Avoiding Design Problems with Geometric Dimensioning and Tolerancing

Geometric Dimensioning and Tolerancing (GD&T) has been around since the 1940s, yet it is unappreciated as a way of finding bad designs before parts get made. • Tolerances are a vital part of engineering a product, even one described with a solid model. • Drawings toleranced poorly or incompletely are subject to more than one interpretation, and untrained users may feel there are multiple interpretations. Geometric Dimensioning and Tolerancing is a well-established enabling technology, so you might think it would be widely used. But in fact, few managers understand how valuable this technology can be. GD&T can't make a bad design good, but the proper use of GD&T within a well-defined product development effort can identify a bad design before it results in a lot of needlessly expensive parts. There are two components to the technology of GD&T. First, it is a precise communications tool. It uses a set of symbols, rules, and definitions to mathematically define part requirements. Second, it is a design approach that lets the engineer define parts based on customer requirements and part functions while allowing maximum tolerances for manufacturing. This combination, properly executed, results in high quality and low costs. GD&T is part of a larger effort, a product-development process (PDP). Some companies may not even realize they have such a thing. But they do execute a series of steps in the process of developing a new product and bringing it to market. Usually, the more complex the product, the more defined the PDP. A PDP serves many purposes. First, it defines a series of activities that translate new-product concepts into customer requirements that drive engineering designs and testing. Second, a PDP distributes and harmonizes activities between different departments (marketing, purchasing, engineering/design and manufacturing). Third, a PDP provides a plan for all these activities. One of the interesting things about working with companies of all types is seeing how people try to blame GD&T for problems at their organizations. Here are some of the more common myths surrounding GD&T: Myth: We don't need GD&T because our drawings are good enough without it. Reality: Without GD&T you cannot accurately create tolerance stacks. You cannot accurately inspect your parts. The dimensioning cannot represent the product requirements. Myth: GD&T is confusing — everyone has a different interpretation. Reality: There is a kernel of truth in the statement, but it is not the fault of the language. Incomplete or poor tolerancing on drawings is subject to more than one interpretation. Untrained drawing users may feel there are multiple interpretations. Proper drawings and a skilled workforce can greatly reduce this problem. Myth: It takes longer to apply GD&T and we don't have the time. Reality: If your engineers have the right skill set, it is quicker to specify symbols in place of lengthy notes. Myth: Our suppliers don't understand GD&T so we don't use it. Reality: Why would you change your drawings to have less tolerance and be less clear to accommodate unskilled suppliers? Would you stop using e-mail or solid models if your suppliers didn't have these capabilities? The use of these technologies became requirements for suppliers. GD&T has been around for many years. Require your suppliers to understand drawings. Myth: Using GD&T raises part costs. Reality: When properly specified, GD&T has a number of tools that provide larger tolerances to reduce part-manufacturing costs. To name just a few: Round tolerance zones, Bonus tolerance, Rule #1, Composite tolerances, Functional dimensioning, Separate requirements. Myth: With solid models, I don't need GD&T. Reality: This may become true someday, but for now, we still need tolerances to functionally describe part features, allow maximum tolerances, analyze new designs, analyze production or field problems, and to inspect parts. In short, a good PDP reduces the time it takes to deliver a quality product to market.