What to Consider When Purchasing a CMM

By Barry Rogers

 

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 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.

CMMs at a trade show

By some estimates, the global installed base of CMMs is around 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 easily be 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.

Multi-Sensor Technology and Software

Advances in sensor technology are leading to improvements in metrology, thus enhancing quality control and inspection methods and resulting in more precise manufacturing processes. CMMs are becoming more flexible with multi-sensor technology, which combines 3D non-contact laser scanners, five-axis scanning with a touch probe, optical systems and surface finish measurement. Tool racks enable sensors to be changed automatically to continue measuring the same part.

Currently, customers often dedicate machines exclusively to touch-trigger or scanning measurement processes. While one CMM operator may specialize in touch-trigger scanning, another operator may specialize in laser scanning and yet another in optical or five-axis scanning. Different metrology techniques may require specific software to drive the data collection routine, while additional software may be required to analyze the data. Software may also be tailored for the application and the types of parts being measured, such as plastic, sheet metal, aerospace or automotive. 

Software enabling the full capability of the sensor type is essential. It’s also important for sensors to be fully integrated with metrology software across multiple platforms. Programming a CMM should be intuitive and operator-friendly. For example, programming a scanning probe to sweep each blade of a rotor is a complex task, but this process can be dramatically simplified with automated software features. Similarly, data analysis and reporting for each sensor type should produce uniform and compatible results.

Both software and hardware for metrology equipment are being developed to interact, thanks to interoperability standards. Generic software compatible with single devices will give way to an integration of all devices. Simplicity and ease of use are key benefits of this trend.

Likewise, open-platform controllers are available to enable any front-end software package written with an I++ interface to talk to any control system that supports an I++ client. This provides complete flexibility to run virtually any software package on any CMM with almost any sensor. Operators and programmers are no longer required to learn a multitude of software programs to run various machines and sensors.

Shop-Hardened CMMs

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 where 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.

CMM Calibration

Producing measurement data about a part to verify its accuracy and validate the integrity of its manufacturing process is a prime function of the CMM. However, to meet this function as well as to satisfy traceability requirements, CMMs must be properly calibrated periodically at a frequency set by the user. The two most common standards by which most CMMs are calibrated are ASME B89.4.1 and ISO 10360-2. The two standards are very similar.

The ASME B89 standard involves a ball bar artifact. Its use is generally confined to North America, whereas the ISO 10360 is favored in Europe, where it originated. This ISO standard is based on the measurement of certified length standards.

The B89 standard uses a ball bar of an uncalibrated length and is essentially a length measured repeatedly throughout the full working volume of the CMM. The axis scales are first calibrated by a separate test (Linear Displacement Accuracy) using a calibrated artifact such as a step gage or laser interferometer. Once the axis scales have been adjusted to measure properly, the ball bar is then used to ensure that the measurement of this length parallel to any axis will repeat anywhere within the volume of the CMM. This ball bar test yields the maximum range of results from all measurements.

The ISO 10360 standard applies to articulated-arm CMMs using tactile probes and optionally optical distance sensors (also referred to as laser line scanners or laser line probes). It specifies the acceptance tests for verifying the performance of an articulated-arm CMM based on a calibrated test length as stated by the CMM manufacturer. It also specifies the tests that enable the user to periodically re-verify the performance of the CMM. Details on tests for scanner accessories are also given. Part of ISO 10360 specifies performance requirements that can be assigned by the manufacturer or the user of the articulated-arm CMM. It also specifies the manner of execution for the acceptance and reverification tests to demonstrate the stated requirements.

The Future of Production CMMs

Almost 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.

Industry 4.0

Quality control is often approached as a reactive function. In other words, if a part is found to be out of tolerance, the only option is to retrace prior steps to find the cause and fix it. However, this approach has been likened to driving a car forward using only the rear-view mirror to steer by. Manufacturing must move from a reactive to a planned and then to a proactive approach. This is far more difficult than one would expect. The final step of achieving a predictive approach is even more difficult, but progress is underway. In the emerging world of Industry 4.0, metrology data from CMMs is likely to play a key role.

High-performance, high-output manufacturing relies on control and predictive action through the active use of measurement data. CMMs are ideal tools for providing data that flows through the business via factory information systems. This will enable ultimate control across the entire manufacturing process.

The demand for higher quality and greater consistency of parts can’t be met unless tools such as CMMs improve in capability, speed, accuracy and throughput. With the increasing versatility of CMMs and advances in sensor technology, along with more automation, CMMs will become an ever more essential part of the overall process.  For example, CMMs are likely to be used to digitize objects so this data can become part of the digital thread that is unifying the supply chain.

CMMs Add Value

When inspecting parts with simple holes for which location and diameter are most important, a touch-trigger probe will be most appropriate for the job. Parts requiring form, best fit or true position calculations will require a scanning probe to capture adequate data for analysis. 3D scanning for modeling, reverse engineering or comparing to the CAD file are functions easily done on a CMM. Deciding which sensor to use is often determined by the required part accuracy and speed. If you’re manufacturing aerospace turbine blades, for example, five-axis scanners or 3D non-contact laser scanners will perform the work far more quickly and with less programming than touch-trigger probes. Fixed-head camera sensors, which are new to the market, are supposedly capable of scanning leading and trailing edges of turbine blades very quickly. When the profile of a surface or surface flatness is important, non-contact white light and blue light sensors or cameras can be used. There are also probes available to measure surface finish on a CMM.

CMMs add great value. In fact, producing data about a part to verify its accuracy, validate manufacturing processes, satisfy traceability requirements and serve other product lifecycle interests adds value as much as cutting chips does, so CMMs are, in fact, moneymakers—a good reason to select a new CMM with great care and forethought.

 

 


ABOUT THE AUTHOR

Barry RogersBarry Rogers recently retired as global director of sales for a major machine tool OEM. Prior positions include director of global sales and marketing for Sunnen Products, and national sales and market manager for Renishaw North America. He also has served as general manager of Cincinnati Milacron’s LK CMM division in Detroit, Michigan. Barry recently started Alpha Strategies, a Chicago-based consulting firm he serves as president. alphastrategiesconsulting.com