Identification of primary datum features

[waqasmalik]

Hi all, please look at the attached figure.
Its an assembly having two parts;crankshaft and coupling.Both have planer faces as primary datum features and diameters as secondary datum features.
My question is that what difference will it make if i do the reverse. If i take diameters as primary datum features and planer faces as secondary datum features and give a perpendicularity tolerance of 0.01( equal to flatness value in given example) for planer features with respect to diameters.?Will it mean same as original callout?
Thankyou

 

[3DDave]

It's an interesting question. Why do you ask?

It make little sense to change the order. The diameters are relatively short and incapable of orienting the two parts as long as the flanges are there. Without the flanges, the FCF order can't be inverted.

 

[CheckerHater]

From ASME Y14.5-2009: Para 4.8 DATUM FEATURES:

“A datum feature is selected on the basis of its functional relationship to the toleranced feature and the requirements of the design…”
Well, design requires two parts to fit together.
Further in same paragraph: “To ensure proper assembly, corresponding interfacing features of mating parts should be selected as datum features.”
We have flat surfaces that are supposed to lean against each other first (PRIMARY requirement) and two cylindrical surfaces to center parts to each other afterwards (SECONDARY requirement).
Further in same paragraph: ”However, a datum feature should be accessible on the part and of sufficient size to permit its use.”
Lucky us! Our flat surface just has sufficient size and is accessible, so we pick it as our primary datum feature.
Problem solved.

 

[gabimo]

On linkedin someone asked almost the same question awhile ago and below is Mark Foster's answer (GD and T expert from Applied Geometrics, Inc)

Begining of Mark's post:
"Whenever you begin a question with, "CAN..." the answer is always, "Yes."

Now, if you are asking if it is a good idea to do so, then some subjectivity enters the equation.

I don't like to use the words, "always" or "never" unless I really mean it. So I would not say that one could "never" have a primary datum feature at MMC(MMB) that was a "short" cylinder. I would just say that it is highly unlikely that the part actually functions that way, and so yes, in general, if the designer did so, he was probably just wishing (or imagining) that the part would work that way.

Of course, ultimately (one could argue), if there is enough clearance between the "primary" cylinder and the mating part, there would be no difference between the cylinder primary (at MMC/MMB) and surface secondary or vice versa because you would be giving back enough mobility in the primary for the secondary to "act" like the primary anyway. But why rely on that happening? Why not just call it out the way that it *is*? That would be my point." End of Mark's post