Shell help
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 From:  Alexander (ALEXCAD)
3965.9 In reply to 3965.8 
Michael,
That's exactly the result I wanted and many thanks for the detailed explanation.

I'm really enjoying using MoI and learning new modelling techniques.

Regards
Alexander
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 From:  Alexander (ALEXCAD)
3965.10 In reply to 3965.9 
Michael,

Sorry to bother you again regarding my problem, I did exactly as you said which worked perfectly thanks. But I have had to change the profile very slightly and now it won't shell properly. It seems to take forever to calculate and then jumbles everthing up. Its driving me crazy.

Please help...

Regards
Alexander
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 From:  Michael Gibson
3965.11 In reply to 3965.10 
Hi Alexander,

> It seems to take forever to calculate and then jumbles everthing up.

Usually a result like that means that one of your surfaces may be bunched up and sort of folding back over top of itself in a small area.

The way shelling and offsetting work, they produce a result that follows the surface normal a distance away from the original surface.

But if your surface folds back over top of itself, that means the surface normal (the perpendicular coming off of the surface) jumps around wildly in that area, which interferes with getting a good surface offset calculated from it.

That's just some general stuff to look for, I haven't checked out your model yet but I will take a look at it now.

- Michael
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 From:  Michael Gibson
3965.12 In reply to 3965.10 
Hi Alexander, so one problem I see right off the bat with your model is that it is not a fully closed solid - it has some unjoined edges in it.

I can tell this because when I select it, the type indicator in the upper-right area of the window says it is a "Joined srf" , if it was a totally closed solid it would read "solid" there instead:



So that's one indicator that the model is not cleanly constructed - it's got some pieces that are not joined up properly.

- Michael
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 From:  Michael Gibson
3965.13 In reply to 3965.10 
Hi Alexander, so the problem area of your model is in this area:



If you zoom in closely to that area you can see that there are some microscopic little surface fragments there, and it appears to have some pieces stacked up right on top of each other there:



That kind of messy area will be difficult to get a clean offset from - the offsetter will actually try to offset those little micro slivery surfaces, and try to extend them and intersect them with neighboring surfaces and stuff like that.

You really want to have a more simplified and clean model structure in order to get a good shell on it.

One thing about filleting is that you usually want the pieces you are filleting to come to more of a sharp angle and then let the filleter produce the rounded shape. In this case you've tried to make the initial surfaces come be smooth to each other, but you can see that this actually makes the filleter's job harder because it makes for a little tiny sized fillet surface when it tries to smooth out that area that you made pretty close to being smooth initially.

So that's one thing to make a better model - don't make part of the surfaces come close to being smooth to one another to start with if you want to fillet them.

Then another tip for shelling is you may want to apply the fillets _after_ you do the shell instead of applying them before. When you apply them before it means that the fillet surfaces are then some more stuff that need to be processed during the shell operation as well.

So that's what I'd recommend for this model - do you have a version of it before you applied the fillet? If so then try shelling that first, and then apply the fillet after the shell.

Also to get a proper fillet you may need to not have the initial surfaces come close to being smooth to one another as you have done here.

- Michael

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 From:  Michael Gibson
3965.14 In reply to 3965.10 
Hi Alexander, I was trying to untrim and retrim your model to get rid of the fillet, but I also ran into another problem area with it as well.

This surface here folds back over top of itself right near its ends:



If I separate that surface from the others and then turn on its control points, look at this area here which seems to be bunched up:



If I zoom in on that area, it appears to be in a kind of hook formation:




This kind of surface that folds back over top of itself is particularly bad for shelling or offsetting calculations. The surface normal that the offset tries to track along slams back and forth to opposite sides in a small area there.

It looks like this surface was created as an extrusion, but that the original curve being extruded has that kind of folded over hook at the ends.

Do you happen to have the original curve that was used to create that particular surface?

You want to be careful to make sure the original curve structure that you start with are nice and clean and don't have bad formations like this. If your original curves have bad structures in them, then those will tend to get transferred to surfaces created from those curves as well.

- Michael

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 From:  Michael Gibson
3965.15 In reply to 3965.10 
Hi Alexander, so basically this model will need to be reconstructed to remove some of these problems - that extruded side piece will need to be constructed from a better initial curve so that it doesn't have those folded-over hooks at its ends.

Then once that part of the model is cleaner you may also try doing the shell first and applying the fillet after that.

- Michael
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 From:  Alexander (ALEXCAD)
3965.16 In reply to 3965.15 
Michael, many thanks for your feedback.
I will have another go at re-modelling the part tomorrow.
Just out of interest could I create the same geometry with a different technique. The sides are rail sweeps and the top is an extrusion trimmed. I suspect there is an easier way to create the same part?

Thanks again Michael.
Alexander
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 From:  Michael Gibson
3965.17 In reply to 3965.16 
Hi Alexander,

> I suspect there is an easier way to create the same part?

Well normally you want to try and let Fillet create the rounded corners, instead of trying to model the corners directly into your starting surfaces as you've done.

Check out these previous threads for some examples:
http://moi3d.com/forum/index.php?webtag=MOI&msg=1002.1
http://moi3d.com/forum/index.php?webtag=MOI&msg=3137.1

You can use Sweep to build chunks of the model, but normally it's better if each surface gives only one element to the final shape, like one broad surface patch and doesn't have any tight bends in it - the sweep going through a tight bend is usually an indication that you're trying to combine more than one underlying form into a single surface instead of building them separately and then filleting them where they join.

Here's another example of that kind of thing of building some broader sheets of a model out of sweeps and then having a fillet provide a smooth juncture between them:
http://moi3d.com/forum/index.php?webtag=MOI&msg=2164.2
http://moi3d.com/forum/index.php?webtag=MOI&msg=2164.4

It tends to be hard to control things when you try to do too much of the model including tight bends and irregular boundaries all in one single surface.

- Michael
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