Common Datums 1

[AMontembeault]

We're having a bit of a debate here at work.

Suppose I had a piece of plate with 2 holes in it. Each hole is specified as a datum (in this case, datums B and C, where datum A is the large flat face of the plate).

I have another feature with a positional tolerance with respect to A and a common datum defined by B and C (B-C).

The question we're debating is if B-C defines 1 datum wherein the axis of both B and C must lie (call it a horizontal datum), 1 datum between the axes of B and C at an implied center(call it vertical), or actually 2 datums orthagonal to eachother (controls both the horizontal and vertical datums).

Previously, we've been operating under the assumption that it only defines one plane, however reccently I had a conversation with an outside source that claimed that because the datum simulator would basically be 2 pins, it in fact defines 2 datum planes. Unfortunately, I cannot find an example in either the ASME GD+T standard or the ISO standard which can confirm or deny that either way is factual - almost every example I've seen only relates to 2 planar surfaces or 2 nominally coaxial surfaces, not 2 holes at some distance apart. Could anyone shed some light on this, or perhaps show me a published example?

Thank you

 

[CheckerHater]

Then, yes, Evan pretty much covered it - something legal, but not very useful.

On the other hand, if we use datum targets, we could grab the part in more "gentle" manner, possibly giving it freedom to get in contact with MMB simulator.

weavedreamer, do you have any particular part in mind?

 

[weavedreamer]

The part is a cylindrical can which slides over an assembly until it presses onto a diametrical feature of the asembly which holds it in place.

Hopefully this description is as helpful as the confirmations provided in the responses.

Thanks.

 

[Tarator]

I do have a question regarding that example from Tec-Ease site where the part has 2 datum features on both ends, Datum axis A and Datum axis B. I understand that we need to find the compound axis A-B by shrinking the collets on both ends, and we find the A-B... My question is how do we measure the total runout on both ends? Aren't the both ends inside the collets? How can you measure that?

 

[pmarc]

My question is how do we measure the total runout on both ends? Aren't the both ends inside the collets? How can you measure that?

I think this was discussed here a couple of times before. There are at least 3 different workarounds:
1. Instead of chucking-up both ends you can use CMM and analyze the collected data.
2. Use a pair of V-blocks to simulate datum features A and B. This will be decent approach as long as one is pretty much confident that the datum features are not procuded with severe form error.
3. Inspect runout of both datum features with respect to any other axis, and not to the datum axis defined on the drawing. If the sum of the readings is not greater than the runout value defined on the drawing, the relationship between datum features is good. Machine centers, for example, are good features to use.

 

[Tarator]

thanks pmarc!