market shift to portable arms.
The CMM is also being attacked in the CNC machine-shop by the increased use of on-machine probing whereby part inspection is being performed as part of the metal-cutting cycle. The ever increasing accuracy of metal-cutting machines is now on par with CMM structures, and the arguments of the CMM being an independent audit facility are outweighed by the virtues of real-time adaptive control and minimizing of defective parts.
The comparative gauging technique employed by Renishaw’s Equator provides a real-world solution to measurement in a non-perfect environment and offers a return to early production gauging principles of using master parts or reference gauges on the production floor eroding traditional CMM applications for small production parts. The recently introduced closed-loop feedback with CNC machine-tools moves coordinate metrology in the production machine shop firmly into Industry 4.0 territory.
Over past decades the CMM industry has been reduced to a few large suppliers with a global reach; the one exception is China where a ‘cottage industry’ still exists comprising numerous regional CMM suppliers all producing tens to a few hundred units a year. All of these Chinese domestic manufacturers use probing, measuring scales, and software components from Europe and USA with limited product differentiation and thus the CMM industry consolidation, that occurred throughout Europe and USA in the 1990’s, will invariably come to China in coming years. To-date no Chinese CMM manufacturer has successfully found a foothold in export markets, probably since CMM assembly labor-cost is their only advantage, which is outperformed by the manufacturing efficiency of western suppliers, most of whome also now have Chinese manufacturing operations. The recent communication from Nikon Metrology announcing their departure from CMM manufacturing could be an further indication that the CMM industry ‘shake-out’ is not yet complete.
The Industrial Robot has advantages over the CMM in its pay-load capability and continuous motion wrist; to compete in the future the more accurate CMM structure will need a heavy duty continuous motion wrist to handle latest generation of optical sensors.
No one can argue with CMM accuracy; the CMM can still perform and play a significant role in the future if it can overcome today’s limitations with respect to carrying the next generation of sensors. A heavy duty continuous motion wrist on a 3 axis Cartesian structure will outperform a 6 axis robot for accuracy and repeatability and could open the door to allowing the advantages of optical sensors to penetrate a much wider field of applications.
Computed Tomography is also starting to make inroads into the CMM market, particularly for intricate plastic parts and complex parts produced using Additive Manufacturing techniques albeit at a price-point currently far higher than a CMM.
In addition the CMM will continue its role in supporting the production of high accuracy parts in small shops, continue its audit role, particularly in critical medical and aerospace industries, and for sure CMMs are too ingrained in most companies to not simply just go away….
In summary the CMM is under threat on many fronts from the many emerging new measurement technologies. In order to survive it must fight back with more productive seamless programming software, higher performance motorized wrists and the ability to adapt to new optical sensing technologies.
The CMM, and its future role in manufacturing, is set to change. How long it will take and what size the eventual CMM market will become is anyone’s guess at this time….. the current reluctance to discard yesterdays CMMs may mean it stays around for another few decades.