This information provided by Bryan R. Fischer, of TDP360 LLC.
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GD&T is a system of rules, tools, and methods to define perfect product geometry, the relationships between geometry, and the imperfection allowed on the geometry of actual manufactured parts and assemblies.
GD&T specifications are defined in drawings or annotated models. Engineering drawings and annotated models have perfect geometry. Thus, the specifications are defined in the context of a perfect part. The challenge is to define clear, complete, and unambiguous requirements that can be reliably recreated on actual, imperfect, manufactured parts and assemblies.
Engineering specifications and their lifecycle can be envisioned as occurring in a series of steps and activities.
The product is designed and modeled.
Drawings or annotated models are created.
GD&T and other specifications are applied to the drawing or annotated model by the designer.
The drawing or annotated model is released.
People in other departments or companies (e.g. suppliers) read the specifications on the drawing or annotated model.
Their goal is to determine what the specification means, the requirements it imposes on the final product, and to convert that into how it affects their role in the product lifecycle.
For example, an estimator needs to understand the tolerancing requirements so they can create an accurate quote for manufacturing, inspection, assembly or another activity.
A machinist needs to understand the tolerancing requirements so they can determine which manufacturing steps are required, if special tooling is needed, etc, to meet the requirements.
An inspector needs to understand the requirements so they can determine which measurement process to use, which tools to use, how much sampling is required, etc. to ensure the product meets the requirements.
People in these and other functions usually provide feedback about the specifications and often request changes to the drawing or annotated model.
The designer receives the feedback and revisions may be initiated.
GD&T Sequence
The designer obtains information about the product from stakeholders, such as what it does, the environment it will operate in, how long it must last, maintenance requirements, safety and handling requirements, etc.
The designer obtains information about the manufacturing process and manufacturing process capabilities.
The product is designed and modeled.
Analyses, and possibly prototyping, are done to determine if the design will satisfy its objectives.
GD&T is applied to the drawing or annotated model by the designer.
Analyses, and possibly additional prototyping, are done to determine if the specifications will satisfy the design objectives.
The design is finalized and released.
People in other activities receive the drawing or annotated model. The recipients read the drawing or annotated model and the GD&T specifications to understand the requirements imposed upon the manufactured product.
Specifically, they need to determine the implications of the specifications and the requirements they impose on their activity. See above for examples.
They convert the GD&T specifications into the activities they do, such as creating work instructions, selecting tooling, ordering material, developing toolpaths for CAM and CMM software, etc.
The product is manufactured. All geometry on the manufactured product is imperfect.
The product is inspected to determine if the imperfect manufactured geometry is within the limits imposed by the GD&T specifications.
GD&T allows us to define requirements that can be reliably and repeatably recreated on actual imperfect manufactured parts, without having to resort to informal methods such as guessing or adding missing rigor, such as guessing how best to set up a part for inspection. Properly used, GD&T eliminates guesswork about dimensioning and tolerancing.
GD&T and Design Information in the Product Lifecycle