Joshua Flanagan, Senior Mechanical Engineer
Geometric Dimensioning and Tolerancing, also known as GD&T, is a very powerful tool often referred to as a “language”. When used properly it can reduce production cost and result in more precise part manufacturing. However, when used improperly it can lead to increased part costs, confusion during manufacturing and misinterpretation of design intent. GD&T is widely used in a variety of fields including automotive, aerospace and defense, electronics, and capital equipment. GD&T allows for effective communication of design intent across multiple disciplines from engineering, design and manufacturing, all the way to test and quality. This results in higher quality produced products as well as freeing up tolerance on non-critical features. The key to properly utilizing GD&T is to understand the correct methods of defining specific features and locations to allow for a clear and concise communication of design intent.
Recently, a group of Columbia Tech engineers and Caring Associates participated in a 16 hour GD&T training class that took place over the course of three days. The instructor, Charles A. Gillis, P.E., Consultant, presented the fundamentals of GD&T as a refresher for some and intro to others. The course reviewed the current standards defined by ASME Y14.5-2009. After completion of the course, associates gained a greater understanding of the potential to utilize GD&T and instances when it may or may not be required to define the engineer’s intent for design or inspection.
GD&T is a dimensioning system used for defining and communicating engineering tolerances for features on parts and assemblies. The system utilizes a feature control frame with different symbols to define explicit nominal geometry for parts and assemblies with respect to their size, location, form, orientation, profile, and runout, as well as the allowable variation from nominal. This allows engineers to define the accuracy and allowable variation from nominal with respect to a size, location, orientation, and form. GD&T symbols for dimensioning a part or assembly are defined in relationship to a datum. A datum is a theoretical point, line, or plane on a part or assembly that is used as a reference point for dimensional measurements and tolerance calculations. When a datum is aligned with a dimension, it will create a center plane. In addition to the datum, a modifier can be used to specify the tolerance at Max Material Condition (MMC) typically used to specify material control on the outside dimension and larger, or inside dimension and smaller or, more simply put, the greatest amount of material. Opposing this is the Least Material Condition (LMC), or the least amount of material that can be used. The third option is Regardless of Feature Size (RFS), which removes the bonus tolerance gained by the other two material conditions. These material conditions contribute greatly when defining shaft and hole tolerance or any area of a part that will interact with another.
Certain geometric tolerance symbols are only applicable in a two dimensional space, where others apply in three dimensions. For example, when talking about a straight edge feature, straightness is applicable in 2D with respect to position, where flatness is applicable to 3D and applies to the surface. Although straightness can be applied to a surface, it can never have a material modifier. The same can be said for circularity vs cylindricity when talking about round features. In addition to this datum, targets including area, points and lines can be used to further define the feature being controlled.
Although GD&T is very useful, it is not always required for simple components that utilize basic title block tolerances and standard dimensions. It is often said that low skill, low cost vendors do not like GD&T as it is often thought to be difficult to understand and expensive to implement. This may result in no quote situations or increased quoted component cost. However, the opposite is true when properly used GD&T allows engineers and designers to reduce the tolerance requirements of a feature or assembly while still maintaining the form and fit as required. Columbia Tech’s engineers are knowledgeable and experienced in GD&T and are backed by a leading Supply Chain of Tier 1 vendors who are highly qualified to deliver low cost components with high precision.