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CAD Tips: Nuances of concentric design and GD&T

The use & application of concentric or coaxial features in mechanical design is a common component of the CAD skill set, though it is often misunderstood. As mentioned in an earlier post, GD&T(geometric dimensioning & tolerancing— ASME Y14.5) is a baseline tool and industry standard. Many of the principles of GD&T come into use in the design of coaxial features & parts. Further, one can gain a more direct understanding of their applicability through corresponding aspects within the CAD package itself. In SolidWorks, this is most evident in Relations and Mates. Parenthetical references to sections below refer to the text of the ASME Y14.5 standard.  

(In this discussion,
there is the presumption that most readers
have a certain basic grasp of fundamental geometric terms & principles.
Links have been provided in certain instances to augment this.)

Relations are exactly what the word suggests — the establishment of constraints on how two or more elements of a sketch or feature relate to one another. These include such things as concentricity, parallelism, coincidence, tangency, perpendicularity, etc. Relations are the geometrical laws which enforce constraints on a design, thereby controlling certain variables so that your model ends up being fully defined. It is not always necessary for a sketch to be fully defined to move forward with the creation of a feature, but for newbies, it is the safest path.

In GD&T, these principles appear in essentially two contexts:  Limits(2.4) & Geometrical characteristics(3.3.1). Limits are familiar to us in many contexts, most commonly through the use of dimensioning, which establishes limits on size and relative position. Geometrical characterics are represented via symbols which are a language independent means of conveying this information, which was one of the key bases for establishing this standard.

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