Part 1 showed that basic dimensions are theoretically exact values that do not need to be measured and reported for conformance assessment of geometric tolerances.  Part 2 showed that measured values of basic dimensions are commonly being reported in industry for other purposes, with varying success.  Part 3 will examine specific configurations of basic dimensions, features, and tolerances to identify the situations in which useful information can be extracted (and those in which it cannot).

Basic dimensions can be thought of as dimensions that apply to the theoretically exact model.  Because the model geometry is perfect, dimensions have well-defined and unambiguous meanings.  When applied to imperfect actual part geometry, however, dimensions have ambiguous meanings. In other words, the part geometry that the dimension represents is not clear.

 Higher Risk of Ambiguity Lower Risk of Ambiguity Basic dimensions involving surfaces Basic dimension applies to more than one feature Thick parts (more potential orientation error) Large form and orientation errors in considered feature and datum feature(s) Basic dimension applies to more than one geometric tolerance Basic dimension is a radius Basic dimension is an angle Basic dimension is a distance between two considered features (neither is a datum feature) Basic dimension is a bolt circle diameter Basic dimension involves an imaginary centerline, center point or midplane Basic dimensions involving perfect center geometry (axis) Basic dimension applies to only one feature Thin features (less potential orientation error) Small form and orientation errors in considered feature and datum feature(s) Basic dimension applies to only one geometric tolerance Basic dimension is a distance from datum feature to a considered feature

For most basic dimensions directly applied to imperfect part geometry, the risk of ambiguity is high.  There is a low risk of ambiguity only in special situations.  The lowest risk occurs for positional tolerances in which the basic dimension represents the distance between a datum feature and one particular considered feature.

Even on relatively simple part geometry, basic dimensions generally become ambiguous.  On the following drawings, all of the basic dimensions have a high risk of ambiguity on real part geometry:

The basic dimensions all have one or more of the following high-risk properties:

• Involve distance or angle between imperfect surfaces
• Apply to more than one feature
• Angle dimension
• Distance between two considered features (neither is a datum feature)
• Bolt circle diameter
• Involve an imaginary centerline, center point or midplane

Basic dimensions are sometimes extracted from the CAD model and measured directly on the part, in cases where model based definition was used.  These basic dimensions introduce the same risks and ambiguity as when explicitly annotated on the drawing.

Summary

Directly measuring and reporting measured values of basic dimensions introduces high risk of ambiguity in the large majority of cases.  This is because the meaning of the dimension on imperfect part geometry is not clear and could be interpreted in several ways.

JSN Megazine template designed by JoomlaShine.com