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[COLOR=#333333][FONT=Arial]Here is my first forum question: Imagine there is an extruded shape with two flat faces at the ends (a symmetrical thick washer, for example). One of the flat faces is called out as the primary datum feature. Which face is required to be used as the datum feature for inspection?:
1) The most favorable face,
2) The least favorable face, or
3) Both faces (using one face and then the other as the datum feature)?[/FONT][/COLOR]
Well, regardless of the GD&T standard utilized a fundamental rule of datums is that the "Datum feature must be readily discernible on the part".
You can read more about this in ASME Y14.5-2009, para. 4.8 (page 57).
Here's what I recommend:
[LIST=1][*]The engineering drawing should be clarified so that the datum feature is "readily discernible on the part". This can be done in variety of ways (part number on opposite surface, dimple, etc.).[*]If this is a drawing must be inspected by you, I would contact the responsible engineering drawing owner and ask for clarification or alternative instruction. Hopefully the clarification is documented for future use.[*]If you have no engineering drawing contact or support and you must inspect it I would inspect the part in two separate setups. What I mean by this is setup using one of the possible datum features then the other.[/LIST]
I recognize that this is redundant and extra work in addition to doubling the chances of rejecting the part due to an out-of-tolerance condition. But this will ensure that a functional part reaches assembly.
I don't read the standard as saying that "readily discernible" means that a symmetric part [I]must[/I] have an identifying feature on it. The wording simply says that it "may" be necessary to add such a feature (dimple, ink stamp, etc.).
So of the choices given in the OP, I would say #1, which is the most favorable face. Because think about it: if a completely symmetric part is assembled at a factory and doesn't seem to work on the first try, is the operator going to immediately throw the part in the reject bin? No -- he would first try to flip the part over and then if it works, all is OK.