Stumped on GD&T Position Holes Multiple Bends/Planes 1

[Kurtanius21]

I'm having a lot of trouble determining if the below Position callout is legitimate for or not. I've studied a lot of example positions, but there's not much literature out there for referencing datums not immediately adjacent to the position callout.

Almost all hole position callouts I've seen, the primary datum is perpendicular to the cylindrical tolerance zone. In my case, I've got a primary datum that doesn't contact the hole's cylindrical tolerance zone and is not perpendicular to it either.

I'm torn between understanding that the primary datum should be perpendicular to a hole's axis or datums can just be pictured as how you attach the part to gauges.

The position in question is the red question mark. Theoretically all degrees of freedom are covered for using datums A, B, and C. I'm picturing a flat gauge with two pins sticking out to lock the part in place. But the gauge isn't 'touching' the hole. So is that question mark position callout correct? If it is, can someone explain how it works? What is the meaning behind the datums relationship and position? (Ignore that no basic dimensions locate the holes)

 

[Garland23]

There's no rule saying that the primary datum must be perpendicular to a position tolerance zone. Datums are simply about constraining degrees of freedom, so that seems to be correct here.
My comment would be that datum C isn't needed. You've already got a pair of holes tagged as datum B, so all 6 deg. of freedom are taken care of by datums A and B.

 

[Burunduk]

The datum feature selection should usually correspond to how the part is placed in the assembly the part is going to. If it's positioned for inspection similar to the way it connects in the application, then features that are measured good will also be located and function well in the application.

You said "I'm picturing a flat gauge with two pins sticking out to lock the part in place" and that is right. So if the primary mounting feature for the assembly is also your datum feature A and the part is located and locked in rotation by two pins that stick out from the base this surface contacts, this datum choice makes sense. The two holes in question, which you are controlling relative to A, then B, then what seems to be a redundant (as mentioned by Garland) C, could be used for attaching another component, which needs to be at a certain height from the base and oriented properly.

 

[Kurtanius21]

Thank you so much Burunduk and Garland23. I figured my understanding about fixturing was correct. I'm also aware that when it comes to GD&T, design intent is critical.

Thanks for the clarification that the primary datum does not need to be perpendicular to the tolerance zone. Helpful to know that. Plus, it's great to see why a third datum here is not needed. Thanks.

 

[Kurtanius21]

Another question. I've seen position callouts with only a single primary datum plane, Like datum B up above in the image. Does that mean the position of the hole is not controlled relative to any translation in the X or Y direction (x-y plane coplanar with datum A)? This question assumes there are no basic dimensions controlling the hole position.

So wouldn't it be better to change the position tolerance to peprendicularity tolerance? I'm guessing that if a hole has no secondary or tertiary datum, it must be because all over dimensions will be measured in reference to the hole, not the hole measured to walls or edges. Sound right?

 

[ghostdigital]

Without knowing what the target installation assembly looks like - it looks legit. All the holes are constrained and defined. Just bear in mind the the datums selected and the order of precedence is best selected by understanding how the part fits into the target assembly.

 

[Burunduk]

"I'm guessing that if a hole has no secondary or tertiary datum, it must be because all over dimensions will be measured in reference to the hole, not the hole measured to walls or edges. Sound right?"

That is absolutely right. The position tolerance you are asking about only controls the orientation of the holes relative to datum A, and the mutual location between the two holes (the distance between them).

"So wouldn't it be better to change the position tolerance to peprendicularity tolerance?"

Not if the design intent is like you mentioned - "all over dimensions will be measured in reference to the hole(s?), not the hole(s?) measured to walls or edges".

 

[Kurtanius21]

"I'm guessing that if a hole has no secondary or tertiary datum, it must be because all over dimensions will be measured in reference to the hole, not the hole measured to walls or edges. Sound right?"

That is absolutely right. The position tolerance you are asking about only controls the orientation of the holes relative to datum A, and the mutual location between the two holes (the distance between them).

"So wouldn't it be better to change the position tolerance to peprendicularity tolerance?"

Not if the design intent is like you mentioned - "all over dimensions will be measured in reference to the hole(s?), not the hole(s?) measured to walls or edges".

Thanks. Another area of confusion is how to apply linear dimensions to a datum axis? Say datum axis B above now has a linear dimension to the right edge. How can I have a linear dimension or ordinate dimension to an invisible datum axis? It doesn't seem to make much sense to me.

 

[Burunduk]

There is no fundamental issue with a linear dimension given from a theoretical datum axis. Remember that the theoretical datum axis is the center of a pin located in the measuring fixture. Such a dimension is useful if it is a basic dimension. For example, if it locates the center of a profile tolerance zone for a peripheral surface. Such a profile tolerance would reference the datum.

 

[jassco]

Hi, Kurtanius21:

You are confused because you said "datum axis B". Do you understand degrees of freedom (DOF) that datum feature B restraints? Datum feature B consists of an axis and a plane. You can locate any features to the axis anyway you want.

Best regards,

Alex