Reference Dimensions on True Position

In this Question Line video, Jason walks through an assembly drawing to explain the scenario where a feature being controlled by position must have a reference size dimension rather than a tolerance on the size of the feature.
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#gdandt #trueposition #referencedimensions

[@EricIrwin]

You didn’t talk about the modifier. How can you have a MMC without a tolerance of size?

[@user-nh7gk1bw7d]

The size has already been insepected during the machinging process, so don't need to recheck it for the wellding assembly , just use the data recorded from the machinging.

[@EricIrwin]

@@user-nh7gk1bw7d Which would mean that the MMC modifier should not be included in the feature control frame.

[@user-nh7gk1bw7d]

@@EricIrwin A tolerance of size for the hole already included during the machining process, and considering the size of the hole will not change after welding process.For simplicity and save money (double check will cost extra money), use a reference size is a good choice for this purpose, that doesn't mean the size have no tolerance though. On the other hand, when MMC modifier is applied for the welding asselmbly, we can use a clylinder pin gauge to check the position tolerance.

[@EricIrwin]

@@user-nh7gk1bw7d I think the error in your logic is that the datum reference frame has changed from the part to the weldment. I still don’t believe it would be good practice to apply the material condition from the part to the weldment. I agree that you could get an answer as you have described but I don’t believe the answer is the correct interpretation of the applied tolerances.

[@user-nh7gk1bw7d]

@@EricIrwin The tolerance is determined by the ability of manufactruing process, it dosen't matter if it changed from one process to anoter process unless the size of the feature will be affected by the subsequent process