Basic Elements: Zero tolerance: A new way of looking at precisionBasic Elements: Zero tolerance: A new way of looking at precision

Invest in GD&T training. Hetland started the IIGDT to help standardize geometric tolerancing through in-house seminars and programs. He takes these on the road to groups and companies that need to know how GD&T can improve their products and their bottom line. He’s also teaching college professors in an “educate the educator” program. “People coming out of colleges are not being trained in geometric tolerancing, even at the most basic level,” he says.

“People using plus/minus tolerancing assume they know what it means,” Hetland explains. “The plus/minus tolerancing is intended to identify the variation, but the problem lies in the fact that it does not control the inter-relationship between other plus/minus-toleranced dimensions.”

Investing in GD&T training results in higher profit margins by increasing capabilities, competencies, and efficiencies; boosting yield and reducing scrap/rejects through the ability to predict and control processes; and decreasing misunderstandings and production delays by means of increased confidence levels—a direct result of skills enhancement.

Learn the language of GD&T. Hetland finds that part/mold designers and moldmakers generally don’t understand geometric language. Moldmakers tolerance off many areas including holes, surfaces, and guide pins, but because of the system, there’s always ambiguity in the mold build (see sidebar, left). “The plus/minus language isn’t precise enough to allow a moldbuilder to manufacture cores and cavities to exact dimensions,” he says.

Yet, most moldmakers say, “Give me an exact dimension and I can hit it,” notes Hetland. Mold shops, for the most part, have the equipment to perform these exacting tolerances in a repeatable scenario for multicavity molds, where interchangeability is critical. True interchangeability requires exact dimensions and unambiguous tolerancing.

Invest in the right equipment. Invest in computer-aided, high-speed machining equipment with the capability to hold the tolerances and a CMM that can help you measure GD&T. “We’re assuming the company has the ability to produce the cores and cavities to GD&T and then measure it,” Hetland says. In fact, interchangeability of cores and cavities practically becomes a nonissue with the implementation of GD&T.

One of the big problems is mold shops using measurement devices that in no way can measure the components to the tolerances specified. “In one instance, a mold shop had the manufacturing capability, but when I asked them how they measured these features they said, ‘We don’t—we can’t,’” Hetland adds.

Learn how to measure GD&T using the equipment. These plus/minus tolerances have been a means of process control, Hetland explains. “But process control doesn’t mean anything if you don’t have the overall tolerancing capability to begin with,” he says. “I suggested they look at a coordinate measuring machine that is capable of measuring the tolerances that were required; to get beyond plus/minus tolerancing and learn how to measure it. Without precise definition of design requirements and truly capable measurements, it’s not clear they conform to the true engineering requirements up front.