Critical Dimension

I am a QA Engineer and i have to highlight the critical dimension on the Mechanical drawing for QA Incoming Inspection, so that the QA Incoming inspector can inspect only those dimensions to accept or reject the part. My question is, what is considered as “Critical Dimension” and how should i select it. Any information regarding this subject will be greatly appreciated.


I do not understand this concept of critical dimensions. On my drawings, all the dimensions are critical. If the drawing is sloppily prepared, we can assume that many of the tolerances are complete nonsense. The critical dimensions are the ones where the tolerances are not complete nonsense. There is the manufacturing issue that some tolerances are difficult to achieve. If some manufacturing process is running at or near its maximum capability, then that feature of the part must be inspected systematically.

I am thinking this through as a designer, not as a plant manager or inspector. I am the guy who makes the drawings. I am not the guy who has to read them. As the designer, I need to try very hard to ensure that critical features are well within the capability of the manufacturing process. As drafter, I look at every dimension and tolerance, and I make they are appropriate.

I worked with a guy who wrote notes on his drawings stating that special attention was to be paid to certain dimensions. To me, this implied that he did not care about the other dimensions. If you are handed a package of drawings by a CAD operator who clicked on all the features and applied the default dimensions, then this discussion is relevant. You are going to have to track down someone who gives a s**t, to tell you which dimensions are important, and what the required tolerances are.
The whole point of good drafting is that you see all the information you need on the drawing.

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