A randomly started discussion about the necessity of the PD single solid rule

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wsteffe
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Re: #4828 PartDesign: inverse for pocket

Post by wsteffe »

The important think is to allow the creation of of multiple solids in a body with a single pad (or other similar operation).

I agree that the "body" term could be become a little bit inappropriate because in common speech body is a single solid.
In fact in the same thread of the post I cited before, I was suggesting to rename Body into Part and the current Part would
be an assembly. That would be a natural choice because it would reconcile the FC document structure with Catia structure.
If you export a FC document consisting of a Part containing several bodies into a step file which is then imported in Catia you will
see that the part is imported as an assembly and each Body becomes a Catia Part.

My proposal was not accepted at that time and I am quite sure it will not accepted today. I would anyway prefer a little bit imperfect english terminology than the preclusion of a useful design flow.
GeneFC
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Re: #4828 PartDesign: inverse for pocket

Post by GeneFC »

user1234 wrote: Sun Mar 12, 2023 12:47 pm A Body is always a SchizoBody.
In English a physical "Body" is never a "SchizoBody". It is always a single contiguous object. Of course there are other uses for non-physical concepts such as "the body of opinion".

"Multi-body" or similar is always used for two or more distinct objects.

This is not really related to what Part Design should or should not allow. However, the use of the term "Body" for multiple separate objects would be incorrect in English.

Gene
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adrianinsaval
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Re: #4828 PartDesign: inverse for pocket

Post by adrianinsaval »

wsteffe wrote: Sun Mar 12, 2023 2:25 pm That would be a natural choice because it would reconcile the FC document structure with Catia structure.
there is no reason why mimicking CATIA should be considered "natural"
I would anyway prefer a little bit imperfect english terminology than the preclusion of a useful design flow.
+1
wsteffe
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Re: #4828 PartDesign: inverse for pocket

Post by wsteffe »

adrianinsaval wrote: Sun Mar 12, 2023 4:19 pm there is no reason why mimicking CATIA should be considered "natural"
A said Catia because it is available at the company where I am employed but I am quite sure that you would get the same result with any other commercial CAD.

The real point is that Parts and Bodies are exported from FC as a hierarchy of PRODUCTs. Here are a few line taken from a step file exported from FC:

...
#7 = PRODUCT('CMP_Filter_6_2','CMP_Filter_6_2','',(#8));Y','',(#42));
...
#41 = PRODUCT('DIEL_CAVITY','DIEL_CAVITY','',(#42));
...
#3630 = NEXT_ASSEMBLY_USAGE_OCCURRENCE('92','DIEL_CAVITY','',#5,#39,$);
..

CMP_Filter_6_2 was the name I had assigned to the part
DIEL_CAVITY was the name I had assigned to the body in this part

I do non know all the syntax of the step file format but I am quite sure that the hierarchy of PRODUCTs is a well recognized standard and that all CAD will import the lowest level PRODUCTS as parts and the upper levels which are defined by NEXT_ASSEMBLY_USAGE_OCCURRENCE(...), as assemblies.

The step file may include many other data structures which are defined below the PRODUCT hierarchy.
In example I have seen that a geometric solid is defined by MANIFOLD_SOLID_BREP(..).
While there is not a standard way to export/import these lower level geometrical entities, all CAD systems will agree that PRODUCTs hierarchy correspond with the hierarchy of assemblies/subassemblies/parts.

And I think you will admit that the STEP file format is a well recongnized standard...
wsteffe
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Re: #4828 PartDesign: inverse for pocket

Post by wsteffe »

Just to show that not all CAD systems are purist regarding english terminology. Catia allows to pad multiple solids from a sketch placed in a Body:
test.png
test.png (105.3 KiB) Viewed 1144 times
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FBXL5
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Re: #4828 PartDesign: inverse for pocket

Post by FBXL5 »

wsteffe wrote: Mon Mar 13, 2023 8:38 am Just to show that not all CAD systems are purist regarding english terminology.
Is it because the developers were/are mainly French? :ugeek:
wsteffe
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Re: #4828 PartDesign: inverse for pocket

Post by wsteffe »

FBXL5 wrote: Mon Mar 13, 2023 9:24 am Is it because the developers were/are mainly French?
I do not think so. It was just a pragmatic decision.

If several customers (paying more than 40 k$ each one) are saying that they need to pad more solids from a single sketch I do not think that your answer will be: "Sorry I can't allow that because according to the english dictionary body means .."
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saso
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Re: #4828 PartDesign: inverse for pocket

Post by saso »

The real reason for this is that in Catia the user manages the bodies and their geometry manually in the tree, while most (all?) other parametric cad systems manage them automatically and because of this they need this logic of a single shape relation to the body because this is how the automatic management of them works. While in Catia this decision is given to the user, in a sense this gives the users more freedom and more power but with more responsibility. And yes, we had discussion about this for FreeCAD and we came to the conclusion that we could make the change to allow for multiple solids in a single body and this change was then also implemented in realthunders version.
wsteffe
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Re: #4828 PartDesign: inverse for pocket

Post by wsteffe »

saso wrote: Mon Mar 13, 2023 10:49 am And yes, we had discussion about this for FreeCAD and we came to the conclusion that we could make the change to allow for multiple solids in a single body and this change was then also implemented in realthunders version.
And what about porting it also to Master ? Is it planned ?
chrisb
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Re: #4828 PartDesign: inverse for pocket

Post by chrisb »

I will not follow you into the Land of Guessing. And I'm afraid that you miss the point, if you think it's only a matter of linguistic details. A body and a Multibody are different concepts, especially when multiple solids in a body can describe an intermediate state.
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