As a designer of devices with plastic components, you create engineering drawings with the hopes that you have captured the critical requirements that will ensure the long-term success of your program. During my years as a CMM metrologist and college professor, I have seen countless engineering drawings that confuse the audience and overcomplicate project requirements. This happens because the engineering drawings create a disconnect between the functional intent and the design requirements of the product.
Your stakeholders from R&D to manufacturing rely on well-defined engineering drawings to communicate the design intent so they can do their jobs. Sadly, miscommunication on bad engineering drawings lead to non-conformances in manufacturing in the form of false rejections of perfectly functional parts. As you can imagine, some of the costs are obvious, but most are hidden and can last for years.
GD&T offers a universal language for communicating design intent as outlined by the ASME-Y 14.5 guideline. GD&T uses 14 geometric characteristics that emphasize specific feature requirements in terms of Size, Form, Location, and Orientation. These GD&T control symbols emphasize important feature characteristics in engineering drawings in lieu of simple but less meaningful coordinates.
I have supported many of my clients through these problems by helping them adopt the tools of GD&T. One of the first things I do is help them understand the mistakes they are making in their engineering drawings so they can identify them and avoid repeating those mistakes. The three most common mistakes I have observed over the years include over-dimensioning, incomplete specifications, and over-constrained tolerances.
1. Over-Dimensioned Engineering Drawings
Engineering drawings saturated with callouts lose sight of true intent. Any part holds an unlimited number of dimensions for any part. Only a few of those dimensions affect how the part works. You want to focus your engineering drawings on those most impactful dimensions.
First, you need to clarify your functional needs. After selecting your control and your functional feature, you can establish your tolerance. You want to ask how much variation your part can tolerate. And in most cases, you will establish a reference for reproducible results. We call these datums.
2. Incomplete Specifications in Engineering Drawings
Some design shortcuts might save time but introduce new problems. For example, using general tolerances on engineering drawings leads to unwarranted failures. One size does not fit all when you define tolerances. This issue magnifies in the tolerance stackup.
Another design shortcut, coordinate dimensioning, does not emphasize form or orientation control of the feature. It actually leaves an incomplete specification of the feature. The quality inspector may have to reject good parts.
GD&T precisely controls form and orientation. Some controls can include multiple elements of functionality within one callout. This significantly reduces dimensioning, specifies functionality, and alleviates overly tight tolerances.
3. Over-Constrained Tolerances in Engineering Drawings
Over-constrained tolerances in engineering drawings overburden manufacturing and inspection processes, which escalates costs. Sometimes engineers make tolerances over-tight due to a lack of awareness of what matters.
Without defining some geometric control, coordinate dimensioning becomes limiting. You can solve this by defining location control with a true position. True position implements a diametric tolerance zone around a feature. A true position evaluation opens up the boundary and does not confine your dimension to a square tolerance. Do you really want a square tolerance zone for a round feature? Probably not.
GD&T true position defines a diametric tolerance zone of movement that can allow near 50% more variation in the hole position than with coordinate dimensioning. You can increase the allowable tolerance here while maintaining a focus on how these components insert into one another.
True position in your engineering drawings gives your manufacturers and quality personnel more room to work. True position applies the same tolerance to a pattern of holes or bosses to constrain the entire pattern by a uniform amount of allowable variation.
Well-defined engineering drawings should translate functional requirements into your design intent. GD&T helps to unify the intent of your engineering drawings and the function of your product. GD&T creates a clear representation of how features of a part integrate in the product assembly. It allows for simulation of functionality through measurement. It conveys functional design intent through geometric feature control rather than coordinate size and location of a feature.
I have helped many of my clients develop their skills in GD&T application, GD&T evaluation, and GD&T metrology. They gain confidence in their ability to achieve repeatability and reproducibility in their engineering drawings. I can help you validate your requirements, improve your quality, and embed meaning into your part drawings by circumventing the mistakes of over-dimensioning, incomplete specifications, and over-constrained tolerances.