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February 21, 1999

6 Min Read
Verifying CAD models against tooling

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Figure 1. CAV analysis can scan both cavity and core side details of a physical part, then compare the dimensions to a 3-D CAD file. The core side details of a cordless phone housing, shown here, exhibit several areas where dimensions are either over or under the ones designers specified. Using the color-coded key to determine magnitude, red indicates positive deviations, blue is negative, and green areas are correct.

Design engineers are caught in the middle of a paradox, according to John Jarvela of Product Development Technologies (PDT) of Wauconda, IL. Switching to 3-D solid modeling software has allowed them to revolutionize molded plastic designs with aerodynamic, spline-driven shapes, complex geometries, and nonlinear parting lines. With CAM integration, these products are now tooled directly off the 3-D database, which unfortunately eliminates the need for fully detailed 2-D drawings.

First article inspections using traditional CMM touch probe technology require these drawings, so complete inspections today are being replaced by fit, form, and function checks. As a result, designers get incomplete or inconclusive data, and many sophisticated parts are failing these 3Fs.

"With first shots off a tool, you often find the blatant errors and fix them by reworking the tool, then discover other deviations that may or may not need to be corrected with a second round of reworking. In effect, a process that starts out at the cutting edge of technology," says Jarvela, a former OEM designer, "is being compromised by incomplete inspection methods for verifying the tool. The result is time-consuming and expensive on the manufacturing side."

Rather than going backward-spending weeks or months creating 2-D detailed drawings to check against a tabulated inspection report-designers need a fast, accurate, and comprehensive method of comparing a physical part directly to its CAD database.

"The answer," he contends, "is to digitize a part and compare the 3-D multimillion-point scan cloud directly to the CAD model. PDT developed such a method using a highly accurate laser scanner, which is repeatable, automated, and noncontact to eliminate the need for conventional CMM operating skill. Our software does the comparison, then generates results as a series of FEA-style images that are color-coded to show dimensional deviations, sink, warp, and contour errors in the part."

Why Verify Via Computer?
PDT offers this service, which they have trademarked CAV (computer-aided verification), to OEMs and molders looking for an alternate means of identifying all tooling deviations upfront. Developed 21/2 years ago, the process begins with complete laser scans of a physical part that generate more than a million measurements to identify all deviations in one pass, regardless of surface complexity. The resulting point cloud database is accurate to .001 inch and aligned to the 3-D CAD data using select reference datums or computer best fit alignment. CAV analysis then computes the distance from each point to its closest CAD surface and creates a color map illustrating all positive (red) or negative (blue) conditions (Figure 1). Color intensity represents the magnitude of dimensional deviation.

"We are seeing an increased demand from both OEM designers and molders, many of whom must now guarantee correlation between their customer's electronic database and supplied parts," Jarvela says. "A combination of geometric complexity, the absence of 2-D drawings, and multiple vendors providing tools for production parts makes this process a highly viable alternative for first article inspections. The turnaround time is days rather than weeks, and the color map output is annotated to clearly identify potential production issues." Customer-specific pilot programs designed to introduce new users to the technology have received positive feedback, according to Jarvela.

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Figure 2. Complex geometry that could not be defined dimensionally on a 2-D print made traditional CMM inspection of this cellular phone housing impossible. During assembly, the clear lens window would not seat correctly to the housing. The CAV process uncovered dimensional inaccuracies on the seating surface, as well as others on the earpiece and main keypad face.

Cellular Seating Solved
Engineers at one PDT customer facility discovered a lens seating problem on the phone housing shown in Figure 2. For weeks, they assumed the cause of poor seating came from the bonding agent used on an adhesive gasket that connected the lens to the housing. During thermal cycling tests, the lens repeatedly lifted up, causing the entire product to fail quality control tests. However, the seating surface was created with spline-driven geometry and could not be defined dimensionally on a print. As a result, traditional CMM inspection was out of the question.

After digitizing the entire phone, PDT used a combination of best-fit alignment and secondary justification to the main parting line to generate an inspection report. The CAV analysis uncovered dimensional inaccuracies in the seating surface, which were traced to an incorrectly machined electrode used to EDM the tool cavity. The main keypad face and earpiece geometry were also identified as incorrect. "Our customers were able to reconstruct the mold cavity at a more capable toolmaker to solve their problems," says Jarvela.

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Figure 3. Styling complexity for this Compaq laptop end housing became a stumbling block for first article inspection. Part of the problem was that the datum plane is off in space because it is associated with an assembly. Using best-fit alignment, the CAV process identified several key errors-shifted snaps and walls, warpage, and a missing groove.

Complex Laptop
Compaq engineers, intending to differentiate their products with a sleek new style, discovered their strategy created its own set of problems. The end housing for a laptop computer shown in Figure 3, for example, failed fit, form, and function tests during first article inspections.

A 3-D contoured cosmetic surface on the housing starts on the bottom and rolls into the part shown here. The main stepped flat surface that forms the gusset between cosmetic surfaces is situated at a slight angle and is attached to a cantilevered wall. Besides the non-orthogonal nature of the part, its main coordinate system datum plane is off in space because it is associated with an assembly. Again, CMM inspection proved to be inadequate.

Using best-fit alignment, CAV found several key errors in the part-three shifted snaps, two shifted walls, one warped wall, and an entirely missing groove. Compaq engineers spent about 15 minutes flipping through the 18 images contained in the CAV analysis and identified every error in the part.

Contact Information
Product Development Technologies
Wauconda, IL
Suzanne Maicke
Phone: (847) 487-2800
Fax: (847) 487-2250
Website: www.pdt-cav.com

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