The question has come up regarding the primary datum listed in the True Position Feature Control Frame. It has been stated that the Primary or 1st datum listed in the Feature Control Frame would be the datum that is perpendicular to the hole being drilled. Other views state that once you define the datums on the part, you just always state datuams A B C in that order regardless of what plane the hole is perpendicular to. Which view point would be considered correct?
Well, the two generic rules you are refering to I have heard before. Technically speaking, neither pont of view is most correct...
Now, to be fair in many (some) applications the first (1.) rule of thumb can work relatively well, but eventually it will fail the designer and the design intent.
The second (2.) rule of thumb sometimes works well in many simple application, but then again it will eventually fail the designer and the design intent.
To be "MOST CORRECT" datums should be assigned and selected based on either DFF (Datums For Function) and sometimes DFM (Datums For Manufacturing) principles and practices.
Datum's for Function (DFF) - General; these are the features which facilitate primary contact with the target assembly on the selected part. The primary datum is the feature which will have the most influence on a particular features relationship with a mating or similar relevant feature on the mating assembly part. The secondary datum is the second most important mating feature, and the tertiary datum is the least important feature relevant to fit or mating requirements.
For example and using the hole feature you mention - the datum’s that would be selected on the hole feature part would be those features that would touch (or nearly touch) and do the alignment, location and orientation of the hole feature to the target mating feature(s) within the target assembly part. The flange (perpendicular feature), if there is one, could in fact be the most influential to the orientation, location and orientation of the hole to the mating feature - but not in all possible designs.
Datum’s for Manufacturing (DFM) - Often, datum’s are selected to simulate and simplify manufacturing and process requirements. This strategy works best in selected manufacturing environments when the manufacturing processes mimic or manufacture such that the assembly alignment and fit requirements are simultaneously met.
As far as always using datum letter "A" for primary, "B" for secondary and "C" as tertiary - there is no rule that states as such. However, in simple design it can be a logical and reasonable practice to keep your engineering drawings similar or logical.
Keep in mind that with respect to DFF and DFM – it is possible on simple parts that the datum feature identified as “A” is the most logical primary datum for all features on a part. However there could be a feature that uses the same datum’s (features) but the most influential datum feature to alignment, orientation or location may in fact be the “B” or “C” datum feature.