As defined in the standard ASME Y14.5M-1994, a tolerance is "the total amount a specific dimension is permitted to vary. The tolerance is the difference between the maximum and minimum limits."
Before specifying certain types of tolerances, you must first define a datum (or datums) as a reference from which the tolerance can be measured. The tolerance types available are the following:
• Tolerances of Form:
◦ Straightness
◦ Flatness
◦ Circularity
◦ Cylindricity
• Tolerances of Profile:
◦ Profile of a Line
◦ Profile of a Surface
• Tolerances of Orientation:
◦ Angularity
◦ Perpendicularity
◦ Parallelism
• Tolerances of Location:
◦ Position
◦ Concentricity
◦ Symmetry
• Tolerances of Runout:
◦ Circular Runout
◦ Total Runout
For all tolerance types there are a number of parameters, such as zone size and modifiers, that can be specified for each definition of a tolerance symbol.