Hi there..

i've got some problems with the attached object. Actually it's just a poly-something representing a gem. There might be something wrong with the curves or surfaces, because after shelling, i will end up with a solid-look-a-like, but actually separated into single surfaces...thats not the normal shelling-behaviour, is it?

maybe someone could give me a piece of advice on this issue

I'm on moi 2beta..

cheers,
marco

This is what I got and is as I would have expected?
Were you expecting something different?

Brian

Hi Marco,

The surfaces of the model appear to be joined OK, but MoI is struggling with a shell and leaving unjoined surfaces.
You can select the object and then select planer which will close and solidify the object, then make shell on the planer surface, but that appears to be limited to around 3 thickness.

One for Michael to look at.

Hi Brian,

> This is what I got and is as I would have expected?
> Were you expecting something different?

If you look closely (circled in your picture) after shelling it doesn't produce a nice solid piece the inside surfaces are overlapping.




Hi Steve,
I tried the planar surface at first, as well, but I could only get it to shell at 0.4, how did you manage 3 ?

Cheers

thanks guys..

actually, the shelling would be just around 2mm ..but i had major troubles when i exported the part via STL. the guys from the rapid prototyping company said, that the data was somehow corrupted or unuseable for them (missing surfaces)...the original surface itself seems to be ok..pitty there is nothing like a "whats wrong here"-button in MOI..

cheers,
marco

Hi Danny,

>>I tried the planar surface at first, as well, but I could only get it to shell at 0.4, how did you manage 3 ?


I just used planer then shell. Have attached file, this is with a 3.3 normal shell.


- Steve

Hi,

>>but i had major troubles when i exported the part via STL

STL??

I shelled your object to normal 2. Then exported to Sat and Step. Sat did give some model errors when importing into another CAD, but that was due to limits and where easily repaired. Step had no problems.


- Steve

hi..

export as a STL...ups..sorry..stereolithography

usually the rapidprotos favour this 3dformat..at least in our part of the world..muahmuah

best wishes,
marco

Hi Marco,

>>export as a STL...ups..sorry..stereolithography

Sorry,.. I do know what STL is, it is just I did not notice it in the export options (not something I have ever needed to use). I have now found my glasses.

I have just exported using STL and seeing no problems.

I have attached the STL output from the model you posted (with a normal 2 shell)


- Steve

Hi Marco, it is actually a lot more difficult to calculate the shell on such things that you might think at first. MoI's sheller is not very advanced, it just does not have the facilities inside of it to calculate an offset where the result has a different vertex structure than the starting shape.

Here is a bit of an explanation for what I mean. Say you take just these 5 separated surfaces:



Then if I offset them by 10 units as individual surfaces, I will get this:



Here is another view:



Note how there is not an equal amount of spacing in the gaps between each piece - that means that these pieces do not meet up in a single point even though the original surfaces met at a single vertex point.

If I untrim all of these surfaces to get their underlying planes, and look at it from the underside, you can see this:



So this is the kind of structure that would need to be created in this case:



I will take a look at it and see if I am missing something in using the shell function that would handle this kind of "deviating vertex" type situation, but I think that you may need to use a different program that has a more advanced shelling function than MoI for doing this.

If you offset by only a very small amount, then those 2 deviating vertices that I showed above do not get separated by very much distance, and if it is not too big of a distance that is not too much of an error to have them represented by just one vertex, that is why using a smaller distance can produce an ok result, although probably the one that you had problems doing STL with had too big of a gap between some of the trim boundaries and the vertex.

- Michael

hi michael,

thanks for this detailed explanation!!..

cheers&thanks again,
Marco

ps: MOI is really saving by butt here at our design department.

Hi Marco, I looked into it some more, and the problem does seem to be that the geometry library that I'm using currently just assumes that the final result of the shell will have just one offset vertex to match each vertex of the original object.

That is the case for 3-sided corners like the corner of a box, or for things that are totally symmetrical but not for cases where many pieces of different angles touch at a single point like in your case - like I showed earlier in this case there should be 3 vertices created in that central area instead of one, even though the original shape has just one vertex in the center.

I have sent this example to the people who make the geometry library that I am using.

Hopefully they will be able to improve it to handle this case as well, I'll let you know what I hear back from them.

- Michael

ioco

I saved the 3dm file from MoI as an .obj file and rendered in Carrara.

Is there, visually, a problem enough to be, in most circumstance, seriously concerned?

Brian

Hi Brian. I guess it depends on who makes the final judgement of what is acceptable or not. In your render, those interior surfaces are reflecting light. Paying clients would probably not accept it. Especially in the jewelry industry where beauty is found in the small details. Everything must be perfect...

jonah

I think Marco mentioned exporting as an STL for rapid prototyping?

That definitely needs a well formed mesh - the rapid prototyping machines form slices from the mesh, it tends to need a different set of stuff than what can maybe be acceptable for a rendering...

- Michael

Hi Marco, if you want to shell to the inside then one way to get it done is to use Edit/Separate to break your object into individual surfaces, then shell all of those to produce a bunch of individual solids, then boolean union those all together, then a bit of trimming off the bottom part and then a construct/Planar on the open edges and you should have a valid shape.

That's what I did to get the attached version - this one should now generate a proper STL file for you.

Let me know if you need more details on these steps or if you just need it of a different thickness let me know and I will make that for you.

- Michael

buh..hi everyone..

8am local time and almost 6000 replies to cover..;]

i had no troubles whatsoever getting this poly-thing into Hypershot or into Metasequoia (thats quite an odd workflow..:]]..but Metaseq comes in handy for fast poly modeling and than maybe a quick clay-rendering in Parthenon with GI), but as - in this case - for the RPs(rapidprot..) i really needed "hart facts" as in IGES, STL or SAT.

the part is actually already in production but because of a rather tight time schedule, we had to give it to a "traditional" model maker, because none of the RPs were actually capable of producing these parts(there are more of them) in time and quality.. i will post some pictures of the final object(s) next week..done in PMMA

thanks to all the helping hands&best greetings,
marco

btw. are there any plans for the future to support the DXF-format natively?..at least as an import-feature..i spotted the 3dmCurves2DXF, but that just covers the output..

Hi Steve

> I just used planer then shell. Have attached file, this is with a 3.3 normal shell.

PHEW! you mean millimeters, the original file was in cm so I was quoting .4cm

Thanks for that.

Why use shelling? So no problem :)
One 3D "Scale" + "Planar" of the base and you have a perfect gem :)
Thickness as you want
Of course the "original" must perfect before the Scale ;)
The trick is draw an helpers line between larger extremities and Scale is picking from the middle of this line to the end

Hi Pilou - scaling is certainly an easy way to get a result, but it will give you a different shape than shelling.

Shelling uses an offset calculation, which creates a shape that has the same thickness everywhere.

The scaling result will create a shape that has a varying thickness to it, except in a few special cases like for a circle or a sphere.

Here is a bit of an exaggerated example to illustrate.

Say you have this shape:



If you use an Offset on it, it will create a shape with a constant thickness:



Notice how the proportions of the shape have changed in the offset? It is not just a scaled copy of the original.

Scaling only would do something like this:




For shapes that are absolutely symmetrical about a center point like a sphere or circle, then that is a special case where scaling and offsetting are equivalent, but for the general case they are not the same thing.

- Michael