CMM Arm Proves Specifications Are Correct

May 19, 2003
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What does a contract manufacturer do when its biggest customer says its parts don't measure up? That was the question faced by Hughes Bros. Aircrafters Inc. (Southgate, CA).

The supplier of compound-contoured sheetmetal details and assemblies to Boeing, Lockheed Martin, Northrop, and other aerospace companies turned to Faro Technologies Inc. (Lake Mary, FL). Hughes Bros. purchased a FaroArm coordinate measuring system that it felt could produce the reports necessary to prove that the parts did, in fact, meet its customer's specifications.

Hughes Bros. produces doorframe corners for the assembly of C-17s at Boeing's facility in Macon, GA. Mechanics had discovered a gap of approximately 0.1 inch between the rounded corners and straight sections of the doorframe.

"Boeing's engineers were telling us that the components we were supplying were not conforming to specifications," recalls Jim Hughes, president of Hughes Bros. "Because the tolerance was + or - 0.03 inch, we felt that we would have caught the discrepancy right away." Hughes and his manufacturing and quality control teams had difficulty believing that their parts were causing the fit problem.

Boeing's engineers gave Hughes Bros. a chance to prove that its parts were within specification, but the timing was tight. By the next day, the supplier was to send them a computer-generated inspection report from a coordinate-measuring device. "We had to prove our case electronically," says Hughes. "The engineers wanted us to measure the questionable areas with a coordinate measuring ma-chine (CMM) and transfer the file to them, so they could pull the data up on their computers and match them with their own programs."

Although Hughes Bros. did not have a CMM, the company did have the Gold Series FaroArm. Boeing agreed to accept measurements from the device. Optical encoders at each of the 4-foot arm's six joints report a point's position and orientation within + or - 0.002 inch, which was 15 Arial better than was necessary for doing the job.

Many of the Hughes Bros. quality control technicians had never used a CMM or other computerized measurement tool. Most were use to calipers, micrometers, and other hand tools that they had been using. Hughes decided that he and his team needed help for the sensitive and important task at hand, so he called on a local Faro representative.

With the help of the representative, the discrepant areas of the doorframe parts were scanned and compared to the measured points of corresponding locations on the original computer-aided design (CAD) model that Hughes Bros. had received from Boeing. An analysis of unrestrained parts--parts sitting on a surface plate and not clamped into a fixture--showed that the parts were within + or - 0.01 inch, which was within the specified I0.03-inch tolerance. Using the more accurate method of clamping the parts into a fixture was unnecessary.

Hughes Bros. later met with Boeing's engineers and reported that measurements made in an unrestrained state were within tolerance. Moreover, Hughes Bros. was able to identify the cause of the mismatch as a mating part made by another supplier.

In addition to using the FaroArm to convince a customer of the quality of its work, Hughes Bros. also uses the FaroArm for more routine jobs, such as checking fixtures and tools, conducting first-article inspections and measuring production parts.

To measure parts, the inspector brings them to a granite measuring table, sets up the arm, and opens the CAD file containing the original CAD geometry, which the company imports as an IGES file directly from the customer. After orienting the arm to the part and CAD file, the inspector can touch the part with the probe at individual points or drag the probe along a surface to collect a stream of points anywhere in an 8-foot spherical working diameter. As the probe touches the features in question, the CAM2 software compares their actual locations and dimensions to the corresponding locations and dimensions on the CAD geometry and shows any discrepancies graphically.

"Measuring the length of a part takes seconds, so we can do a quick setup just to tell whether a dimension on a work in process is close," says Hughes. "Accuracy is much better than by hand, at least by a couple of thousandths." The arm and software not only measure length and width fast, but they also streamline the measurement of contours and angularity at any location and orientation Hughes says.

According to Hughes, the benefits are numerous. Not only are inspection jobs 75% faster than with the old, manual techniques, but also computer-aided manufacturing measurement cuts man hours in half by allowing one person to complete them.

"You don't need one hand to hold the arm and the other hand to manipulate the probe," observes Hughes. "No matter what angle you create, it will stay in the air without falling."

Faro Technologies Inc.
(407) 333-9911
www.faro.com

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