Buying a Coordinate Measuring Machine: CMMs Past and Future
While there has been much speculation that CMMs are becoming obsolete, the opposite is true.
Coordinate measuring machines (CMMs) have been around since the 1960s. In most precision manufacturing facilities, the CMM is the company’s chief dimensional measurement device that is traceable to the standards maintained by National Institute of Standards & Technology (NIST). Most CMMs are used to verify dimensional accuracy for quality control (QC) and first-article inspection, the function of checking the first part produced by a manufacturing process to verify that it is meeting tolerance requirements. Inspection with a CMM provides data for in-process measurements to ensure dimensional integrity from operation to operation and to satisfy traceability requirements. CMMs are also used to perform gage repeatability and reproducibility (R&R) studies, 2D and 3D scanning, part sorting, reverse engineering and a host of other tasks.
By some estimates, the global installed base of CMMs is about 150,000 units. Because of the typical air-bearing design of many CMMs, the structure of these devices seldom wears out. Therefore, the lifespan of a CMM may exceed 30 years. Many 30-year-old CMMs are used daily throughout the precision manufacturing world. Older CMMs can be easily upgraded with new electronics, drives and the latest software, adding an extra 10 years to a machine’s usable life. In fact, a CMM can be retrofitted multiple times to extend its life even further.
During the past several decades, CMMs have become faster, more accurate and more affordable. While there has been much speculation that CMMs are becoming obsolete, the opposite is true. Today, CMMs are more versatile and functional than ever before.
For all their advancements, CMMs have long been the source of bottlenecks and frustration on the manufacturing floor. Machine tools may sit idle for hours, even days, waiting for the inspection results from the QC lab prior to the production. It’s not hard to understand why many production managers would like to see CMMs taken out of the QC lab and put onto the manufacturing floor.
Unfortunately, CMMs utilizing the traditional air-bearing construction with granite tables do not perform well in an open shop environment. Unlike temperature-controlled QC labs, shopfloor conditions rarely can maintain a constant 68°F temperature considered optimal for CMM performance in a lab. When the ambient temperature varies, measurement results also vary. The laws of physics underlie these variabilities. Furthermore, holes in air bearings tend to clog with oil, dust and debris floating in the air when machine tools are nearby. In these conditions, a CMM will eventually stop working properly. Today, shop-hardened CMMs have emerged to resist these problems. For these devices, air bearings have been replaced with mechanical bearings and linear guideways. Machine components manufactured from thermally stable materials have resulted in stiffer, lighter structures and thermal compensation for use outside temperature-controlled settings. Machine footprints are smaller to take up less valuable manufacturing floor space. Cantilever designs, which offer the most effective measuring volume, are making a comeback for shop-hardened applications. Interestingly, cantilever designs were once the most common type of inspection machines in the early years of CMM technology.
The Future of Production CMMs
As for the future, I envision that there will come a time when all CMMs will be fully integrated into the manufacturing process as manufacturing companies continue to demand in-line measurements for immediate feedback. This will enable timely, in-line adjustments to eliminate scrap and improve quality.
CMMs will be treated like any other machine tool as an integral part of the manufacturing process. The same operator machining the part will also load the part onto the CMM and push the button. Simplicity and ease of use will be required for this integration. Developments advancing this trend include color mapping to show where a workpiece is in or out of tolerance. Easy-to-read reports will be the norm. Because machine operators are not metrologists, the information from the CMM must be displayed in easy-to-understand reports. Systems capable of performing measurements at different stages throughout the process without human intervention are emerging, and manufacturing processes will self-correct based on these automated measurements.
This is part one of a three-part series about buying a CMM.
Find more insights about acquiring a new machine tool by visiting the Techspex Knowledge Center, “Guide to Buying Machine Tools.”
While countersunk and chamfered holes are similar in appearance, functionally they are quite different. Consequently, different gages exist to serve these different functional requirements.
Measuring workpiece dimensions is relatively simple for machine operators but measuring workpiece geometry which involves more complex comparisons of part shape to an ideal shape--is now also practical on the shop floor. The gaging equipment for doing this is coming down in price while becoming easier to use.
A laser scanning system helps this shop capture the free-form surfaces on a hand-sculpted original. The resulting digitized models are the basis for CAM applications such as programming a CNC machining center.