32 Geometric Dimensioning and Tolerancing Copyright Goodheart-Willcox Co., Inc. As previously discussed, model-based definition (MBD) and CAM are commonly used together. Annotated models created with PMI tools provide a number of advan- tages in a CADD/CAM system. Directly applied PMI is software-readable and can be used by CAM software to generate machine tool paths. This improves efficiency in a number of ways. It allows the production process to be precisely controlled and reduces waste in manufacturing because machining information is developed directly from the model data. It also reduces errors in machine programming by removing the need to refer to both the model and an associated drawing. This saves time in produc- tion and lowers costs. Implementing GD&T into a CADD/CAM system is critical for communicating the function and relationship of part features. The following design guidelines are for use with dimensioning and tolerancing in conjunction with CADD/CAM: • Major features of the part should establish the basic coordinate system but are not necessarily defined as datums. • Subcoordinate systems related to major coordinates are used to locate and orient features on a part. • Define part features in relation to three mutually perpendicular reference planes and along features parallel to the motion of CAM equipment. • Establish datums related to the role of the part and relate datum features in order of precedence as a basis for CAM usage. • Completely and accurately dimension geometric shapes. Regular geometric shapes can be defined by mathematical formulas. A profile feature that is defined with mathematical formulas should not have coordinate dimensions unless required for inspection or reference. • Use coordinate or tabular dimensions to identify approximate dimensions on an arbitrary profile. • Use the same coordinate dimensioning system on the entire drawing. • Continuity of profile is necessary for CADD models. Define contour changes at the change or point of tangency. Define at least four points along an irregular profile. • Define circular hole patterns with polar coordinate dimensioning. • When possible, dimension angles in degrees and decimal parts of degrees. For example, dimension 45°30′ as 45.5°. • Specified dimensions are at the mean of a tolerance because the CNC part programmer normally splits a tolerance and works to the mean. While desirable, one cannot predict where the part is made. Always base dimensions on design requirements. If you know that a part will be produced by CNC methods, establish dimensions with limits that conform to the CNC machine capabilities. Bilateral profile tolerances are also recommended for the same reason.