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Wall thickness or tolerance control on a tube or ring is dependant on functional requirements with an eye on manufacturability.
If the tube is just carrying fluids from one place to another then a minimum and maximum wall thickness need only be specified. When measuring, manufacturing would not necessarily measure every possible wall thickness but would measure enough locations to have confidence in the as-manufactured part and manufacturing processes utilized.
“basterdize runnout” – I’m not sure what you mean by this, again mechanical tolerance should by applied to meet functional requirements so that the end item part does the job it was meant to do.
“It is odd to think of a moving datum but that is what we have.” – Moving Datum? From your description that is not what is going on. When a diameter is specified as your datum, the datum or “zero” is established most correctly from a datum simulator. For an external diameter one would collapse a ring gage or equivalent onto the as-built external diameter. The ring gage would collapse until the three outer-most areas of the as-built external diameter are contacted. Then, a zero or a measurement is made from the Ring gage surface itself to the concerned feature elements to establish the surface variations.
So, you see the datum is not really “moving”.
Can you post an example on this forum?