What is GD&T?

Geometric Dimensioning & Tolerancing, or "GD&T", is a standardized language of symbols that define the allowable type and amount of geometric variation that a mechanical part may have.  A "mechanical part" is any physical part.  It could be a camshaft, or an artificial hip component, or it could be a printed circuit board, especially if that circuit board has something like an encoder reader on it.  Any part that must be dimensionally "correct enough" to function is a candidate for GD&T.  Deciding whether GD&T is applied or not on a given part will be discussed in an upcoming post.

The geometric variation can be the size, form (shape), orientation, or location of any feature on a part, such as a planar surface, or a cylindrical hole, or some irregular or compound curved feature.  The complication in specifying tolerances to control these things is often blamed on the GD&T language itself, but it comes from mother nature, rather than from GD&T.  GD&T is simply the messenger that helps us see, and also deal with, all the imperfections that create risk of functional problems.  Even size tolerances can seem complicated after the functional requirements they address are considered.  We need well written and complete standards to define GD&T.

While ISO standards also exist, the GD&T standards that we use and teach at Validate-3D LLC are ASME Y14.5, titled "Dimensioning and Tolerancing", and also some other ASME Y14 standards.

So, the ASME Y14.5 standard defines GD&T for us.  It includes rectangular frames called "Feature Control  Frames" that are used to specify the type of tolerance, the shape of the tolerance zone, the amount of tolerance, and any constraint in location, or maybe just orientation, of the tolerance zone.  Feature control frames look like this:

When people see something like this feature control frame that specifies circular runout they may say that this is GD&T.  I agree.  What about this one though?

are all elements shown in this picture GD&T, or just the feature control frame and the datum feature label that hangs downward from the feature control frame?  I will say that even the size tolerance for the diameter of the feature that is placed above the feature control frame is "GD&T".  The reason I say this is because ASME Y14.5 includes specific requirements that define what the size tolerance means.  These requirements definitely address the dimensioning and tolerancing in a geometric way, and they come from the standard that defines GD&T.  So, everything shown above must be GD&T, right?  In fact, the size and position tolerance that is shown here are combined to define a worst-case boundary that is useful in design and also for analysis of tolerance stacks.  I think this reinforces my assertion that all tolerances defined by ASME Y14.5, including size tolerances, are GD&T.

We have shown a tolerance specification examples for circular runout and for a combination of size and position.  The other types of tolerances, called "Geometric Characteristics" in ASME Y14.5, and just a few words about each are also provided, for future discussion: