GD&T Theory Question

[JacobCheverie]

I have attached a stripped down print of a part - A cube with two holes in it. Top face is datum A, Center hole is datum B, Side face is datum C.

The question that I have comes down to the theory behind the dimensioning I guess. I have listed three datum reference frames for a Positional tolerance.

1. [A|B(M)|C]
2. [A|B(M)|C|>]
3. [A|C|B(M)]

In #1, datum B constrains translation in the plane of A. The MMB modifier allows datum displacement upon departure from MMC. My question is this - is "shift" allowed in the direction of the basic dimensions indicated while having datum C as tertiary? Would we need to add the translation modifier as is seen in #2? Or would the better approach be #3, where we use C to constrain translation in the direction of the basic and only allow the small hole to be controlled to B in one direction?

Functionally, C will constrain rotation about the axis of A if used as a tertiary datum, so how would that work in case #1 if the part shifts from datum C simulator? Would the datum displacement be ignored while making constant contact with C? That would override the can-may-must rule.

Any thoughts are welcome and appreciated.
Block.JPG

[Matthew D. Hoedeman]

CAN ~ MAY ~ MUST

#1 "A" does 2 rotational and 1 directional degree of freedom. "B" does 2 directional degrees of freedom, leaving C to only control 1 degree of rotation. All 6 DOF taken care of. You can shift in the direction of "C", so you can shift in X & Y (if "A" is "Z")

#3 "A" does 2 rotational and 1 directional degree of freedom. "C" does 1 rotational and 1 directional degree of freedom. Leaving "B" to only control 1 directional degree of freedom. In this case, you need a tolerance for "B" as "C" locked a directional degree of freedom. You can not shift in the direction of "C"

[Matthew D. Hoedeman]

yes, it is valid. Guessing from the print, they want the smaller hole dimensioned to both the "B" (as master) and to "C" (as master). 2 different dimensions being used to control 3 features as a stackup, more or less. If the small hole to the "B" (as translation master) is within +/-0.001 (which is what it boils down to) and if the small hole to the "C" (as translation master) is within +/-0.001 (which is what it boils down to) then "C" is within +/-0.002" to "B", which they COULD have called out instead of doing it the way they did. ("C" to "B", profile 0.004)

[JacobCheverie]

This isn't actually a print that somebody drew - this is a basic print that I made up. The three Position callouts are not being applied simultaneously. I'm trying to determine if callout #1 is valid on it's own. If not, does #2 make it valid?

In callout #1, the MMB on datum B could cause a departure from the surface of datum C. Is that allowed? Must the part always keep in contact with it's datum feature simulator?

[Matthew D. Hoedeman]

It looks good to me, since in #1, "C" only constrains rotation, it has no 'location'. Good to go
#2 is also good to go.

#1 and #2 will give different results, but both are valid callouts.

I think #2 and #3 will give the same results.