In Part 1 showed examined basic dimensions from a standards and quality assurance perspective. For conformance assessment, there is no requirement to report measured values that correspond to basic dimensions. Nevertheless, this is being done in industry in various ways and for various purposes. This article will examine and different strategies being used to generate measured values for basic dimensions.
Strategy 1 – Direct Measurement
One strategy is to simply measure each basic dimension, as if they were directly toleranced dimensions. This means that each dimension is examined individually, without any use of datum reference frames. This simplistic approach has the following disadvantages:
Strategy 2 – Measurement of Location Components for Position Tolerances in a Datum Reference Frame
Basic dimensions can be involved in the reporting of additional information for geometric tolerance inspection. The components of the feature’s true position in the datum reference frame are reported, and in some cases these components directly correspond to basic dimensions on the drawing. In this context, measured values corresponding to basic dimensions can provide useful information:
In some cases, a basic dimension on the drawing directly represents one of the basic feature coordinates in the DRF. This is the situation in which a measured value corresponding to a basic dimension can provide useful information.
Strategies 1 and 2 described above represent two extremes. Strategy 1 focuses solely on the dimensions, and Strategy 2 is fully based on the geometric tolerance definition. Other strategies are possible, that use the geometric tolerance information to a limited degree by directly measuring the dimensions in a particular coordinate system.
Several different strategies for obtaining measured values for basic dimensions are possible. In some cases this can provide useful information and in other cases many problems arise. Part 3 will analyze the specific basic dimension, feature, and tolerance configurations in which useful information can be extracted and those in which it cannot.