Advancements in digital vision systems make measuring parts simpler, more repeatable

You may be in the mood to pitch those cumbersome Mylars for measuring parts on an optical comparator, if you haven’t done so already. This shop-cleaning activity could occur after acquiring a digital video measuring (DVM) system that enables electronically superimposing a CAD system’s DXF file of the part to be measured over a live video image of the part.

“What you’re doing is replacing the old Mylars used on comparators,” said Mike Provenzano, applications engineer for Nikon Metrology Inc., Brighton, Mich.

Nonetheless, he emphasized that the biggest benefit a digital vision system provides is the machine’s ability to automatically measure parts, whereas the human operator influences a manual analog measurement device, such as an optical comparator.

Greg Maisch, product development manager for Starrett Kinemetric Engineering, measures a part on the company’s new HVR100-FLIP benchtop vision measurement system, which can also measure parts in the horizontal orientation. Image courtesy of Starrett Kinemetric Engineering.

Based on his studies with different types of metrology equipment, Provenzano concluded that a skilled technician can accurately and repeatably measure a workpiece from one point to another using a manual measuring device, but that accuracy and repeatability don’t extend from skilled technician to skilled technician. The repeatability for one individual might be within 0.25µm, while the variation might be four to eight times as much from one technician to another.

“If you don’t have good, experienced people who’ve been doing it for 20 years, then forget about it,” he said.

Having the ability to superimpose, or overlay, a DXF file over a live video image of a part really shines when part complexity increases. If end users don’t use an overlay, they can only see lines, curves, circles and a constant radius with any machine running the Metlogix M3 software suite, said Greg Maisch, product development manager for Starrett Kinemetric Engineering Inc., Laguna Hills, Calif. With an overlay, it’s possible to compare significantly more complex curves, polylines and other compound features on a machined part to the actual part design and obtain quantitative data about geometric tolerances.

“You can watch that data migrate over time and make corrections as necessary,” Maisch said, adding that suitably complex parts include knee implants and turbines for jet engines.

Going Digital

Rather than just having a technician look at a Mylar and determine whether a part is good or bad, Maisch said the main benefit of a DVM system is the ability to generate quantitative data to know which specific part features are in or out of tolerance or drifting toward bad. “You’re going to see those trends much more clearly,” he said, “and you can analyze that data to find the root cause. Root-cause analysis becomes much more powerful.”

Cutting tool wear is one possible reason for producing out-of-tolerance parts, but the problem is preventable. “As the tool wears, you can monitor for tool wear and make a change before parts get out of tolerance,” said George Schuetz, director of precision gages for Mahr Inc., Providence, R.I. “That what’s the whole process of monitoring is all about.”

The addition of a rotary indexer on the VMA-4540 vision machine enables parts to be rotated into the field of view. Image courtesy of Nikon Metrology.

Ineffective workholders are another root cause to be considered. “We have a saying here that ‘in order to gage it, you have to stage it,’ and it’s the same thing when you’re manufacturing a part,” Schuetz said. “If the fixture that’s holding the part while it is being manufactured is becoming worn, then it doesn’t hold as well, and you are more apt to get variation in the parts.”

Unlike an optical comparator, a DVM system can measure hundreds of points in the field of view, Maisch said. The system runs the entire part measurement program with very little interaction, enabling a technician to perform other productive tasks.

Advancements in the sensors and cameras for these systems have enabled metrology equipment manufacturers to provide large field-of-view images that were typically only available on an optical comparator-type device, Maisch explained. He added that the latest sensors and camera have become considerably more sophisticated than past generations. “Because you have so many pixels, you can really cover a large field of view.”

In addition, being able to better process data via newer computer interfaces, such as USB 3.0 with a data transfer rate about 10 times faster than USB 2.0, enhances the functionality of digital vision systems, Maisch said.

For a machine of comparable size, he said an optical comparator costs about half to a third as much as a DVM machine,
depending on the accuracy requirement. But the gap should shrink even as the digital side’s technology advances. “As computers and sensors continue to get more sophisticated,” Maisch said, “you will still see the cost come down.”

Provenzano doesn’t necessarily see prices coming down, but he said the capabilities of the cameras and lasers on Nikon’s digital vision systems, such as the high-end VMZ-R series, continue to improve while the price tag remains the same. “Everything tends to go up in price, so, if we don’t raise the price, that’s almost like a cost savings.”

And how capable are the cameras getting? “When you get into the high-resolution cameras,” Provenzano said, “you are really just about knocking on the door of imaging equal to a microscope.”

He added that the company is also making major strides in the level of available magnification. “I have machines that go way beyond what anybody needs.”

Best of Both Worlds

Although end users are gradually shifting to the digital vision side of metrology, don’t dismiss analog equipment. “There is still a big demand for traditional optical comparator technology with some enhancements,” said Mark Arenal, general manager of Starrett Kinemetric Engineering, noting automatic edge detection as one such improvement. “A lot of our optical comparator products have very capable metrology readouts.”

Maisch added that an optical comparator, which is a bit more shop-hardened and less susceptible to thermal cycling, probably is more appropriate for a small machine shop that only needs to spot-check parts. “It’s very important with the vision machines that you keep them in a fairly controlled environment.”