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Kodak model comparator, 1945
Edward C. Polidor (left), founder of Optical Gaging Products,
and Bill DeBoer of OGP

__________________
by Kennedy Smith

Click here for a list of optical comparator vendors

Since their invention in 1920, optical comparators have changed very little. Improvements during the years have added higher accuracy and user-friendliness, but all in all, optical comparators look and function much the same as they always have.

 "The fact that they haven't changed much over time is a major reason why they remain so popular," explains Fred Mason, marketing communications manager at Optical Gaging Products. "Familiarity and experience breed confidence." Simply put, people are sticking to what works.

 So what's in store for the future? Is it even possible to improve upon a workhorse that's hardly changed in 80 years? This article attempts to answer these questions.

The Evolution of Optical Comparators

1920s

The optical comparator is invented by James Hartness, president of J&L Machine Co., in 1922. It projects the shadow of an object onto a screen a few feet away and can be compared with a chart showing tolerance levels for the part. By the end of the decade, the optical comparator is an integrated machine, small enough to fit on a desktop. Comparators also begin to be used to examine wear of a part as well as for setup phases in manufacturing.

 

1930s

J&L Machine Co. weathers the Great Depression by exporting optical comparators to the Soviet Union, which finds itself undergoing rapid industrialization under the lead of Josef Stalin. At home, comparators are being used more and more in small-part manufacturing plants, including those that produce razor parts, toothbrushes, dental burrs, bottle molds and more. Comparator sales reach a little more than 300 per year.

 

1940s

Optical comparator sales skyrocket as optical comparators are adopted as a standard for U.S. artillery specifications. They're used in the manufacture of just about every part used in World War II, including rivets and firing pins. About this time, comparators become commonplace in the automobile and aircraft manufacturing industries. Accuracy of the optical comparator improves because of better glass grinding technology, creating better lenses for image projection.

 

1960s

Automatic edge detection is introduced, making it possible for the machine, rather than the operator, to determine the part edge. This provides more accuracy by eliminating subjectivity. Accuracy is also improved with the introduction of Vernier encoder-type scales on the comparator's stage, which converts the stage into an additional measurement instrument with which to measure the part.

 

1970s

Digital readouts are introduced, as is programmable motorized stage control. As machines become more automated, developers start to incorporate programmable functions into the optical comparator. This paves the way for complete automation of an optical comparator machine.

 

1990s

Incorporated software becomes standard optical comparator equipment. Computers can be interfaced with optical comparators to run image analysis. Points from manual or automatic edge detection are transferred to an external program where they can be directly compared to a CAD data file.

 

Sources: Fred Mason, Optical Gaging Products; Peter Klepp, Dorsey Metrology; The Optical Comparator and the Fight Against Scrap ("Tools & Technology" newsletter, Winter 19981999) by Ruth Ann Groh.

 

Evolution of the optical comparator

 It seems that James Hartness had it right the first time when he introduced his staff at the Jones and Lamson Machine Co. to his invention, the shadow graph (now known as the optical comparator). Hartness developed the optical comparator out of a need to standardize screw thread sizes. Serving on the National Screw Thread Commission, Hartness who would later become governor of Vermont set out to create a machine that could measure the complex curves of a screw. This first optical comparator projected the shadow of the object onto a screen a few feet away. Hartness and his employees could then measure the shadow image of the screw thread against draft designs. By the end of the 1920s, an optical comparator was a single integrated unit able to fit on a table top.

 In the 1940s, it became obvious that optical comparators were indispensable tools of the design and production process. As the United States became involved in World War II, the defense industry used optical comparators for weapons and special equipment. This, and a booming automobile industry, helped the optical comparator become a staple in part measurement by the 1950s.

 Automatic edge detection was added in the 1960s, and the 1970s brought about digital readout capabilities. About 15 years ago, the issue of backlash was resolved when manufacturers started using linear scales. Before then, measuring accuracy relied upon threads on a rotary encoder (using a hand crank to turn and move the stage). "When you're using threads and turning a gear, the threads can actually wiggle a little bit between the meshing of the gears," says Mike Metzger, measuring department manager at Nikon Instruments. This problem was eliminated when operators switched from rotary encoders to linear encoders.

 Despite a host of technological advances and dozens of sizes, magnifications and special features, the optical comparator still relies on a relatively complex optical assembly that accurately magnifies and projects the image of the part with minimal distortion. "Plus, because it's such a mature technology, there are a number of manufacturers," says Mason. "This means a wide range of prices and capabilities."

 Peter Klepp of Dorsey Metrology agrees, adding, "This basic principle has remained unchanged since its invention. Manufacturers continue to apply emerging technologies to the foundation to increase the accuracy and versatility of their instruments."

Dorsey Metrology International
Ph: (845) 454-3111
www.dorseymetrology.com
Recent developments

 In the last decade or so, changes to optical comparators have focused on additional functionality, improvement of the quality of imaging, creating fully automated machines and integrating computer technology into the system.

 By now, most measurement equipment operators are proficient with computers, so using them with an optical comparator system doesn't add any difficulty. "As computers have become reasonably priced and the average operator has become comfortable with them, they've become almost standard equipment on an optical comparator," says Klepp. "It's very seldom that a comparator without a digital electronics package is requested by a customer." In addition, software increases the capabilities of the optical comparator and makes it easier for operators to use special features of the system, such as transferring selected points of measurement into a program that can directly compare it to CAD file data.

 Software aside, Klepp notes two major developments in comparator technology at Dorsey. In 2000, the company released an optical comparator with reflective scale technology, which is capable of eliminating any backlash from linear encoders. Additionally, Dorsey recently released the first comparator with three-axis touch probing. "Historically, the Achilles heel of an optical comparator has been its limitation to measure only two axes," says Klepp. By integrating either a touch probe or a laser noncontact device, the optical comparator is now capable of measurement in the Z axis.

 Nikon's Metzger says the best (recent) improvement to optical comparator technology has been to the quality of lenses. "This is a result of computer-controlled manufacturing of the lenses. If you make a lens better, more light transmits through it and your image becomes brighter." This allows surface illumination so the operator is capable of seeing not only a profile of the part but features on the surface of the part as well.

 "The latest comparators feature motorized stages, automatic edge detection, geometric measurement computers and 0.5-micron or better stage measurement," adds Walt Wardzala, precision measuring instruments product manager at Mitutoyo.

 The size of today's optical comparators typically range from 12-inch screens to 30-inch screens, although there have been exceptions (at one time, Optical Gaging Products produced a 60-inch comparator). These can be tabletop models or floor models, depending on the needs of the company. Small, low-end manual machines are available for a few thousand dollars, while large, fully automated, high-quality machines can cost upward of $100,000. Other factors that affect price are the size and weight of the part that can be measured. For example, a 14-inch machine designed to hold heavy machined parts will cost more than a comparably sized system designed for sheet metal parts.

Predictions on new developments

 Although the optical comparator has changed little over time, its original design is still subject to constant improvement. There's always something in the works for a better and more efficient product. Some experts speculate on what's in store for the future of optical comparators.

 "Removing operator subjectivity from the measurement process will continue to be a goal," says Mason. "So, automation of edge detection, stage motion, etc., will be an emphasis for comparator developers."

 Klepp predicts that optical comparators will move toward three-axis measurement, incorporating video and touch probing systems. "These will become standard options, and three-axis software development will further improve operator capabilities."

 Perhaps the most noteworthy prediction, however, is that optical comparators will be replaced by video measuring systems.

Next-generation optical comparators

 "The video measuring tools that are out today are the hottest products on the market, and they've been hot for about 10 years," says Metzger. Some experts even go so far as to say that video measurement systems will make optical comparators obsolete in the not-too-distant future.

 Bipin Mukherji, president of VideoGage Inc., says his company's product is slated to become the next generation beyond optical comparators. "Comparators are nice basic tools, which have done their job over the years," explains Mukherji, "But, with industry evolving the way it is, there is a need for something superior to optical comparators."

 Among the advantages to utilizing a video measuring system, Mukherji notes:

  Fully corrected (i.e., not inverted) image

  Image processing capabilities

  Variable zoom lens, offering multiple magnification levels

  Variable illumination with a cool fiber-optic light source, plus three light sources

  Color and profile images

  Capability to capture images of work pieces or part features

  Third-axis measurement

  Capability to archive and document measurements

 

Vision Engineering Inc.
Ph: (800) 644-7264
www.visioneng.com
 With a video measuring system such as this one, the operator can click the mouse on any given feature on the part and because it utilizes a video camera and computer technology measure every pixel in that area. "One could never do this with an optical comparator," says Mukherji. "It's so quick, you can do this with the blink of an eye."

 "Video measuring is booming right now," agrees Metzger. This is partly because such systems are reasonably priced and offer more advanced capabilities than do their optical comparator counterparts.

 Another product combines optical comparator technology with video and microscope technology. Rick Coyle of Vision Engineering Inc. describes the company's product as a more advanced alternative to the optical comparator. Like the optical comparator, Vision

Engineering Inc.'s DynaScope series of products utilizes a ground-glass screen. However, the company has patented a method of rotating the glass at 2,800 revolutions per minute, which reduces graininess and produces a smoother image for accurate edge detection. Additionally, Coyle notes the DynaScope is capable of advanced illumination of the part, easy upgrades to include photographic capabilities for documentation, and several magnifications.

Optical comparator phaseout?

 For several reasons, companies still have a need for optical comparators. Comparing a part profile to a chart is easy to understand, easy to teach and easy to implement. Newer machines have more capabilities than before. Electronic geometric processors allow more accurate dimensional and angular measurements. Moving the image of a feature to a particular location on the screen, zeroing the readout and moving to the next feature provides a direct readout of distance. Some edge-detection systems remove subjectivity in the process. They also provide a low-cost solution for operators interested in quick go/no-go measurement. "They're ideal for someone with a limited budget, an extreme variety of parts or just looking for a quick check of a feature," adds Wardzala.

 Despite the prediction that optical comparators are on their way out, experts agree that they've proven their place in industry. There will always be people who want the nonautomatic, basic tool. "Those markets will always be there, although they're not the growing markets," says Metzger. "They're easy to use, they're reliable and they work for the job that they're intended to work for."

 

About the author

 Kennedy Smith is Quality Digest's assistant editor. E-mail her at ksmith@qualitydigest.com . E-mail letters to the editor regarding this article to letters@qualitydigest.com .

 

Optical Comparator Companies

A.A. Jansson Inc.

Bley Metrology

Deltronic

Dorsey Metrology International

Micro-Vu

Mitutoyo America Corp.

Nikon Instruments Inc.

Optical Gaging Products Inc.

S-T Industries

The L.S. Starrett Co.

Suburban Tool

VideoGage Inc.

Vision Engineering Inc.

VISIONx Inc.

 

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