This information provided by Bryan R. Fischer, of TDP360 LLC.
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GD&T is a standardized system and graphical language used to describe product geometry and its allowable variation. The system of GD&T is defined in the ASME Y14.5-2018 standard. The language of GD&T is used in specifications to define nominal part geometry and its allowable variation.
GD&T provides rules, tools, and methods to define nominal (perfect) part geometry. Perfect part geometry is defined with dimensions or 3D model geometry.
GD&T provides rules, tools, and methods to define different types of tolerancing features, tolerances, and tolerance zones, and contextual relationships between features.
GD&T provides rules, tools, and methods that allow people to recreate and evaluate requirements imposed on actual, imperfect, manufactured product geometry. GD&T bridges the gap between the perfect world of drawings and CAD model geometry with the physical world of actual, imperfect, manufactured product geometry.
Plus and minus tolerancing does not and cannot do these things.
Why is GD&T Needed?
GD&T was invented long ago to solve a long-standing problem. That problem still exists today.
The problem: Without GD&T, specifications on engineering drawings are unclear, incomplete, and ambiguous, and the requirements imposed on the product are also unclear, incomplete, and ambiguous. This problem also applies to specifications on annotated models.
Without GD&T, people reading a drawing or annotated model to understand requirements defined by tolerancing, to provide a cost estimate, manufacture, and inspect parts and assemblies are not sure what the specifications mean. They are not sure which requirements are imposed on the product by the specifications, as the associated conformance criteria are inadequately defined. Thus, when determining conformance to the specifications, people must guess. Many people in industry have built their career around guessing what these requirements are and what conformance means. Unfortunately, in many cases people are so used to unclear specifications that they expect specifications to be ambiguous and incomplete and believe completing the definition is part of their job – they no longer expect design to send them high-quality product definition data – they think their superpower at work is making sense out of nonsense. Many experienced people outside of design don’t know that they are receiving inadequate product definition data – they complete the definition without recognizing they are adding missing constraints and requirements to the product.
GD&T was created to solve this problem. There are several layers to the problem.
The first layer is the specification itself. Specifications are defined on drawings, annotated models, and in specification documents. Specifications exist in the theoretically perfect world. The drawing views show a perfect part. The dimensions define the perfect part, the geometry in drawing views and 3D models represent the perfect part. So, the first layer is the specification. Specifications must satisfy these criteria. A specification must:
Be clear and understandable
Be unambiguous
Be complete
Have one meaning
Be explained by a recognized standard or other official document
Work with and be consistent with other specifications that apply
Be reproducible on actual manufactured parts, which have imperfect geometry.
The second layer is the requirements imposed by the specification on the product. The requirements must be completely and clearly defined, and they must be based on a legally-applicable standard or specification document. The meaning of the specifications must be understood by everyone who reads the specification, and everyone must have the same understanding. A specification must not mean different things to different people. However, specifications affect people differently. For example, a machinist reads a specification to determine which manufacturing process and tools to use to machine a surface, whereas an inspector reads the specification to determine how to measure the surface to determine if it conforms with the requirements imposed by the specification. The key is that the requirements and what they mean in the context of an as-produced part or assembly must be clearly understood by everyone. And while their activities may differ, their understanding of what the specification means must be consistent.
The third layer is being able to recreate the requirement on an actual imperfect as-produced part or assembly. This is necessary for evaluating conformance to the specification. Of these three layers, this layer is the most challenging if GD&T is not used. GD&T includes rules, methods, and tools for establishing repeatable setups for geometric tolerancing. GD&T provides a bridge between the perfect world of drawings and CAD models and the imperfect world of actual parts, assemblies, and shop practices.
The constraints and requirements imposed upon a product by a specification are defined in the perfect world of drawings and models. In this perfect world, the imperfections of the actual product are missing. The drawing, dimensions, and the CAD model represent a perfect part. The constraints and requirements imposed by the specification are typically not geometrically or graphically represented – specifications are generally represented symbolically. Numbers, text, and symbols define the allowable imperfection and its limits.
While specifications are defined in the product definition data set, their meaning is determined by comparing the specification with an applicable standard.
Determining conformance to the requirements imposed by specifications occurs in the context of the imperfect part. Many of the specifications on engineering drawings and models are inadequate, as they are not based on a method that can be uniquely and repeatably recreated on an actual imperfect as-produced part. The specification seems okay on paper, but the specification does not provide the correct context to reliably recreate its requirements in the physical world. GD&T was created to solve these problems.
GD&T provides a means to address items 1, 2, and 3 above.
Most important reasons to use GD&T
GD&T is unambiguous
GD&T is based on a rigorous standard
GD&T specifications have a standardized meaning, they are consistent, common, and universal
The requirements imposed by a GD&T specification are standardized, they are consistent, common, universal
Requirements imposed by GD&T are reproducible on actual, physical imperfect parts and assemblies in the physical world
Direct tolerancing (plus and minus) methods do not and cannot satisfy the criteria above. Direct tolerancing is ambiguous, incomplete, does not clearly define requirements, and the requirements it defines cannot be recreated in a standardized manner on actual as-product parts and assemblies. Direct tolerancing requires people to guess which requirements are imposed by the specification and how to determine conformance to the specifications. If it isn't clear what the requirement is, obviously it isn't clear how to determine conformance. Products defined using direct tolerancing (plus and minus tolerancing) are incompletely defined and require guessing in analysis, manufacturing, and inspection.