When Johnson Controls won the job to supply headliners for General Motors Corp.'s 2002 Chevrolet Blazer and TrailBlazer sport utility vehicles, however, GM imposed a new measurement requirement. In order to ensure that the headliners would not cause interference with certain components in the roof areas of these vehicles, GM wanted Johnson Controls to measure thickness in the center of the headliners, Stewart says. "Because the distance to the area we had to measure was roughly 21¿feet inside the headliner, there was no way that we could do that with a snap gage," he relates.
To meet the GM requirement, Johnson Controls turned to the MiniTest 3100, a handheld thickness measurement unit supplied by ElektroPhysik USA Inc. (Arlington Heights, IL). The unit is actually sold as a coating thickness gage for measuring the thickness of nonmagnetic coatings on steel or ferrous substrates. But the MiniTest 3100 works equally well for measuring the thickness of the headliners -- which are made of nonmagnetic materials -- when the headliners are placed on a steel plate supplied by ElektroPhysik.
The MiniTest 3100 works using more than one mode, including magnetic-induction. In this method, a probe that contains a primary and secondary winding is placed atop the material to be measured. When the primary coil is supplied with an alternating current, it induces a signal in the secondary coil, the strength of which varies according to the thickness of the nonmagnetic material that separates the probe from the ferrous substrate or plate.
Johnson Controls workers are able to measure the thickness at the center of the Blazer and TrailBlazer headliners by placing an ElektroPhysik N-100 probe on top of the headliner. The headliners are placed on a steel plate within a holding fixture while the measurements are taken. The probe is attached to the handheld MiniTest 3100, which provides a digital readout of the measured thickness in about one second, Stewart says. The measurement values are stored in the device and can be downloaded to a computer for later analysis. The Blazer headliners are specified at 19 millimeters thick, + or - 1.5 millimeters at the center. The TrailBlazer headliners must measure 20.2 millimeters thick, + or - 1.5 millimeters. So both are well within the thickness range up to 4 inches (101.6 millimeters) that the MiniTest 3100 can measure.
The headliners are manufactured from multiple layers of fiberglass, foam and other materials that are heated, compressed and bonded together, and then placed in the holding fixture where they are measured. Johnson Controls measures the headliner thickness during the initial set-up of the compression machine, and takes subsequent measurements periodically during production runs for process control purposes, Stewart says.
The company checks thickness four times per hour for the TrailBlazer headliners and twice per hour for the Blazer headliners. "If we find that a headliner is out of spec during the process, then we shut the line down and make corrections on the compression tool to bring it back into spec," Stewart notes.
ElektroPhysik USA Inc.
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