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Machine Design

Vantage Point: You’re probably paying too much for quality

Chances are the first thing that comes to mind when product quality is mentioned are dimensional specifications with lots of zeros after the decimal point — the more the better.

Larry Stockline
Promess Inc.
Brighton, Mich.

But every added zero challenges machine-tool builders, control makers, and tooling suppliers to develop ever more precise, consistent, and costly equipment to build the parts. This approach to product quality has been going on for several decades now, though people are beginning to question whether it is really the most effective way to get the job done. To be fair, there are some products for which the goal of dimensional perfection is reasonable and necessary; engines and transmissions come to mind. But, there are numerous products for which quality can be measured functionally rather than dimensionally.

Automotive control arms, for example, need a high level of precision to function properly, yet most are comprised of heavy-duty stampings with press-in rubber bushings — hardly the stuff of precision manufacturing. What auto companies typically do is define the required geometry of the finished assembly, but leave the job of figuring out how to deal with so-called “phantom” dimensions to the supplier.

One approach is to build highly precise tooling and fixtures, and then constantly adjust them to deal with unpredictable part variation. Another is to simply press-and-hope and then measure-and-sort and accept the scrap and rework as a cost of doing business.

A third approach being used by a Big 3 automaker is based on electromechanical- assembly-press (Emap) technology. Emap uses force and position sensors in a push-measure-push process. Here, system software compares the “signature” of a control arm being assembled to that of a known good assembly, then adjusts process variables so the two signatures match. The system boosts functional quality of the control arms it assembles with no change in dimensional specifications of the component parts. In fact, the approach may let manufacturers loosen tolerances on component parts to lower manufacturing costs without compromising functional quality of the finished assemblies.

Ultimately, consumers care more about function of finished goods than the precision of component parts. In that regard, we believe manufacturers should begin to think more like consumers. Naturally, manufacturers are reluctant to do this because of their huge investment in equipment and systems to produce ultraprecise components. But changing the focus from part tolerances to product function will not obsolete that investment. There are probably tens of thousands of components and products that could be made better at a lower cost simply by changing the way quality is defined and measured.

Promess ( is a maker of in-production monitoring systems.

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