Weighing cost vs tolerances
By Greg Reed, Contributing Technical Editor -- Test & Measurement World, 6/1/2006 2:00:00 AM
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| Larry Stockline, President, Promess Courtesy of Promess |
Automotive manufacturers have long sought to improve quality by making components that are closer and closer to perfect, but the trend toward tighter tolerances also drives up the cost of production. Assuming the ultimate goal is a reliable product, as measured by function rather than dimension, might an approach that accommodates looser tolerances improve reliability? Conventional wisdom says "no," but practical experience says otherwise.
In a recent chat with Larry Stockline, president of Promess, I learned how the company improves manufacturing quality in automotive products by using looser tolerances. While it sounds counterintuitive, Stockline's concept is validated by the fact that Promess's systems produce high-quality parts for demanding Big 3 and transplant automakers.
Q: Can you provide an example of automotive components that require functional precision yet seemingly defy close tolerance specification?
A: Control arms are a good example. Not only are they stamped, but they are also mounted with rubber bushings. Nevertheless, the automakers have a very tight specification for exactly where the ball joint has to be positioned by the control-arm assembly. They just don't tell the suppliers how to meet it.
Until very recently, the only way to improve reliability was to specify a tighter tolerance. Yet, for many automotive components, we have long since passed the point where specifying tighter tolerances is the best success path. Using intelligent assembly technologies, it's quite practical to loosen the tolerances on many individual component parts and let the assembly system compensate for variations and still produce quality products. Think of the cost savings that could be realized if the industry took this approach rather than blindly pursuing perfection regardless of cost.
Q: How can loosened tolerances aimed at a phantom dimension actually improve quality?
A: OK, let's look at that control arm. In pursuit of perfection, the engineer will build a set of precision tools and fixtures to install the bushings only to discover that stampings and rubber parts are not predictable. Regardless of precise tools and fixtures, the process inevitably degenerates into "press and hope" and then "measure and sort" with scrap and rework written off as a cost of doing business.
A better answer is to stop making stampings and rubber bushings into precision products, and design an assembly system "smart" enough to compensate for the variations. Instead of "press and hope," you "push, measure, push" until the assembly is functionally correct, regardless of component variations. Isn't that what the automaker really wanted?
Q: What kinds of force or precision measurements take place in a typical press operation? How do you monitor the press process?
A: Let's stay with the control-arm assembly. It uses four electromechanical assembly presses (EMAP) equipped with external sensors that measure the final assembly in real time. The EMAPs are arranged in opposing pairs at the bushing end of the control-arm assembly. The EMAPs use the data from these sensors as feedback to control final positioning.
By comparing this data to that of a "good" operation, the control determines when the bushing is close to final position, and the EMAPs are switched into precision "push, measure, push" mode for the final location. This system produces upper and lower control-arm assemblies within tolerance, independent of the stack-up of the individual components.
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