In this video, we respond to a question asking how using Geometric Dimensioning and Tolerancing can affect internal or vendor-provided manufacturing and inspection processes.

It is important to remember that the goal of GD&T is to better communicate the functional requirements of the part. Functional requirements are often related to how a part is assembled to other parts, and how it functions in that assembly. Therefore, we should choose datum reference frames that will simulate the assembly of the part. Note that the order that the datums are referenced in the feature control frame should mimic the order of assembly.

The datum reference frames that we choose also establish the inspection requirements for the part. In Brandon’s example in the video, the bores for the clevis pin are referenced to datum A, the flat surface of the clevis bracket, and datum B, the bolt holes where the bracket attaches to the cylinder. To properly inspect the clevis bracket, the location and orientation of these bores must be established with respect to datum A first, and then datum B.

In contrast, machining or manufacturing processes and the order in which they are performed is not dictated by GD&T. These processes are entirely up to the manufacturer. As long as the part produced is within the tolerance requirements that we establish on the drawing, the part is good. It is up to each manufacturer to work out how to best leverage the equipment at his disposal to make the part. In fact, if a part is manufactured from a coordinate dimension drawing and then from the equivalent GD&T drawing, the same processes can and likely will be used. GD&T is simply another way of communicating the design information. However, keep in mind that GD&T does have advantages over coordinate dimensioning. For example, datums can take the place of reference edges used in coordinate dimensioning, allowing more permissive tolerances where possible. The net effect of converting a coordinate dimension drawing to GD&T should be to increase the amount of tolerance allowed for non-critical features, while maintaining the proper amount of tolerance where it is required.

As a practical matter, one of the most important things is ensuring that the vendor or manufacturing group has a thorough and correct understanding of GD&T rules and notation. A discussion regarding dimensioning methods should take place with your manufacturing group or vendor. You may want to ask if they would be willing to have some of their personnel attend a GD&T training course. Ideally, this conversation would begin when the engineering and design group first adopts GD&T. If the vendor or manufacturing group does not have the required knowledge to properly interpret GD&T, they may increase prices, refuse to bid, or (worse yet) manufacture poor quality parts.

Since GD&T should not affect manufacturing processes, but does dictate inspection requirements, you may encounter a situation where the quality group doesn’t have the ideal tools for inspection of the part produced. In that case, the design and quality groups should work together to establish a method of inspection that will ensure proper functionality of the part. For example, if you produce a part infrequently that has a surface profile feature, it may not be practical to purchase a CMM to inspect that part. Depending on the functional requirements, series of caliper measurements, or coordinate readings on an end mill at defined intervals may suffice instead.

In conclusion, keep in mind that GD&T is intended to communicate the functional requirements of the part, while eliminating tight tolerances from non-critical features. The goal of design with GD&T is not to complicate the drawing, but to simplify it. Also, keep in mind that our team here at GD&T Basics is here to help with any questions or training needs that you may have. Please don’t hesitate to reach out to us by clicking the “Contact Us” button below!

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