CAD vs CMM Basic Dimensions

In this Question Line video, Jake answers the following question from a CMM programmer regarding conflicting basic dimensions: 

I’m a CMM programmer, my manager asked me to change CAD nominals to match drawing basic dimension to conform to true position actuals. Is this right? Drawing refers me to ASME Y14.41 but I don’t have that spec.

When a dimension pulled from a CAD model doesn’t match its 2D drawing, how should you address this mismatch?

Jake begins by clarifying that the 2D drawing is the legally binding document by default, giving it the function of the master. However, some drawings or agreements may include a note stating that the CAD model takes precedence, making it the master. When in doubt, it’s important to verify which should govern.  

He then points out three common causes for why this mismatch occurs: 

1. Rounding in the CAD model 
2. Overridden basic dimension in the drawing 
3. Incorrect measurement 

By walking through several examples, Jake highlights key checks to help identify the cause of mismatched basic dimensions between the CAD model and the drawing. 

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