Using CMMs for Surface Finish Measurement
Advancements in motors and sensors improve CMM surface measurement capabilities.
When explaining surface finish measurement, it can be helpful to start with the classic metaphor of a desert, as often described by gage maker Taylor Hobson. Imagine grains of sand as roughness, the ripples of sand as waviness, and then the undulating dunes as the surface profile.
Traditionally, metrologists have used a dedicated surface finish gage to measure the three categories, but new tools such as Zeiss’s Rotos roughness sensor are making CMMs an increasingly attractive option for the task. Combining multiple measurement functions into the same machine is beneficial when shop and lab floor space is at a minimum, explains Brett Labadie, applications engineering manager, Zeiss Industrial Metrology. He adds that consolidating efforts into a single machine can also provide faster cycle speeds and cost savings on fixtures.
Why Surface Measurement Matters
The auto industry provides many straightforward examples of why surface finish measurement is important. In an age of 100,000-mile-plus powertrain warranties, it’s imperative that engine block cylinder bores are made with the correct surface roughness in order to retain oil for lubrication, guaranteeing low wear and long life.
Labadie also points to an example from the 1970s, when body panels on cars would suffer from the “orange peel” affect.
“What it boiled down to is there wasn’t enough roughness—or enough tooth—to actually hold the paint to the panel before it would dry,” he explains. “In the process of drying, it would run and create that orange peel effect. And that’s something that people are cognizant of today. We expect high-quality end products, and surface finish plays a role in telling us if that product is going to have the aesthetic, or even the longevity, that we expect as a consumer.”
When manufacturers are concerned with waviness, they’re often concerned with how well a part is going to seal. “Waviness is the long wave form,” Labadie explains. “It removes the roughness characteristic. Let’s say you’ve got a lathe, and you’ve turned the surface. But the bed of the lathe had some issues and it had some undulation, and that undulation of the lathe is going to completely transfer into that turned part. It might have enough of an impact that the part does not meet that functional intent.”
Modular design and rotation in three axes—each featuring ±180 degrees—allows a stylus to reach all areas of a part for inspection. Source: Zeiss Industrial Metrology
Surface Measurement with CMMs
CMMs have offered surface measurement solutions for several years, but newer sensors and better flexibility have made them more practical.
Some of the biggest challenges with surface finish are environmental factors such as vibration, he explains. Since CMMs are typically air-bearing systems, there are prevalent harmonics or frequencies that can unintentionally have an influence over surface finish measurement results.
This has caused problems in the past when measuring waviness with a CMM. Surface measurement nosepieces (pickups) are either skidded or skidless. Skidless surface finish measurement allows waviness evaluation. With skidded measurement, the skid will dampen vibration or noise that might be inherent in the environment. With skidless measurement, the stylus that moves across the surface is more susceptible to vibration from the environment. Continued improvements in CMM design have minimized influences with regard to skidless measurement, Labadie explains.
“As technology continues to improve and change, you have better ways to control some of that inherent noise that did have an influence at a greater level in the past,” he says.
Increased range of motion with sensor design has also enabled CMMs to better access all areas of the part. With Zeiss’s Rotos sensor, all three axes allow for full rotations of plus-or-minus 180 degrees, allowing a CMM to perform surface measurement inspections immediately following dimensional measurement inspections without re-fixturing the part. This eliminates the need to take the part from the CMM to the dedicated surface finish gage, therefore allowing for a possible cycle time reduction and less part handling by operators.
It was important to design a product that allows for accessibility to the different features that need to be measured, whether they are small holes, or recesses, he explains. “Or let’s say you have a large part and you’re holding it in a fixture, and there’re features on the bottom of the part that need to be measured, but the customer doesn’t want to re-fixture the part, so allowing for flexibility in the surface finish pickups that are available to access those different features was a big consideration.”
All of this is also made possible by automatic probe change.
“So we might have a traditional CMM sensor that does all the dimensional checking with a stylus system, different ruby styli, or other traditional CMM probe tooling,” he says. “What happens is, we drop off the traditional probe tooling in the rack, and then we pick up the surface finish sensor that has the appropriate surface finish pickup attached to it. Then we go through and do all of our surface finish measurements. So it’s just another tool that goes in the rack that gives greater capability and flexibility. And with that, the software has the analytical tools needed for the surface finish evaluation, as well.”
A wide variety of sensors are available to perform surface finish measurement with a CMM. Source: Zeiss Industrial Metrology
As technology advances across the board, Labadie expects to see more and more metrologists using CMMs for surface finish measurement.
“Everybody has a CMM, and if we can incorporate more things into a CMM and maintain the accuracy specifications that customers want, I think the additional flexibility and capability is attractive to them,” he says. “I went to a quality show several years ago, and one of the customers came up and said they wish metrology companies would invent a box, where you would simply put a part in and it would tell you everything you need to know about the part.
“Well, CMMs are driving in that direction with these different types of sensor technologies that are now available. They provide the option to further make the CMM even more flexible.”
That being said, dedicated surface measurement gages, at least for the foreseeable future, will still have a place.
“Vibration is the enemy of surface finish. And with the CMM, there’s always going to be some inherent vibration that has an influence at some level,” he says. “Not all surface finish gages are going to be replaced. But I think in a broader sense, if you’re thinking about mass production, and where a lot of these bigger companies have gage benches, and point-of-use gages that operators are using to measure surface finish on various geometries, serious consideration can now be given to using the CMM for these measurements.”