Using the all-around symbol to specify a Datum

[[Je...]]

A few parts I measure have prints that use the all-around symbol to establish datums (as well as apply profile controls). Usually, there is a note on the print that states, "ZERO IMPLIED AT THE CENTER OF GEOMETRY OF DATUM A." Sometimes this mimics the mating relationships of the part, but other times it is done because "that's our standard practice."

Is this appropriate?

Non-standard geometries, those that are not an axis, line or plane, are expensive to inspect. Also, the only way I know of in Calypso to assign freeform surfaces as datums is to use a best-fit alignment. When doing so, are the print's basic dimensions obsolete, since Calypso defines freeform nominals entirely from the CAD?


Here is an example (not proprietary, feel free to use):

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center_of_geometry_datum.pdf

[[Ma...]]

I don't know if this can be by any standards, but i would in this case run one curve around and use bestfit to get base A ( one curve due thickness ).

[[Je...]]

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Thanks, Martin.

I don't have a problem measuring it. What I am wondering is if it's best practice to design a print this way. The reason I ask is because I've been asked to advise on print generation, and this particular issue is one I am uncertain about.

[[Ma...]]

Perhaps this part is fitting somewhere so this would be reasonable.

Anyway - for CMM it is fast to check - for CNC machinist it's simple to make program. It's WIN-WIN situation 😃

[[Je...]]

Martin,

Where would you place the center of the geometry on this part? I see a center of mass, but it does not have a centerpoint or axis.

[[Ma...]]

I think - regardless of text - that after setting BA from known features ( base B,symetry from 2 lines and circle ) run curve around a part - then make Alignment from curves, where you put just XY and rotation Z ( if base B is laying on Z ) that will do it.
Basicaly it is bestfit alignment.

If they were thinking of another point, they they should draw it 😃