What Is GD&T?
Geometric Dimensioning and Tolerancing (GD&T), when made an integral part of the Product Development/Delivery Process through drawing previews/reviews, is the tool that can have the greatest impact on improving quality, cost and delivery of your product. GD&T is not new, but there is newness about it. Recent developments in CAD and CMMtechnology, ISO Standards and Statistical Process Control have brought GD&T once again to the forefront of design, quality and manufacturing. GD&T is a common language that may be used to improve communication between design, quality, manufacturing, suppliers and customers. For nearly two hundred years, industry has been struggling with the problem of clear part definition. The latest and mostsuccessful approach has been to employ Geometric Dimensioning and Tolerancing (GD&T). After nearly sixty years of refinement, industry now has a Standard which will handle virtually every situation. The Standard is published by the American Society of Mechanical Engineers and is designated ASME Y14.5M-1994. Some of the advantages of using GD&T on engineering drawings and product data sheets are thatit: • • • • • Uses datums and datum systems to define dimensional requirements with respect to part interfaces. Specifies dimensions and related tolerances based on functional relationships. Removes ambiguity by applying universally accepted symbols and syntax. Provides information that can be used to control tooling and assembly interfaces. Expresses dimensional tolerance requirements usingmethods that decrease tolerance accumulation.
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. Interchangeabilitydoes 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 amanufacturing 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 orfunction. 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.
GD&T Glossary and ResourceSymbols and Terms
Use this quick reference to find definitions of common GD&T symbols and terms. Our Pocket Guides are great resources for your desk, workbench or pocket. Be sure to check out our GD&T Tips! Straightness - a condition where an element of a surface or an axis is a straight line.
Flatness - is the condition of a surface having all elements in one plane.
Roundness - describesthe condition on a surface of revolution (cylinder, cone, sphere) where all points of the surface intersected by any plane. Cylindricity - describes a condition of a surface of revolution in which all points of a surface are equidistant from a common axis. Profile of a Line - is the condition permitting a uniform amount of profile variation, ether unilaterally or bilaterally, along a line element...