Full Version: Shelling

When I try to shell this pot so that the top is open, a thickness of 0.5mm fails, 1.0mm works, 1.5mm fails, 2mm fails, 2.5mm fails. It seems only 1mm works. Why? Note that the text is not part of the pot so the shell is only working on a simple shape.,

Thanks, Andy

Attachments:
Test3.3dm

Image Attachments:
Untitled.png 

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Hi Andy, over here both 0.5 and 1.0 are working ok. You can't go much larger than 1.4 because the fillet on the bottom will be folded over top of itself, see here at 1.4 how small the fillet has become:



If you need to shell it to a larger thickness it would be good to put the bottom fillet on after the shell rather than before.

- Michael

Image Attachments:
andy_shell.jpg 

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In 2D MoI can correct calculate what happens with offsets and a fillet - why not 3D?

Andy

Image Attachments:
Untitled.png 

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Hi Andy, unfortunately the calculation in 3D to determine collapsing and self intersecting areas is far far more complicated than it is in the 2D case.

- Michael

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OK, so your reasoning is that because it is complicated it won't happen in MoI?

Alibre Design has no problem with it - see screenshot. Because Alibre Design is parametric it's less of an issue there than in MoI because I could just swap the order of operations around. With MoI not being parametric (understandable) it becomes more important IMO for the application to do what the user expects as it may be very difficult to undo multiple steps and change the order of things (e.g. shell before fillet).

An added complexity for MoI is that it forces the user to consider the order of things depending on the values used, for example:

Wall thickness = 3mm, fillet = 5mm : do fillet then shell
Wall thickness = 6mm, fillet = 5mm : do shell then fillet

Andy

Image Attachments:
Untitled.png 

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Andy -

Just my $0.02 worth of experience:

I try to avoid fillets on simple shapes because yes, they are difficult to un-do. Instead I'll first try to build the fillet into the construction curves.

In the case of the capped, shelled cylinder, you can draw a 2D profile curve, apply the 2D fillet, and Rail Revolve into a cylinder having a thick wall. The advantage is you can simply move a couple of points and re-do the Revolve to get a new fillet dimension.

I also tend to save making fillets to the very end of the model creation so I don't paint myself into a corner.

Another thing I do on more complex projects is take a copy of the objects when they are in a less developed condition and paste them off to one side (or into another instance of MoI) as backup to an earlier revision.

Ed Ferguson

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Thanks Ed - good suggestions. Andy

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Hi Andy,

re:
> OK, so your reasoning is that because it is complicated it won't happen in MoI?

No it's simply an explanation for why it has not happened _yet_ in MoI.


> Alibre Design has no problem with it - see screenshot. <...>

Yes Alibre Design uses a geometry library called ACIS which has had something like 50 more man-years of effort put in to developing the shell and fillet geometry processing than the library that MoI uses.

I would recommend using Alibre to do your shelling instead of MoI, that way you won't have to worry about the ordering.

Shelling in MoI is pretty primtiive. When I started MoI I had hoped that the shell and fillet functions in the geometry library that I used would improve over time but the authors of the library have gone in a different direction than that.

I would like to license a different library to better handle those areas. That requires a lot of work as well though even just to investigate it.


> it becomes more important IMO for the application to do what the user expects

I totally agree! Unfortunately it will require a lot of work to make this area behave like the user expects. I have not managed to scrounge up an extra 50 years of work time to make it happen as of yet.

- Michael

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