Quality 101: CAD Overlay Inspection Basics

January 1, 2011
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As the need for manufactured part inspection becomes more critical, automated inspection plays an increasingly important role.

CAD chart overlay measurement has become one of the most popular ways to inspect samples. Source: Nikon Metrology Inc.


As the design for industrial manufactured parts becomes more complex and the requirements more stringent, the demand for faster, more accurate and highly automated inspection becomes more urgent. The necessity for tools that aid in finding quick and correct results to problems has become increasingly important.

For example, the design of fuel injection systems for internal combustion engines in automobiles has become more complex in recent years. This is due to industry requirements to design and manufacture cars with increased fuel mileage, reduced emissions and greater efficiency. This trend toward more complex design is not limited only to the automotive industry, but is becoming more prevalent in other industries, such as aerospace and consumer equipment.

While chart overlay measurement using traditional optical comparators might remain a core component of how engineers pass or fail manufactured parts, there is now a highly automated and precise option gaining traction in the field: computer-aided design (CAD) chart overlay measurement.

It should come as no surprise that as computers play a larger role in the design and manufacturing process in industry, computer programs designed to facilitate inspection become more prevalent, as well. To that end, CAD chart overlay measurement has become one of the most popular ways to inspect samples, thanks to its ease of use and the faster throughput it offers.



Solutions such as CAD overlay allow teams to solve manufacturing issues in real-time. Source: Nikon Metrology Inc.

Why It Works

The use of CAD has a long-standing history in architecture and engineering. CAD overlay allows engineers to study a sample much like a blueprint, translating a sample into an onscreen rendering and allowing the engineer to view it within its correct dimensions and with tolerance lines.

Unlike an optical comparator, which optically projects the part profile on a frosted glass screen, utilizing a dedicated overlay chart set at a specific magnification, a vision measuring system has the ability to digitally overlay any CAD file chart at varying magnifications and tolerance levels. This means that an operator can essentially get a digital rendering of the CAD file and study it overlaid against the actual sample.

Vision measuring systems also have options to easily output quantitative data for statistical process control, and to record digital images that can be shared and studied anywhere they are needed.

CAD overlay is flexible and can be used in multiple ways. With a vision measuring system’s high-resolution optics, an engineer can zoom in for a closer look, and the CAD overlay also zooms in along with it. Alternatively, one can zoom out for a larger view and the chart also zooms out, all while maintaining the calibrated magnification scaling.

Chart overlay measurement with a conventional optical comparator is limited to a fixed magnification because the overlay charts are designed for a specific magnification. The magnification can be changed on the optical comparator by changing the objective lens. However, the size of the chart will not match. Additionally, the sample size will be restricted by the screen size of the optical comparator. For example, if a 10X objective lens is used on a 20-inch screen, the maximum sample size that can be displayed would be 2 inches.

If one wants to traverse the part, he can manually drive-or automatically position-the vision measuring system’s stage, and the CAD chart moves with the stage’s translation. The vision measuring system digitally coordinates the CAD chart, the live digital image magnification and the stage position to produce a coordinated e-CAD overlay anywhere on the part.

With an optical comparator, one would need to move the stage and manually perform a best fit to the chart. This is typically done at low magnification, which limits visibility and translates to reduced accuracy.

If one simply needs an image, click the “capture” button and the image, with the overlay, is easily saved to the PC. The only thing that is required is a CAD file with the tolerance zone depicted.

While several top equipment manufacturers make vision measuring systems, the goal of all metrology software programs-as well as the equipment systems that use it-is to create something compatible across a wide variety of industries that can be both easily learned by the individual and shared across teams. This ensures that the output can be sent to anyone who is taking part in the testing process, ensuring quicker response times for evaluating problems and issues.



How It Works

For engineers already working with advanced computer systems, the process of using CAD overlay should be a familiar one. Programming of the system is easy and powerful and can usually be completed in few steps. For example:

Create a part coordinate system-also known as the reference points-at the same location as it is on the CAD file.

  • Open the CAD file.

  • Set up the tolerance zone.

  • In the chart measurement window, denote deviations from nominal or tolerance zones.

  • Inspect. At each location, the operator can provide information such as deviation data, time interval and the ability to choose pass or fail.



    After images are stored, engineers can easily inspect the feature areas using traditional imaging software programs, such as Microsoft Paint or Windows Picture Viewer. This ensures that images can be passed among multiple operators for inspection without adding new software to his computer. Many programs also allow for multiple images to be stitched together, so operators can get a complete portrait of how multiple parts fit together and can then view them on the screen. As a final option, the results and accompanying data sets also can be printed.



  • CAD Overlay in The Future

    In today’s global workplace, solutions such as CAD overlay allow teams-no matter where the individual members are located-solve manufacturing issues in real-time, which saves countless hours of manpower. The automation of CAD overlay means that repetitive and time-consuming tasks are reduced. At the end of the day, this type of streamlining results in bottom-line savings, which-regardless of industry-is a highly desirable outcome.



    Michael Kireyev is an application specialist at Nikon Metrology (Brighton, MI). For more information, e-mail michael.kireyev@nikonmetrology.com , call (847) 428-0980 x223 or visit www.nikonmetrology.com .

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