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The Feature of Size Side of The GD&T Hierarchy Pyramid
The previous Tips provided you with The GD&T Hierarchy Pyramid that you could fold up and keep as a handy reference and an explanation of the profile side of the pyramid. I suggested you print it on glossy paper with a weight of from 24 to 100 lb. Some folks have printed it on B or C size paper to make it easier to read. This Tip will explain further how to use the Pyramid.

The feature of size panel shown supports the three steps to applying GD&T:

  •  Identify the datum features to establish origins of measurement.
  • Control datum features. This qualifies the datum features.
  • Locate all other features to established datums. Of course, all features may be located using profile of a surface if the feature is defined by basic dimensions or the CAD file is basic. In addition, features of size may be located using position or in the case of cylinders, circular or total runout

Rule #1 of the Standard tells us that if size is measured according to the Rule, the form of the feature is automatically controlled. In practical terms the feature is checked with a full form element, such as a plug gage for a hole, to make certain MMC is not violated. In addition, two point measurements are required to be certain the feature does not violate size at any cross section. Therefore, the form controls of straightness, flatness, circularity and cylindricity are usually not needed since their deviation must be less than the total size tolerance.

Position may then be used to locate these features of size. If the feature is cylindrical and shown coaxial to a datum axis, like the 10 hole on this drawing, circular or total runout may be used instead of position. Position and runout both control the orientation of the feature. Therefore, the orientation tolerances are usually not necessary but may be used as a refinement if necessary. This drawing illustrates how features may be located. Datum feature A does not need a locating tolerance since the other features are located and oriented relative to the datum axis it establishes. Datum feature C establishes the origin of all axial measurements and therefore does not need to be located.


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