Hoffman on Testing

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Cheap Fixtures Cost More in the Long-Run

When I travel the world to assist assembly plants that are having difficulties achieving gauge R&R standards, I can usually spot the extent to which testing requirements are misunderstood by taking a quick look at the test fixtures. Off-the-shelf fixtures are a big red flag that immediately suggests that containing upfront costs was the most important driver in designing the system.

Everyone wants to minimize costs. That’s understandable with or without a worldwide recession and unprecedented global competition. It’s ironic, then, that the very things most engineers do to contain costs in test-centric assembly systems are the cause of out-of-control production costs. Cheap fixtures that don’t work well are the culprits in more assembly plants than I care to count.

A recent case was in a plant that needed to test a fuel injection component at very high pressures—3,000 psi—and to very low leak standards—0.03 standard cubic centimeters per minute. To their credit, the in-house engineers knew they needed specialized leak detectors to measure these low leak rates at these high pressures, and they acquired leak detectors that they assumed would do the trick.

But, these superior leak detectors did not deliver the consistent test results they had expected. Why? The 16-micron roughness average of the test fixtures was nowhere near as precise as the real-world requirements. Very precise fixturing with a roughness average of 10 microns or less was needed. Fixtures with a 2L lapped finish (a roughness height of 2 microinches) fixed their problems, but they cost more than the originals. High-precision fixturing always does. But, you pay for better fixtures to achieve gauge R&R and keep the expense of quality problems out of the equation.

Frankly, thinking you can do tight-tolerance leak testing with fixtures bought or built on the cheap is magical thinking. It doesn’t work, plain and simple.

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