JHG If my space vision is still intact there should be an angle in the mix to establish the beginning and/or the end of the radius location.
That would be the tertiary datum even if you resorted to a datum target with exactly the same but you get the idea I don't know anything but the people that do.
This is not always convenient, and alternate dimensioning procedures sometimes show relationships you are interested in.
Then again, if you are interested in these relationships, maybe you should apply datums accordingly.
Does it matter if the product function dictates that the end of the part (a part of the profile) the controlling point?
Is there any part of the ANSI spec that requires/recommends that the referenced datums be the same point/plane from which basic dimensions are originated? I think it makes more sense that the tertiery datum be the centerline of the hole - it just seems more logical - but I know from a product function standpoint it doesn't make any difference. mtncrawler, Never apply datums to imaginary features like centrelines. Your hole is a legitimate datum feature as per ASME Y14.5M-1994.
However, our inspector was using the hole - because that was the origin for all the basics.
Essentially, it amounted to a shift in that axis; nesting to the right against the current specified datum
We are currently inspecting/nesting per the Datum -C- as is.
In that reference, the tertiary datum feature is a planar feature specifically set at a basic angle to the other datum features, whereas here the datum feature is an arc.
Either way, the point of the Tec-Ease graphic was to show that the tertiary datum (regardless of actual datum feature orientation to the primary and secondary datums) is always mutually perpendicular to the primary and secondary datums.
A model called TTRS for technologically and topologically related surfaces, has been developed and its application to dimensioning and tolerancing is presented here.
According to this model, any part can be represented as a succession of binary surfaces associations forming a tree.