Avoiding Design Problems with Geometric Dimensioning and Tolerancing

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.

In short, a good PDP reduces the time it takes to deliver a quality product to market. So it is useful to review the role GD&T plays within a PDP. A typical PDP consist of the following stages:

• Requirements setting
• System/conceptual design
• Component/detailed design
• Manufacturing design
• Component validation
• System validation
• Manufacturing validation
• Production

Product variation from manufacturing is a fact of life. The allowable variation is communicated through a GD&T feature control frame. The Quality Management Process focuses on how this variation is to be controlled and managed. It is useful to examine a GD&T feature control frame and illustrate how it is used through the PDP. The consequences of not using GD&T in a PDP can be costly.

Datum selection is the best place to start when managing variation during the PDP. A robust datum scheme uses datums that best prevent the parts from moving out of position, minimizes the number of items in a tolerance stack, is well controlled within the part (location tolerances and rigid features) and is shared by as many manufacturing processes as possible.

The datum scheme is largely driven by the product-build strategy and system requirements: Thus it is important to consider the effect of datum selection early in a PDP (requirements and system design phases). The design-and-build strategy defined at this stage will have the largest effect on product quality with the least cost. Mistakes made at this stage will be expensive to fix at later stages, if they can be corrected at all.

The requirements-setting phase of a PDP is where you identify product features and performance requirements which dictate what needs to be controlled and how tight to hold the part tolerances. The part-appearance requirements dictate where there are critical dimensions, as well. Remember, the best datum reference frame is only relative to the specific product features that must be controlled.

You must select functional datums. A functional datum is simply one that uses the product features that physically locate the part to the final product. Using any other datum will add variation in the final tolerance stack up.

Also remember the importance of datum priority. The primary datum should be the functional datum that controls most of the allowable degrees-of-freedom of movement. The secondary and tertiary datums, if needed, are functional datums that control succeedingly fewer degrees of freedom. During the design of the manufacturing process, you must select features used to locate the part during manufacturing operations — features that determine where and how the part is held while it is manufactured. These features must be either identical to the product datums or extremely close to them. When locating features don’t coincide with functional datums, variations invariably arise.

Finally, by definition, the validation phase of a PDP employs the GD&T datums in all fixtures used for checking dimensions, or for CMM (coordinate-measuring machine) routines that gauge part variation. You should check fixture designs and CMM routines to ensure they locate parts to the specified GD&T datums and conditions (restrained or unrestrained). All in all, you’ll want to identify functional datums and plan for GD&T early in a PDP to maximize part quality.

Symbols and tolerances
With product features and datum references settled, it’s usually a simple matter to choose a geometric characteristic symbol. The type of product feature to be controlled and the type of control (form, profile, orientation, location, or runout) will usually narrow the field to one or two symbols. The geometric feature being controlled is directly related to customer requirements. ASME Y14.5M1994 is a good reference for a complete description of each symbol and its use.

One of the largest mistakes made in product design is copying tolerances from a previous drawing without reevaluating how well individual part tolerances and the datum scheme meet product requirements. Too often we have heard “…but the part is within specification,” when the part tolerances specified were never evaluated to see if their combination met product requirements.

Tolerances should be allocated to each part based on product system requirements and manufacturing capability. Several methods exist to allocate tolerances. These are the same tools used for tolerance analysis (Monte-Carlo simulations, root sum of squares and limit stacks). In each of these methods the relative contribution of part tolerances to the final system variation can be calculated. Part tolerances need to be balanced against product and manufacturing costs. If system requirements cannot be met, first try reducing the part tolerance that contributes the most to system variation. If the cost of reducing a tolerance is high, try reducing other less costly tolerances instead.

Tolerance analysis must include part-fixture tolerances when a fixture-build is used in manufacturing. Otherwise the final tolerance analysis doesn’t consider the full range of variation. Note that use of GD&T allows additional tolerance for manufacturing (see GD&T Myths sidebar).

Of course, most companies are not building completely new products in which they have no prior experience. So they can use past data to help allocate requirements. It is helpful to look at the build strategies of past products and the resulting dimensional capability. This exercise will help show whether current requirements are in the realm of prior experience or if new techniques are in order. Ideally, your company has kept data on past part-tolerance capabilities. These data are not only necessary for continuous improvement, but also are useful early in a PDP. They help determine your process capabilities and what tolerances you must hold for specific manufacturing processes.

Tolerance analysis should be performed whenever build strategies or part tolerances change. This analysis is the final chance to predict and avoid problems before finalizing the part GD&T and ordering hardware.

Benefits of GD&T
One benefit of using GD&T is its ability to precisely and clearly document the part requirements. This results in a part that can be outsourced to anywhere on the globe. The mathematical precision of the part description makes the part easier to manufacture and inspect. Another benefit of GD&T is the ability to define a part in a manner that protects the part function and allows maximum tolerances for manufacturing.

GD&T is also vital for accurate results in inspection. The datum system communicates what part surfaces are used to create the datums for measurement. The feature-control frame communicates the sequence for relating the datums to the part surfaces. It also gives the amount of tolerance that part features are allowed from their theoretically exact location at the datums.

GD&T is important for calculating tolerance analysis accurately. The mathematical definition of part surfaces and their tolerance zones allows calculation and analysis of the extreme boundaries. This lets you analyze the effect of manufacturing variation before the part is produced, and thus anticipate problems and address them before the design is released.

Of course to reap the benefits, designers must properly specify GD&T on the drawing. In the rush to release drawings, sometimes engineers do not spend enough time on specifying GD&T correctly. The amount saved rushing through the drawing is miniscule compared to the potential savings from defining the part correctly.

There are a few practices that can help a company get the maximum benefit from GD&T. First and foremost, a company should create a policy mandating the use of GD&T according to a recognized standard, such as ASME Y14.5M1994. GD&T use must be consistent among engineering, design, manufacturing, and quality organizations. And all levels of management must endorse the use of GD&T.

Second, all portions of the organization that will be exposed to GD&T should be trained in it. The degree of training will vary depending on how the employees use GD&T (awareness overview, interpretation or application, analysis). Ideally, there should be a certification process. It should assess each employee’s knowledge of GD&T or need for further training and set requirements for each job’s level of certification.