Coordinate system in MBD 1

[Woosang]

Years ago, when my customer adopted MBD, its MBD construction guideline said that each MBD shall have a coordinate system and a datum reference frame as I recall, which sometimes made those useless for simple parts which do not need datum.

ASME Y14.41 Digital Product Definition Data Practice also says that a model shall have one or more coordinate system which corresponds to datum reference frame as paragraphs below (2012 version).

5.1.3 Coordinate Systems
A model shall contain one or more coordinate systems.
11.2.1 Datum Reference Frames and Coordinate Systems
The following requirements apply to the relationship between the datum reference frames on the model and the coordinate systems.
(a) Datum Reference Frame and Coordinate System Correspondence. Each datum reference frame shall be associated to a corresponding coordinate system.

I wonder why do they state such requirement, even when a product definition does not require any datum reference.

 

[greenimi]

Ewh,
I don't (knowledge on MBD too much). That's why I came here.

 

[Burunduk]

greenimi,
ewh's question was addressed to 3DDave.

 

[3DDave]

We never had software that would handle the cradle to grave job - that is a full integrated stack including suppliers like Detroit's Tier 1, Tier 2, where we could dictate what software the suppliers and internal users used.

We also never had a customer who could take delivery solely of such models.

Even so I built all my models so all the geometric data and feature controls were part of the model and were "shown" on the drawing rather than being created in the drawing mode. This was key for using the VSA tolerance analysis software as it worked directly from the model information.

There were large gaps in the software - I don't recall any means of recording finish requirements in a model so that one could select a surface and have it tell the class of conversion coat, the primer, the allowable range of primer thickness, or the top coat and range of thickness and so forth or select the part marking and it reports the epoxy ink color and spec. Recording those in the model were on the big list I tried to get ComputerVision to implement, especially the vexatious paint masking requirements.

Lucky me and primarily government contracts meant the required deliverables to them was mostly PDFs; the USPS also wanted the native files, the PDFs, and DWG files because they used AutoCAD for everything. Suppliers most always wanted a plain STEP file. Can't do MBD if no one wants MBD and the software doesn't support all of the required characteristics to run full factory planning from the model.

On a large job for the IDF corporate had bid on a build-to-print with design responsibility with an Israeli company. Unfortunately the Israeli company had an on site factory crew who figured out how to build things and so no welding information was on the drawings. What was worse is build-to-print meant no budget was allowed to engineering so our manufacturing did what the Israeli company did and let the welders do whatever they wanted and documented none of it; this is a problem when TACOM, the stand-in for the IDF, wants to ensure the production units are just like the qualifying prototype. Can't make them the same if no one knows what that should be.

So I got the job of documenting the 20-30 pounds of weld material - all represented in the assembly model to show the expected extent of the weld. It also uncovered a bunch of areas the Israeli model diverged from the Israeli drawing. They used UG, so we imported STEP. Likewise they did not include materials; apparently there was a misunderstanding and they had the idea that the material spec (ASTM) specified material, not knowing that some ASTMs cover dozens of alloys, from soft to nearly armor plate, but they failed to record what specific ones were used on the prototype.

At the end each part had a model material assigned, the correct ASTM spec, and the weldment had welds modeled at the correct size an in the order the welds were required. The user for that was drafting as the IDF wanted PDFs and the drafters were to add annotation to the drawings to match the depicted welds.

The stress group did sometimes use the native models, but for many they too would STEP out the geometry, cut back features, simplify shells to midplanes.

On the flip side; I've never seen a factory model that includes a statistical model of ability to conform to requirements. Like, for CNC mill XYZ what is the distribution for variation from true position for typical hole diameter statistical bins, and what is the composite range across the factory? Not sure there is software for the purpose of pre-qualifying a factory to produce a part.

So, worked very hard in that direction, but procurement, the factory, and the customer all had separate leadership and couldn't or wouldn't use MBD, just STEP extraction for fabrication/CNC toolpath.

 

[ewh]

Yes, there are holes, some big ones, and until they are successfully addressed MBD won't be widely adopted. But it is not going away...
In reference to the OPs original post, I have had experience with a "proof-of-concept" full MBD project for a major aerospace supplier and this csys requirement was originally written in but was not fully met due to software limitations at that time. All individual datum symbols and FCFs were associated to their respective datum plane/surfaces, but not the individual frames of the FCFs. In the case to threads as datums, a cylindrical surface had to be created to represent the required datum feature.
AFAIK the program was a success in identifying issues and advantages (beyond the obvious software/supplier issue) of such an approach and provided a solid basis for company procedures in future projects.
Contract ended with program end.
[edit: formatting]

"Know the rules well, so you can break them effectively."
-Dalai Lama XIV

 

[Burunduk]

All CAD systems already have coordinate systems. The initial CSYS is not optional and requires no input from the user to create; they are intrinsic. As soon as the NEW button is clicked and the name entered and then OK - that is when the first CSYS is created for the item. Can't avoid it, cannot delete it. Any measurement done at that point is done in that coordinate system.

When someone begins a long post about his expertise at something with a mistake that indicates lack of sufficient knowledge in that field, you know the rest is worth little attention.

 

[3DDave]

I've read the data structure for CADDS and Pro/E (before they encrypted it because Solidworks used the same data structure) and the coordinates in them did not require a CSYS feature to exist. IGES doesn't require a CSYS and neither does dxf.

Bravo! did not have a CSYS feature, but it certainly had a coordinate system.

What does one suppose happens on import of an IGES file that has no CSYS defined into an empty part?
Where in model space does that model go?

When a CSYS is created without any references where in model space does it go? Hint - the internal data structures all have X,Y, Z components - the intrinsic coordinate system.

Perhaps there is a CAD system without that?

 

[Burunduk]

The requirement in Y14.41 which was mentioned by the OP is for a coordinate system feature, and that is the topic. Not the intrinsic coordinates within the data structure of the file.
A coordinate system feature has uses in MBD. Another example of such use is tolerance direction associativity. It can provide a visual response of an axis when a straightness feature control frame controlling a planar surface is clicked (an alternative to visually represented line elements).

 

[Woosang]

First I would like to thank you everyone to get interested in this post.

Then I also have to say sorry that I have left my question unclear and made everyone confused. Due to different time zone, and the urgent work / personnel matters, I missed leaving my response in the right time.

Woosang,
What do you mean by "THEY" ?
Who are THEY?

Quote (OP)
....why do they state such requirement.....

I meant "they" by both my customer and Y14.41. I thought they required same thing, which is : Each model must have a DRF (regardless of whether it is practically valuable or necessary), and a coordinate system corresponding to that DRF.

The logic in 14.41 is, IF there is a DRF THEN it must have a matching coordinate system; the inverse is not required.

My understanding per above 3DDave's comment and Burunduk's other comment is : When a model defines a DRF, coordinate system corresponding to that DRF must be constructed, besides the intrinsic absolute axis in the model. If there is no datum necessary for a part, then no DRF, hence no local coordinate system is needed. That is what Y14.41 states.

Then my mentioned customer's guideline differs from Y14.41, which need more investigation and consultation from my side.

 

[drawoh]

Better question, why create a CSYS at all for any DRF? All datum features would already be identified/labeled

We need to agree on how the part gets fixtured before we inspect it. MBD is inspectable if you have defined datum features, and inspection fixtures and/or a CMM.

--
JHG