CAD, fasteners, projections, and quality

  • 17-May-2012 03:41 EDT
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A key breakthrough in making CAE more accessible to non-experts was tying work instructions developed from CAD into commercially available projectors that light up specific areas on the workpiece with specific work instructions. (Northrop Grumman)

A design residing in the digital world of CAD needs to live in the real world. Skilled technicians need to fabricate, cut, drill, and fasten real materials together. To guide them, traditional aerospace work instructions are paper drawings and visual aids that manufacturing engineers create by interpreting 3-D CAD data, according to Joe Festa of Northrop Grumman’s Manufacturing Technology Development Group.

The process of installing fasteners is both labor intensive while requiring precision and tedious attention to measurements, he said. Today, mechanics refer to drawings prepared by manufacturing engineers, using mark-ups on the part to provide reference features and measurements. Festa points to problems with this approach: drawings that are hard to read, quality measurements recorded on tape, and fasteners that must be removed and reinstalled because adjustments are made iteratively until tolerances are met.

There is also a growing need to record the as-built condition of each plane. Each tail number needs its "digital twin," according to Festa. Why? The real world can be quite different from the CAD design, especially when fastening, say, aluminum and composite stack-ups.

“[Thicknesses] can vary so much that you would have to make a choice between two different lengths of fasteners,” said Festa, the one the drawing calls for and the one needed to ensure a proper grip length. A single skin requiring fasteners could require up to 1000 individual parts distributed between 15 to 20 different part numbers. Step-and-gap measurements are also needed between adjacent skins. These too need to be in the "as-built" data set. Plenty of opportunity for error. To create the "as-built" data set today requires typing manually.

Enter the Fastener Installation Live Link System (FILLS), developed under a Small Business Innovative Research grant from the Air Force Research Laboratory. The system brings CAD directly to the work site by projecting work instructions—developed from CAD—directly onto the work piece.

“FILLS took commercially available optical projectors, mounted them above the product, calibrates to that product so it knows where it exists in 3-D space, and [then] projects work instructions in the form of multiple colors and shapes directly onto the product.”

For example, by highlighting fastener holes with a special "starburst," workmen use an optical, wireless, handheld gage gun to measure the holes drilled in the skins. The thickness of the hole is stored in the as-built digital twin, using color codes. When mechanics later begin installing fasteners, the hole is color-coded for fasteners of the right length.

Other members of the SBIR team included Delta Sigma Corp., Right Hemisphere, VRSI, and Lockheed Martin. “We are looking to take this to the next step,” said Festa. “The current state is a wonderful digital environment, but it is not quite parallel to the problems that we have out on the shop floor.”

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