During the dimensional measurement process in quality inspection, when determining the conformity of a part’s geometric dimensions and geometric tolerances according to drawing requirements, the following concepts typically need to be clarified:

1. Dimensional Terminology

1) Basic Dimensions: These are the dimensions specified by the design, commonly referred to as theoretical dimensions or standard dimensions. Generally, these dimensions are rounded by the designer to conform to standard dimension series, thereby reducing the variety of fixed-size cutting tools and measuring instruments required.

2) Actual Dimensions: These are the measured dimensions;

3) Limit Dimensions: These are divided into maximum limit dimensions and minimum limit dimensions.

2. Dimensional deviations are classified into actual deviations and limit deviations:

1) Actual Deviation: The algebraic difference between the measured dimension and the basic dimension;

2) Limit deviations: The algebraic difference between the limit dimensions and the basic dimension (upper and lower deviations), commonly referred to as the upper and lower tolerances, used to control actual deviations.

3. Dimensional tolerance (abbreviated as tolerance) refers to the allowable range of dimensional variation, i.e., the difference between the maximum limit dimension and the minimum limit dimension; it is used to limit errors and is typically displayed using a tolerance zone diagram.

4. A tolerance zone diagram is a graphical representation of the relationship between dimensions, limit deviations, and tolerances. It consists of a zero line and tolerance zones. The zero line is a reference straight line representing the nominal dimension and defining the deviation; it serves as the starting point for the deviation. Standard tolerances are standardized tolerance values specified by national standards, while the basic deviation is the limit deviation closest to the zero line.

        Generally, limit deviations primarily depend on the adjustment of machining equipment and do not reflect the difficulty of machining; whereas tolerances indicate manufacturing precision and reflect the difficulty of machining. Limit deviations primarily reflect the position of the tolerance zone, affecting the tightness of the fit; whereas tolerances represent the size of the tolerance zone, affecting the accuracy of the fit. The use of tolerance zone diagrams more vividly illustrates the relationship between dimensions and limit deviations, particularly in fit relationships where they can simultaneously present different conditions of holes and shafts.