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Why Do I Need GD&T?
The craftsmen of old could fashion parts in a way that would allow them to slide together and give the impression that the parts fit "perfectly." Today, with the concept of interchangeable parts, credited to Eli Whitney, it is expected that parts will assemble the first time and perform their intended function. Interchangeability does not apply only to mass produced parts. Whenever two parts are expected to fit together and function without rework or adjustment, the parts must be clearly defined. Parts that have been made in other departments, plants, cities or even countries must consistently fit and function even though slight variation from the intended shape and size will exist in every part.
All parts go through a manufacturing process. There is variation in all manufacturing processes. These variations are reflected in the parts. In addition, there must be a way to inspect a part to assure that it was made to the required specifications. As Bob Traver says:
"You can't make what you can't measure because you don't know when you've got it made!"
Most importantly, the part must perform its intended task or function. To accomplish all of this, the part must be clearly and totally defined. In most cases this definition is accomplished on a detail drawing or within a CAD file.
When used properly, GD&T will get the right questions asked early in the program, simplify the engineering drawing, and directly relate customer requirements to product specifications and process control.