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The Profile Side of The GD&T Hierarchy Pyramid
 
A previous Tip provided you with The GD&T Hierarchy Pyramid that you could fold up and keep as a handy reference. 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. The next few Tips will explain further how to use the Pyramid. (Click here to open and print "The GD&T Hierarchy Pyramid".)

The first 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. There are several geometric tolerances that can be used to locate features. There is only one geometric tolerance that may be used to locate every feature on a part. The geometric tolerance is profile of a surface. Profile of a surface is the only geometric tolerance that controls the location of surfaces. Since every feature on a part has a surface, profile may be used to locate every feature.


Most companies still use linear and angular general tolerances found in the titleblock of a drawing. They do not know why they use it, they just do. "That's the way we've always done it." This general note causes much confusion because it does not relate tolerances to a datum reference frame, allows tolerance accumulation and wedge shaped tolerance zones. To clear up the drawing's meaning, many companies are changing to a general profile of a surface tolerance with datum referencing. This requires that the dimensions describing surfaces be basic. Since this will usually be most of the dimensions, it is becoming a common practice to add the note "UNTOLERANCED DIMENSIONS ARE BASIC". This note eliminates a lot of boxes on the drawing. Then apply tolerances directly to those dimensions that should not be basic such as hole diameters and slot widths. Since the general profile tolerance controls location of the entire surface of each feature, the only reason to put any geometric tolerances on the field of the drawing is for refinements of orientation, size and form.

   

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