NOCTI Pre-Engineering Practice Exam

A basic dimension defines an exact value for location or size without providing a tolerance itself. Its purpose is to establish the theoretically exact geometry, which is then controlled by a GD&T tolerance such as true position. In other words, basic dimensions specify where features should be or how large they should be, but the allowable variation comes from the tolerance applied to the feature’s location or form. A practical use is locating a group of holes on a flange. You would mark the hole centers and the bolt circle diameter with basic dimensions to indicate their exact intended positions. Then you apply a true position tolerance to each hole relative to datum features, which tells you how far the actual hole centers may deviate from those exact locations while still mating correctly with the other parts. This distinction is why basic dimensions are not toleranced themselves. They set the precise target, and the true position tolerance defines the permissible deviation from that target.