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Optical Comparators Adapt and Grow

Bruce Morey
By Bruce Morey Senior Technical Editor, SME Media

Beware predictions of the demise of any technology. If the early 1920s saw the dawn of the optical comparator, there has been much speculation about its sunset. That was especially true when vision systems started hitting their stride a few years ago. Many could see optical comparators were superfluous with the use of vision systems. Many thought the sunset of optical comparators was imminent. Many were wrong. Why?

Optical comparators, sometimes called profile projectors, remain a fundamental tool for metrology even though the basics were invented in the 1920s. Improvements like digital readouts and edge detectors have kept them relevant.

“It is a simple tool that is cost-effective and gets the job done,” explained Tom Snyder, vice president–service for Inspec Inc. (Canton, MI), a full-service calibration, value-added reseller, and contract metrology company.

He has a point. To a user, a low-end comparator is simplicity itself. A light projects a magnified silhouette of a part onto a screen. A user then measures dimensions and geometry of parts from the edges of the silhouette, hence its other name of profile projector. In the simplest models, precise graduations marked on the glass screen itself lets users make simple measurements.

More common today is a simple digital readout (DRO) that measures the distance of a cross hair projected onto the screen. Another enhancement is automatic edge detection, using photo-diodes to take out some of the operator variability in measuring the edge of the shadow. Moving from feature to feature gives a precise measurement that is easier than any caliper, micrometer, or height gage. From the beginning, special Mylar overlays have been clipped onto the screen with a trace of how the part should look when projected with tolerances. One quickly sees if a part is in specification. “In either case, extensive training is not required. A typical shop user can start using one effectively in about five minutes,” said Snyder. Typical uses are checks of a few critical components on relatively simple parts.

Optical comparators can also be cost-effective, with the simplest available for under $10,000. Existing units are often refurbished, since it is straightforward to replace screw threads with linear scales and add DROs. “We have serviced machines that are 50 years old,” Snyder said.

Inspec handles three different product lines: Mitutoyo, Starrett, and Micro-Vu. While there remains a place for simple shadowgraphs, Snyder is also seeing a clear trend to adding modern cameras, telecentric lenses, PCs and software to import CAD data in place of Mylar overlays. “There is a full line of variable capable machines that can meet many applications and user levels. Many people in the industry think of shadow graph basic units and we must show them the higher levels of technology that are available,” he said.

John Kaminsky of Inspec Inc. demonstrates the utility and power of an optical comparator.

Range and Applicability

Optical comparators come in two basic styles, horizontal and vertical. While stressing the simplicity and ease of use of either style, Walter Wardzala, major instrument sales specialist at Mitutoyo America Corp. (Aurora, IL) noted that there is some specialization between horizontal and vertical styles. The horizontal style, like Mitutoyo’s PH-A14 series, has its lens mounted horizontally—sideways if you will—and requires the part to be presented “standing up.” Vertical machines, on the other hand, “are more like a giant microscope,” he explained. Parts are laid down on a vertical stage and the lenses looks down on the part. “They are especially useful for small parts, such as what you might encounter in medical device manufacturing,” he remarked.

“In the US, vertical-style comparators are relatively rare. It is not people’s first thought process when they think of an optical comparator,” he said. He believes that most US customers are simply in the habit of thinking of horizontal machines, and tend to discount vertical machines. He compared verticals to larger versions of toolmaker’s microscopes, while admitting they tend to be a little pricier than a comparable horizontal machine.

Light sources used in optical comparators tend to be well collimated and designed for the lenses used. Nikon features a through-the-lens light source for sharp, clear images, according to the company.

A good example is the company’s PJ-H30 Series 303-Profile Projectors. The machine boasts measuring accuracy of (3+0.02L) μm, according to the company. It also features a variety of optional accessories in lenses, filters, reading scales, fixtures and stages. Parts can range from a few millimeters up to 80 mm, depending on the accessories.

He also compares comparators to the wider world of metrology equipment, especially vision systems. He expects comparators to continue to evolve, with LED lighting and better measurement coupled with automatic DROs. A good example of an advanced DRO is Mitutoyo’s QM-Data200 series processing unit, a sort of DRO on steroids. While providing 2D coordinate measuring, it also offers automatic feature detection, tolerance zone measurement, and output of data in a spreadsheet format.

Collimating the Light Source

There are other distinct technical advantages of optical comparators if one wants to compare them to vision or video measuring systems, according to Mike Wolfe, sales manager for Nikon Metrology Inc. (Brighton, MI). It is all about the lenses and the lighting, he said.

“The beauty of a good optical comparator in comparison to vision systems is that the light source can be perfectly collimated and designed for the lenses being used,” he explained. Nikon instruments feature a unique through-the-lens light source, according to Wolfe. “With our optical comparators, you get sharp and clear edges on round or turned parts, such as threaded parts, shafts or pins.” In contrast, most other comparators use external or oblique lighting for surface illumination. “They will shine points of light with fiber bundles or other external light to illuminate the sample. This can produce unwanted shadows,” he said.

The definition of a comparator and a vision system is, in some instances, blurring, such as in the c-vision lite system from CCP (a brand of CVI). It is designed for use on the shop floor, with users walking up and making quick measurements on a digital system as they are used to doing with an optical comparator.

It often boils down to the part. “Vision systems measure many samples well, such as flat parts and stampings, and can provide high throughput. Comparators, on the other hand, measure turned and round parts exceedingly well,” said Wolfe. He also noted that the task of measuring complex features like splines is ideal for comparators. A spline is a series of multiple radii blended together, making it difficult to measure all those radii individually. A complex Mylar overlay, designed with a CAD system, can be used to create a quick Go/No Go reference. Standard overlays can also be used for more common feature measurement and comparison.

Nikon offers two vertical beam optical comparators, the V-12B with a 12″ screen and the V-20B with a 20″ screen. Each can have three different magnifying lenses housed in movable turrets, like a microscope. Nikon also offers a horizontal beam model, the H-14L with a 14″ screen.

Advancements as a Mirror of the Past

“Technology advancements have helped make optical comparators more effective and efficient to use,” said Mark Arenal, general manager for Starrett Kinemetric Engineering (Laguna Hills, CA). These advancements range from more capable DROs to the replacement of halogen lights with LEDs, which are cooler, quieter, and last longer. Starrett offers a wide range of comparators, including horizontal, vertical, benchtop, floor-standing and floor-standing side bed models.

While he noted that advancements are important, there is more than technology involved—training is also a significant factor. Technicians, machinists and operators already know how to use existing comparators. “We’ve also found that people are working longer,” said Arenal. “This means senior employees that have been using this technology for decades are quite comfortable with it and want to continue doing so.” At the same time, he noted that future generations cannot be neglected. The company has optical comparators in community colleges and they have even trained future operators at a local high school on the principles of optical comparators. It is that easy to pick up.

Which is why optical comparators will retain their look-and-feel as they evolve. “Our HDV model comparator is really a horizontal digital video system,” he explained. “It is very much structured like an optical comparator from the way it is built, but it is all digital video. You can import a DXF file created from a CAD file to create electronic overlays, for example, instead of using Mylar overlays.” This is becoming a common set of features for many suppliers.

It begs the question: are they still comparators?

Arenal sees it as an evolving technology stream, as new features are enabled by advances in supporting technology, extending the capability through the video system that is familiar to different generations of users. “The transition takes time, going from a Mylar overlay to a digital format,” he said. “There is also budget involved; some companies don’t want to spend what it takes to move from existing Mylar overlays—at least not yet.”

A good example of the future is the HDV300 Horizontal Digital Video System offered by Starrett. Configured like a traditional horizontal comparator, it mounts a high-resolution digital video camera, coupled with a choice of quick-change telecentric lenses or 6.5:1 zoom for micron-level resolution. DXF CAD files can be imported and 2D Go/No Go gages created, like using a Mylar overlay chart on a comparator screen. It also has video edge detection to remove some of the operator variability in visually selecting edges. The company also reintroduced its HE400 Horizontal benchtop optical comparator model. Arenal noted the basic comparator is one of Starrett’s biggest sellers, with a choice of either Quadra-Chek or MetLogix software control systems.

Keeping Mylar Relevant

One of the reasons Mylars have stayed relevant for so long may be due to a quirk in the permitting process for medical devices. The US Food and Drug Administration has an involved procedure for approving the manufacturing process of the device.

“Many companies have certified their processes with Mylars and find it too cumbersome to go back to the FDA and replace them,” said Tim Fantauzzo, vice president of sales for CCP, a brand of Quality Vision International Inc. (QVI; Rochester, NY). As mentioned above, modern optical comparators display CAD directly on screens by projecting DXF files. “This would allow medical device users and others to migrate to modern technology while still using acceptable methods of measurement,” said Fantauzzo. “Solutions from [our company] include eCAD and vCAD that provide on-screen CAD technology for LED based comparators.”

He also agreed that there is a certain “old guard” that simply likes the way a simple profile projector works. “A lot of our sales appeal to the customer is that we can still do it the old way with a glass screen with clips where you can clip up a drawing—the guys on the floor love that,” Fantauzzo said. The engineering departments are the ones keener to use CAD in a full digital integration with video cameras. “Ideally, you need both,” he said. “A lot of our analog comparators also have a digital video; it is a hybrid if you will. It is a full CNC with a walk-up capability.”

CCP also refurbishes units on trade-in for resale. “It was a bit of an experiment for us when we first started that service eight years ago, but today we do roughly half of our volume with refurbished units,” Fantauzzo said. They even refurbish original Kodaks from 1955—but upgrade them with 21st century optics, scales and DROs.

CCP certifies these refurbished units for accuracy. This is not as simple as it might seem. “While simple in operation, the device relies on a complex train of components, each of which must be accurate, and whose accuracy must be measured,” he explained. The general standard specifications for comparators are stage motion, FOV accuracy, magnification accuracy, and edge detection accuracy, according to Fantauzzo.

Further, the system interactions of these features must be measured and characterized. He also noted that unlike other metrology devices like CMMs, customers do not ask him to provide specs to a standard like ISO 10360. “My best answer to any question from the customer is ‘give me your part and let me run a gage R&R or feasibility study’,” he said. For the machinist using a comparator in a walk-up mode, the question doesn’t really apply. “He just wants to measure a single critical dimension on a part. It is a visual comparison,” said Fantauzzo.

A good example of this is the company’s line of c-vision Video Contour Projectors. CCP states that they combine the speed and accuracy of a traditional video measurement system with the capacity of an optical comparator, and are designed for use on the shop floor. These machines emphasize larger FOVs, better optics, and brighter lighting, along with solid, heavy construction. “I see two different markets evolving. One is for standard, inexpensive, high-quality but simple-to-use systems and the other is for highly capable vision-based systems,” said Fantauzzo.

Bridge Between Old and New

Another company blurring the technical definitions of vision systems and optical comparators is VisionX Inc. (Montreal). The company does not offer standard profile projectors based on shadow illumination; instead, its flagship is the VisionGauge Digital Optical Comparator. “It could be said to be a bridge between old and new, but I would go a step further and say that it can completely mimic an optical comparator,” said Patrick Beauchemin, president and CEO for VisionX. The device even offers an optional Z-axis laser measurement module to add depth measurements of slots and holes to X-Y dimensions. “The system also features patented automatic CAD alignment and automatic pass/fail, with minimal operator intervention,” he said.

Beauchemin related that the market is becoming more accepting of the device’s premise, which is finding its way into manufacturing in general, including aerospace, medical and automotive. “We have a better success rate in applications where very small lots of parts are produced,” he said. “Typically, those are parts with complex geometries, tight tolerances, quick changeovers and where programming a CMM might take longer than making the entire lot of parts.” Beauchemin added that the devices have found their way into high-volume industries as well, such as automotive, but are typically used there for quick troubleshooting. “The typical application is summed up as, ‘I have a part, a CAD file, and one minute to make a measurement’,” he said.

When asked about the future of these devices, either shadow graph comparators or all-digital ones like the VisionGauge, Beauchemin echoed the statements of others. “I see more energy around evolution, rather than future equipment remaining unchanged,” he predicted. Customers want intuitive interfaces. Newer generations of engineers raised on smartphones and tablets demand them. Suppliers will be pressured to deliver easy-to-use equipment requiring little training. VisionX’s latest product is the 700 series with five-axis motion and fully automated options—which Beauchemin fully admits might strain the definition of an optical comparator, but seems a natural evolution from the company’s base technology.


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