Is there a modifier to make an ISO Datum be defined like an ASME Datum? 2

[TopPocket]

I work in ISO but am making a part I wish to inspect with a gauge.

I want to describe the primary datum plane, and I want it to match the simulated datum plane of the gauge. So the granite table top so to speak.

However, I believe that in ISO a datum would be defined by the derived median plane, which would not be the same thing.

I can't use datum targets as they could change position with normal part variation.

Is there a modifier for a datum to indicate I wish the datum to be derived from the simulated datum feature? Something in the same vein as the envelope modifier perhaps; turning ISO to ASME? Almost like a Tangent modifier, but for datums?

 

[CheckerHater]

I am quite sure you need no modifier to derive your datum from flat surface.

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[Garland23]

TopPocket, I don't think ISO defines a datum in that way (derived median plane). If a datum is meant to imitate a mating surface/feature, why would it even make sense to have such a datum?
The only document I have in front of me right now is the older ISO 5459 about datums, but it clearly shows the datum as a tangent plane to the physical datum feature.
Perhaps you are thinking about a datum created from a feature of size? Then it would be the derived median plane.

 

[cr7]

I think there is in the ongoing 5459 draft, adding [G] .
We have to wait until release.

 

[TopPocket]

Yes, I think you're right, got the wrong idea there for some reason.

My part is a little outside of the descriptions in 8888 being a rod bent into an S shape, but I think if you squint real hard, it's sort of a plane, and the spirit is there regardless of the feature.

 

[Germermaman]

The way a datum system is made in ASME vs. ISO is extremely similar.

One difference is in fact how a plane is used. The latest ASME standard will use a so-called L1 algorithm as a default, which is basically a filtered Gaussian plane extended to one or more highest points. The older ASME standards allowed different approaches as well, but the default was a complicated algorithm to find a stable plane from a candidate set of planes. That was not very repeatable, so the default is now the L1 algorithm, which mimics a plane on a granite table that has been leveled with shims to make it stable (since there are so many cases where there are no stable planes with exactly three high points).

The approach in ISO is almost the same, except a filtered Chebycheff plane extended to one or more highest points will be used. In most cases, the difference is absolutely negligible.

The rest of the datum system build is almost the same, only the wording is different. While ASME speaks of a datum simulator, ISO reduces the datum elements to so-called situation elements, which work exactly the same.

Only a few minor details are different. For example, in ASME, datums are fixed in orientation and location (to each other), in ISO they are only fixed in orientation (to each other).