A common question asked by CMM inspectors is whether or not to include basic dimensions on inspection reports. GD&T theory tells us that basic dimensions are theoretically exact and are not directly toleranced. Despite this, reporting measured values for basic dimensions is a common practice and is used by many companies to make adjustments to manufacturing processes. Customers often ask to see measured values for basic dimensions on inspection reports. Is this a legitimate request, or is it based on a flawed understanding of geometric tolerancing? Reporting of basic dimensions is one of the key issues addressed by the ASME Y14.45 standard for Measurement Data Reporting, which is currently in development.
Dimensioning and Tolerancing Standards Perspective
One core concept of geometric tolerancing in the ASME Y14.5 Dimensioning and Tolerancing standard is that features (or their components) are controlled within tolerance zones of a particular shape and size. These zones are built on a framework of theoretically exact geometry defined by basic dimensions. The framework describes the as-designed orientation and location relationships between the toleranced features, as their relationship to datum features. This is a fundamental difference between geometric tolerancing and plus/minus tolerancing, in which dimensions are directly toleranced. With geometric characteristics, the tolerance value is represented in terms the size of a zone – not in terms of dimensions. The Y14.5.1 standard for Mathematical Definitions for Dimensioning and Tolerancing also defines actual values of geometric characteristics in terms of the size of a zone. The dimensions of the theoretically exact geometry are not variables – they are constants.
Quality Assurance Perspective
The AS9102 aerospace quality assurance standard states that design characteristics can be measured, inspected, tested, or verified to determine conformance to the design requirements. Based on the ASME standards, basic dimensions do not meet this definition of design characteristics – they cannot be measured and do not have requirements to conform to. Thus the applicable values for AS9102 Form 3 for Characteristic Accountability are measured values for geometric characteristics such as position and profile – not measured values for the basic dimensions.
Basic Dimensioning Methods
The Y14.5 standard allows the theoretically exact part geometry to be defined in various ways. Basic dimensions can be explicitly specified on a drawing in different layouts such as rectangular or polar coordinates. Certain basic dimensions, such as basic 90 degree angles, are implied. If a model-based definition is used, explicit basic dimensions may not appear on a drawing at all.
Any of these methods can be used equivalently, without affecting the geometric tolerance. The meaning of each geometric tolerance is identical in each of the above cases. Again, this is because the basic dimensions are properties of the theoretically perfect part definition – they are not applied to the real part. This is another strong indication that measured values corresponding to basic dimensions do not have a direct relationship to the specified geometric tolerance.
Application of Geometric Tolerances to a Real Part
Illustrating the geometric tolerances in the context of a real part shows how the theoretically exact geometry provides a framework for the datums and zones. The tolerance zones are constructed at their basic locations, with the zone sizes determined by the tolerance values. Measured values are calculated by constructing fitted zones at these same basic locations, with sizes that just envelop the features. These results are independent of the basic dimensioning method used – this is because the basic dimensions only apply to the theoretically exact geometry, and are not directly applied to the real part. There is no requirement to measure dimensions on the real part – only to measure the features and establish the zone sizes. This is a subtle but important distinction.
From the standpoint of geometric tolerancing and the associated standards, basic dimensions represent theoretically exact geometry that does not vary. With geometric characteristics, the tolerance value and measured value are based on zones and not on dimensions. For conformance assessment, there is no requirement to report measured values that correspond to basic dimensions.
Part 2 of this article will examine basic dimensions from the standpoint of manufacturing process feedback. Since real part dimensions corresponding to basic dimensions are being measured and reported in industry, we will examine how this is being done. This will identify situations in which these measured values provide useful information, and in which they may be misleading and confusing.