Radial Patterns as Datums

In this Question Line video, Jake answers the submitted question about radial patterns as datums: 

In the drawing, I have marked the pitch circle diameter of the 6 through holes as Datum B with perpendicularity to Datum A. Also the angular spacing of the holes is an inspection dimension. Please clarify that this method of dimensioning the hole pattern is correct or not.

 

 

Jake reviews the submitted drawing, above, which features a pattern of six through holes. In this drawing, the bolt circle diameter is identified as Datum Feature B and is defined with a size tolerance and perpendicularity to Datum Feature A. The angle between the holes is also dimensioned with a tolerance.  

Jake clarifies that dimensioning the bolt circle in this way is not a correct method for establishing the datum. Referencing ASME Y14.5, he explains that a pattern of features of size—such as a group of holes—can be used together to establish a datum feature simulator, which is used to derive the 2nd and 3rd planes of the datum reference plane. He then walks through an updated version of the drawing, applying position control to one of the holes and defining Datum Feature B as the hole with six instances. With this approach, the hole pattern is properly defined to function as a single datum feature in accordance with ASME Y14.5 and the original design intent. 

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Watch the video below to see how Jake applies position control to correctly define radial patterns as datums: 

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For more information regarding radial hole patterns, see: 

Common GD&T Student Questions: A Pattern of Holes as a Datum Feature 

True Position on Bolt Circle Diameters 

Radial Hole Pattern as a Datum Feature

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