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:

• The benefits of geometric tolerancing are lost
• Unlikely that the measured values will correlate with the measured value of the geometric tolerance
• Direction of measurement will be different for each dimension
• Dimensions can often be measured in several different ways, leading to different results
• Repeatability and reproducibility of dimensional measurements is often very poor when form error is present
• The meaning of the dimension on the actual part will be ambiguous in many cases
• Some basic dimensions may apply to several features or several feature control frames
• Basic dimensions may involve imaginary centerlines or center points that cannot be uniquely defined on the actual part
• This strategy inherits all of the problems of directly toleranced dimensions, that geometric tolerancing is intended to address.

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:

• The Datum Reference Frame (DRF) is a coordinate system derived from the datum feature simulators and datum features.
• The tolerance zone is located at true position (theoretically exact location in the DRF).
• The controlled component is the axis, center plane, or center point of the feature.
• The measured value is the size of a zone that is located at true position, that just envelops the controlled feature component.

• The actual coordinates of the controlled feature component can be measured, and reported relative to the basic coordinates.
• The DRF provides the coordinate system (axis directions and origin) for the components
• The basic coordinates are defined by basic dimensions on the drawing or model-based definition.

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.

Other Strategies

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.

Summary

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.