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Inclined features as datum (the setting of expert parameters)


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I have encountered several  parts with inclined feature as datum and I have a question regarding it.

I've read one of the articles in this forum and found GOM seems not able to construct coordinate datum system by inclined features.

However, the article was written a year ago and I'm able to create those datum systems so I suppose GOM is able to deal with it already.

Below is my question:

A-B line is the primary datum.

Plane C is the secondary datum.

Plane D is the tertiary datum (incline to the primary and secondary datum).




While creating the datum system, there are 2 options (GD&T and Geometry) available for computation mode.

And these 2 different computation mode result into different datum reference frame.




Datum D controls the orientation of above datum system, and if I chose “geometry”, datum system A-B|C|D is as below.

“Line CD” is the intersection line of datum C and D.

The orientation of datum system A-B|C|D is in the same direction (parallel to) “Line CD”.

It makes sense to me and at least I understand how the DRF is being created.image.png.65e3c8a35ce0cc71141905245bdff684.png

However, if computation mode “GD&T” was chose, datum system A-B|C|D seems to be rotated as below.



I’ve checked the definition of inclined feature as datum in ISO5459 and I don’t think it is clear to me.

Does anyone know how the orientation of this DRF being defined?

I think it might be related to the angle between the tertiary (inclined feature) datum and datum system A-B|C but I can’t find the correct answer so far.


Please let me know if anyone known what the answer is, or share your idea if there is any.




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Nope, during computation the elements are rotated (only for datum system calculation) so that they fulfill the parallelism or perpendicularity constraints that needed from the GOM software. In short GOM software expects TEDs that are 0 or 90 degrees to each other. If your current elements have another angle, GOM software assumes one these TEDs (depending which one are the smaller angle).


Of course we are planing to support inclined features someday in the future.


Hope this helps.

Christoph Schult

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Thanks for reply.

I understand GOM is not supporting inclined features as datum now.

But since the datum system (with inclined feature) is able to be created, do you suggest the users use it accordingly?

or we should just ignore the created datum system?

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I'm still curious about the difference between the computation mode "Geometry" and " GD&T".

Do you know what method (in mathematic way) while choosing computation mode "GD&T" that GOM is using currently?


Reina Chien

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you are correct, at the moment we expect from the customer that he doesn't do things that are not supported. So you could ask the question what are the reason that we still allowing to insert such elements inside the datum system dialog.

There are two reasons for this:

  • Actual datum systems only (i.e. you have no cad); here it is impossible to distinguish between inclined features due to production irregularities and inclined features by purpose 
  • The same is true for sloppy defined CAD parts, i.e. CADs that are not construct correctly (e.g. instead of having an degree of 0° they have something around 1, 2 or more degrees).

Depending you last question if you are choosing "GD&T" we are doing the things that are described inside the ISO 5459:2011 using the selection of the fitting elements as the input data for your datum feature.


For your example given in your original post (assuming that your A-B line is point direction line)

1) A-B line is used as given defining a direction of the datum system and setting the origin to this line

2) Datum C is a plane perpendicular to your primary datum, moved so that is touches the selection data from the outside. Now the origin for your datum system is set but a degree of freedom is still open (the rotation around your primary datum axis)

3) Datum D is the direction of the chebyshev plane, parallel (due to the fact that inclinded datums are not supported) to the primary datum fixing the last orientation degree of freedom.


Best regards

Christoph Schult

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