The plus-minus tolerancing is not sufficient for complete specifications of parts. There is a need to define clear specifications using geometric controls. There are total 14 geometric characteristics symbols in geometric dimensioning and tolerancing. These symbols are in 5 categories. Form tolerances, Orientation tolerances, Position tolerances. Profile of surface tolerances and Runout tolerances are these types.
The form tolerances include Flatness, Straightness, Circularity, and Cylindricity. The form tolerances never use datum references. The inspection of form tolerances should not have any reference feature.
The orientation tolerances are Perpendicularity, Parallelism, and Angularity. They must need a datum reference. Orientation tolerances also control form when they are controlling surfaces or features. Orientation control can be used to control the only orientation and never the location of features.
Location tolerances are Position, Concentricity, and Symmetry. They use datum features based on the type of application. Concentricity and Symmetry have not used by most the industries. Position tolerance can easily replace them without compromising the function. Position tolerance is often used to locate feature of size.
Profile tolerances are Profile of a line and a Profile of a surface. Profile of a line is 2-dimensional control. Profile of a surface is a 3-dimensional control. Profile tolerances can control size, form, orientation, and location. Profile tolerance can control surface or boundary.
The runout tolerances are Circular runout and Total runout. They have mostly used for rotating applications. The runout tolerances are being called as composite controls as they can control the form, orientation, and location of features. They must need a datum axis. The inspection procedure requires the part in the rotation which needs inspection.
Out of all 14 geometric characteristics symbols, Position and Profile tolerances are often used in drawings by engineers. They are known to be powerful controls in the world of GD&T. Hope this blog clarifies about GD&T symbols and their types. Wish you very happy learning !!