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Where Microns Matter: Optical Measuring SystemsWhere Microns Matter: Optical Measuring Systems

By Karen Haywood Queen Contributing Editor, SME Media
QVI’s Tom Groff runs a measurement on a Fusion machine.

Optical measuring systems, which use light instead of touch, are becoming more widely used in manufacturing because of their faster speed, higher accuracy and ability to measure oddly shaped parts.

Optical measuring machines are used in all stages of manufacturing—for proof of concept, mass production and quality inspections. For times when both tactile and optical measurements are needed, manufacturers are offering more flexible, multi-sensor systems with laser, camera and tactile measuring options.

Optical metrology is proving its value in the aerospace, automotive, electronics and medical sectors, as well as in precision engineering across all sectors, manufacturing leaders say.

“In my world, microns matter,” said Jay Elepano, optical coordinate measuring machines (CMM)/surface form and geometry (SFG) manager at Zeiss Industrial Quality Solutions. “A human hair is 100 microns. My world is two to 20 microns. We’re literally splitting hairs.”

‘‘Optical metrology has a part to play both in proof of concept or first articles and when products move to mass production,” said Ryan Toole, product manager, stationary solutions, for Hexagon Manufacturing Intelligence. “With precision engineering, you have extremely small features on a part or features that can’t be accessed by traditional tactile programs methods or you don’t want to physically touch the part because of the material it’s made of.”

“Everyone is always looking at measuring things faster, with higher throughput and the same level of accuracy,” said Tom Groff, VP of sales at QVI’s OGP (Optical Gaging Products) unit. “Cell phones have smaller and smaller components. Even the glass that is on the devices is very intricate in different layers. We have sensors that can measure different layers of the glass thickness, from half a micron to five microns.”

As printed circuit boards and other components are miniaturized, notably in the electronics industry, the ability to get accurate measurements will dictate whether the finished product will work, Elepano said.

Further, products in the electronics and injection molding industries are often characterized by dense clusters of small features not accessible to a tactile sensor, Toole said. An image-processing sensor, on Optiv CMMs, simultaneously measures these densely arranged features within the camera’s field of view and does much faster than conventional tactile probing, he added.

“You can look through a camera, take an image and use software that will immediately tell you what is going on with that part, a printed circuit board, for example,” Toole said. “In precision engineering, you have very tight tolerances and the need for a large amount of data from what you’re measuring. When you’re using an optical probe, you’re getting thousands of points of data, and you can work within the software or export the data for analysis.”

Medical sector especially promising

The medical sector has a high growth potential for optical measurements because of tough FDA regulations, the need for speed, strict accuracy requirements, oddly shaped components and the necessity of measuring parts without risking contamination.

“Decades ago, the best way to measure the part was to actually make contact with the part,” Elepano said. “Nowadays, as video- and camera-based technology has increased, it is becoming more and more likely that you can get the same level of accuracy from measuring the part with non-contact methods.”

For example, tactile measuring systems could contaminate an artificial hip. That hip also cannot be off spec by even a few microns.

Meantime, if the shifter knob on a car is slightly off, consequences aren’t dire. Users can choose the machine with the level of accuracy that works for their manufacturing requirements. Not surprisingly, greater accuracy correlates with higher cost, Elepano said.

“When failure is catastrophic, you need precision,” Elepano said.

“Medical has had growth because of the strict requirements and the tighter accuracy that is needed,” Groff said. “A lot of insertable instruments used have very tight tolerances and are difficult to measure. It’s not like you’re measuring a square part. We have three different configurations that allow for certain types of medical devices to be measured. It’s all about minimizing risk with the FDA. These are what medical companies are required to have documentation on tap for when they get audited by the FDA.”

Optical measuring systems also make it easier to trace an audit trail. Managers can easily add an electronic signature to document their input so that it can be recorded and traced, he said.

“The FDA has requirements for audit trails,” Groff said. “When you are executing and inspecting a medical device, the whole history of the process, who did what, what lot it was, any issues, compliances and user access need to be tracked and logged in an audit log. Our software has the ability to add traceability to the data-collection process, making the FDA-required audit log easier to implement and execute.”

Not only can 100% inspection be realized, but inspection speed and throughput can also be dramatically increased due to automated system measurement routines and, depending on the application, palletized multi-part fixture inspection tables, said Starrett Kinemetric Engineering General Manager Mark Arenal.

“Ultimately, the goal in any good inspection program should be to attain maximum gage repeatability and reproducibility (GR&R) and, in the process, provide comprehensive data for statistical process control and traceability,” he said.

Reliable GR&R is possible with today’s non-contact video and multi-sensor systems and their advanced software, he said. Current display readout and software measuring technology, such as Metlogix M3, provides full qualitative/quantitative profile analysis functions where an inspector can compare a part profile against a nominal CAD model and obtain an actual graphic representation of any deviation from the CAD file.

For example, Starrett HDV (horizontal digital video) comparators offer the ability to automatically find and track the edge, continuously comparing it to a 2D profile, and superimpose the edge on a CAD model, Arenal said. In this scenario, an operator quickly, easily and automatically collects tremendous amounts of data that are all archived and documented with date, time, lot number, job number—removing operator error from the equation.

Speed of inspection is key

Speed also matters when all parts must be inspected. Without the necessary speed, the inspection process becomes a bottleneck during manufacturing, Toole said. The ability to quickly measure a dense cluster of features also is a prerequisite for precise and repeatable evaluations of form and true position, he said.

“We see 100% inspection as commonplace for medical implants,” he said. “When you’re looking for high throughput, optical metrology has an advantage.”

Pallet measurement in an automation setup, where several similar workpieces are measured at one time and the optical measurement is repeated in a loop, also helps speed up throughput, Toole said.

AM will be affected

Additive manufacturing (AM) is another emerging sector for optical measuring, Groff said.
“They are getting to the point where 3D printed parts can have the tolerance levels to require inspection systems like ours,” he added.

Optical measuring machines are deployed about half the time in production and the other half in quality inspections, Groff said. On the production floor, temperature, humidity and vibrations can affect accuracy in measuring.

“If the temperature changes dramatically when parts are getting measured, the measuring machine needs to account for that,” he said. “Even more of a factor is vibration. That’s why people still send parts from the production floor to a quality lab for first inspection.”

In the past, one challenge has been to find qualified operators with a good knowledge of measuring and programming logic, Groff said. Improved software has met this challenge.

“The biggest challenge on any automatic measuring machine is the creation of the inspection program,” he said. “We’ve made our software more intelligent. We have the ability to use the CAD model to define how users want to measure the part. The CAD model now has product and manufacturing information knowledge. We import that knowledge into the model and now we know a lot more about how to measure that part.”

Other big issues include geometric dimensions and tolerances, Groff said. “To apply those tolerances, you have to program the machine correctly. Now the tolerances are embedded in the model and we can rely on our measurement software to report the right results,” he said. “The software creates the whole program for you, including the measurement path and how to avoid hitting the part and fixture. The user doesn’t have to know as much.”

Another challenge has been the need to measure multiple orientations of the same part. In typical optical measuring machines, the video and laser are always pointing down, he said. QVI’s FlexPoint allows the user to orient the sensors in different directions so as to measure different orientations for a part without having to rotate that part.

Sometimes, though, users need to use multiple modalities, optical and tactile, during the manufacturing process. With that in mind, manufactures are using optical measuring machines that add an additional dimension via a tactile probe.

More recently, flexible measuring machines have been geared only toward measuring smaller parts that lend themselves well to benchtop machines, Groff said.

“But small features now exist on larger parts,” he said. “Think about a big turbine engine. Those are large products but they have intricate parts.”

QVI’s FlexPoint and Fusion are families of machines that enable tactile, video and laser measuring of larger parts.

“Instead of moving everything around to take measurements, you can measure everything at once [with] that snapshot by a video camera,” Groff said. “When you incorporate a multi-multi-sensing system, laser and tactile on top of that, it can be hundreds of times faster. If I have thousands of features in that four-inch area, it can be thousands of times faster.”

Hexagon’s Optiv 322 combines optical metrology (a camera and chromatic white light sensor) with contact metrology (contact scanning and touch trigger probes) on one frame, Toole said. Hexagon’s PC-DMIS software allows a fast inspection of the full set of features. Finally, as with other emerging measuring machines, this benchtop system has a small footprint.

The Optiv 322 also offers flexibility to users, Toole said. “You can purchase a machine on Day One that does only optical metrology,” he said. “If your needs change, if the parts you need to measure change, you can easily add an analog scanning probe for tactile scanning. You can also add a chromatic white light sensor later. You can get what you need today and upgrade the system later. The system can easily adapt as needs change.”

Zeiss leveraged a long history making microscopes with its later entry to the field of optical measuring systems when developing its O-Inspect optical measuring machine over the last five years.

Instead of following the traditional method of adding a tactile measuring probe to an optical measuring machine, Zeiss calibrates O-Inspect in three dimensions—just like its traditional coordinate measuring machines, Elepano said.

“Many manufacturers add a tactile probe offset from their camera, which doesn’t give you true 3D compensation” he said. “We did the opposite. We have calibrated the scanning probe in the entire volume in all three dimensions, X, Y and Z, and the camera is offset to that. We then have a camera that is calibrated through the entire 3D volume. That’s the benefit of being newer to the market.”
Challenges remain. Some manufacturers are reluctant to try an unfamiliar technology or they simply don’t know these solutions exist.

“A lot of people are used to traditional CMMs,” Toole said. “They’re familiar with tactile probes, less familiar with optical probes. People shy away because they don’t know the technology. But the learning curve is very low. The software for optical probing is the same as for tactile. Users don’t have to learn a new software platform.”

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