Hi,

did a double-moebius strip last night and had a weird problem with the shell command. (Idea from Rinus Roelofs - http://www.rinusroelofs.nl/rhinoceros/rhinoceros-m06.html ).





You see the original joined surface and my first attempt to shell it, using the centerline option. The surface was made last night so I can't tell you what steps I took to join it. But in the right example I've used the separate command and then the join command and then the shelling works fine.

Any thoughts of what went wrong?

/Lars

Just noticed that something similar happens when I try to Boolean union the shelled object. Half the object is ok, but the other half is not.

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The more crazzy prog in this topogic domain! And i's free!
http://www.topmod3d.org/

Hi renklint,

I did take a look at the tutorial: http://www.rhino3d.nl/tutor/tu02.html
of Rinus Roelofs

He used Loft to create two halves and joined them

Hi Lars, it's pretty hard to see what might have gone wrong just from looking at an image... Can you please post the .3dm model file of the part that did not work right or send it to me at moi@moi3d.com ? That will let me take a look at the actual model shapes and see if I can give any information.

From looking at your image, it kind of looks like your original shape was a single surface, which shouldn't normally do anything different with Separate or Join on it since it is just a single piece, unless you've actually got 2 surfaces there stacked up on top of each other, which if so would be the problem.

- Michael

Paolo - cool, I don't understand dutch, but I'll shure have a look at the tutorial.

Michael - it's originally two surfaces, but in the attached file they're joined together.

Hi Lars - I see I didn't notice the other break line in your screenshot there.

But it looks like the problem still might be stacked up / duplicated surfaces.

If you use Edit/Separate on that object, and then drag some of the pieces away you'll see that there are actually 4 surfaces in there, not just 2.


Then additionally, after you shell there is a kind of internal wall created in this area:



Use Edit/Separate to break the shell into individual surfaces, then hide the exterior ones and you can see the unwanted interior wall:




And there are actually stacked up pieces on that internal wall there too. I'm not sure yet but this may indicate some kind of loop in the trim curves in that seam area, that may be another problem. I'll take a bit more of a look. Because even deleting those internal wall pieces does not allow the exterior pieces to be joined into a fully closed solid.

- Michael

Hi Lars, it looks like an additional problem is that the surfaces don't seem to be completely smooth to one another which complicates shelling.

Probably when you create the initial surfaces it might be a good idea to sort of extend their shapes a bit further past the current ends, then cut them back to their final shape. That could help to ensure a more compatible and smooth connection right where the pieces touch each other.

There seems to be a bug in here somewhere with this kind of moebius topology, even when split into 2 pieces.

I'm going to take a look at fixing that up, but in the meantime probably the easiest way to produce a shell is to shell each surface individually instead of joining them. Then you will have a bunch of pieces that you can work with.

You'll still have some cleanup work to do though because of the surfaces not being totally smooth to one another. After you do the shell of each surface individually, if you examine the results carefully you'll see that the offset pieces do not match up exactly:



Zooming in to the above location:




That deviation in the offset is due to the surfaces not being exactly tangent to one another at their connecting spot. That overlap between the pieces will kind of complicate things, you may need to do something like trim back those areas with a cutting line from the side, then put in blends to connect those pieces into a full solid.

- Michael

Hi Michael, you're right, there were 4 surfaces.. Things look much better when deleting the overlapping ones. I can see the misalignement now, that explains why I couldn't slice up the model by boolean dif with a line, cause it wasn't a solid. Thanks for having a look.

/Lars

Did another try with a double-moebius strip that was just one surface, but with the same kind of misalignment. Maybe that's inevitable with a surface that only has one side.














/Lars

Hi Lars - I suspect the main problem is that the surface is just not quite exactly tangent in that seam area.

When the offset between 2 abutting surfaces drifts away from one another, it means that they have slightly differing surface normals at that location.

Here is a kind of example of that:




I have exaggerated this above example, but I suspect that is the same thing that is happening to you, except that the normals are really pretty close to being aligned so the deviation between the offsets is pretty small.

You'll probably need to make the surface "periodic" in that direction to fully guarantee it is smooth along the seam where its edges touch. I don't right now have a tool in MoI to edit a surface to make it periodic closed, maybe I should be doing that automatically when the result surface is very close but for now if you have Rhino you can use the MakePeriodic command on such a surface to give it periodic closure which will be guaranteed to have the same normal at the start and end.

- Michael

Hi Michael, I have the Rhino trial version so I'll give it a try. The reason I downloaded the Rhino trial was to use their free Grasshopper plugin. I've made the moebius strip with Grasshopper. And by increasing the number of u and v points I got even closer. See attached file.

Update - no attached file, the 3dm-file increased to about 13Mb

/Lars

Hi Lars - that's getting to be a quite awfully dense surface at 13 Mb just for that one shape.

That is probably too dense, the offset surface is usually going to be denser than the original surface so if you try to offset such a complex thing it is going to really be too much.

I'd try the original one that you had but trying to true up the seam by using MakePeriodic in Rhino.

- Michael

Hi Lars,

Here is the single-moebius strip,based
on the example of Rinus Roelofs

Thanks Paolo, I'll check it out when I get home.

/Lars

I had a play with this.

Shelling the strip was OK but I had problems trying to add fillets to all the edges, so instead I created 2 solids using sweep (2 rails), then booleaned out the holes using cylinders. I was then able to add fillets to all the edges.

manz perfect job :)

ciao
torolf

Well I am lost!
Have tried everything and I have no idea how you get from A to Z in this idea.

Brian
(I think it will keep me awake in bed tonight even so----should I make a movie?--2 cameras?)

Hi Brian,

I have attached the solid model with the construction curves, maybe it will help?



Regards,

- Steve

test the SKP format export : ok! :)