An examination of the effect of variation in datum targets on part acceptance

The use of datum targets within Geometric Dimensioning and Tolerancing (GD&T) is commonplace in industry today. Datum targets are used primarily for manufacturing and inspection setups where it is not practical to have tooling that contacts the entire surface of the datum. Establishing a simulated datum from datum targets may be done physically or mathematically. In the process of establishing a simulated datum, there are several possible sources of variation. Any variation or error in establishing the datum will manifest itself as variation or error in the measurement of the distance between the feature and the datum. This paper examines the effect of variation in datum targets on part acceptance using both mathematical analysis and simulation. The focus of this examination is diametric positional tolerance of holes that reference the datum established by datum targets. A three-dimensional mathematical model of the hole position error with respect to the datum target variation is developed. Then, simulation is adopted to identify the major sources of hole position error due to errors in datum targets. It is found that the datum target variation has a profound effect on the measured location of holes referencing those datums. Explanation and validation for accepted practice are offered and some additional basic guidelines are developed for the placement of datum targets relative to hole patterns.