Help doing an extrude cut

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 From:  Bob (APTIVABOY)
7242.1 
Hi all,

I've had MOI for awhile now but really rarely used it. Well, I've finally lost patience with Geomagic and its constant crashes and inability to import other common file types consistently nor well, so I've been really using and learning MOI more lately. I have to say, it handles lofts and organic shapes sooooo much better than Geomagic. Not trying to bash my former favorite program, but there it is. Plus, MOI doesn't crash! MOI has helped me with the following funnel for a Sims class destroyer, part of a conversion kit for a 1/350th scale model that I'm doing. The gussets and lofts at the base of the funnel, as well as the funnel cap, were unworkable with Geomagic, so thank you for this great program.

Now, the big problem that I'm having right now concerns the funnel cap, specifically how to cut out parts of the funnel cap assembly that the model doesn't need. I've gotten the inside or surfaces of the funnel, essentially two vertical walls done. However, I need to have those walls conform to the upper profile of the funnel. Any ideas on how to accomplish this? I've tried sweeping arcs to the top funnel cap sides but so far, no go.


I also need to eliminate the little extrusions on the sides of the funnel cap where the after wall poked through the side of the cap. Funny thing, I was able to do so on the firward extrusions, but can't for the life of me remember how I did it! Ah, the joys of learning new "switchology."

Thank you for any help or suggestions.

Bob
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 From:  Michael Gibson
7242.2 In reply to 7242.1 
Hi Bob, I'm glad MoI is working out for you!

One of the difficulties with this particular model here is the pieces you are trying to cut have their ends bent in a sharp fishhook like shape such that they are basically folded back over top of themselves at their ends, it's difficult to see unless zoomed in pretty closely:





That kind of thing with a surface very sharply turning and folding back on itself (so that it's basically "self intersecting") will tend to cause a lot of problems in surface/surface intersection calculations so you will probably need to throw out those current pieces and re-extrude some from a more simplified and non-folded base curve before they'd be usable.

Then for some strategy on how to approach it - the easiest way to do this kind of thing would be to use solids for the pieces and then there's a new feature in the booleans for v3 where you can do a boolean union with some face sub-objects selected and that will make the boolean done with intersections only happening on those selected faces. There's some discussion of this here which several visual examples which should hopefully help explain it:
http://moi3d.com/forum/index.php?webtag=MOI&msg=6663.1

For a case like you want, if you have 3 solids set up like this, with the 2 thin pieces just sticking out some distance past the back, top and bottom of the thickened base piece (again all 3 objects here are solids):


Then to set up the selection, you select the 2 thin pieces as entire objects, but make a face sub-object selection on the main body piece, with just the 2 inside faces selected, so the selection looks like this (2 full objects and 2 face sub-objects are all selected):




With the selection set up in that way the boolean will only cut things using those selected faces, and so when you then run Boolean Union, you will get this as the result from the boolean:



Basically by making the selection of the inside faces you're telling the boolean to temporarily pretend that the object is super thick and it only tries to cut things using those inside faces.

It's very convenient compared to working with just a bunch of individual surfaces. In order to use this you would want to have all the objects be set up as solids and not just as some individual surfaces. I've attached the example 3DM file so you can see how the objects are structured.

Hope that helps!

- Michael

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 From:  Bob (APTIVABOY)
7242.3 
Manny thanks, I shall give that a try! Just curious, why is it that those lines have a fish hook look to them? I didn't draw them that way.Is that just a by product of the rendering process?

Thank you again,

Bob
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 From:  Michael Gibson
7242.4 In reply to 7242.3 
Hi Bob,

> why is it that those lines have a fish hook look to them? I didn't draw them that
> way.Is that just a by product of the rendering process?

I'd have to watch how you created them to really know how they got the fish hook shape to them, but it's not just a rendering glitch that's a hook in the shape of the curves those surfaces were extruded from.

It looks like you've got some of the curves in your file there, it's difficult to see the hook just by glancing at them but if you turn on the curve control points with Edit > Show pts and move the ones near the bottom you can then see the hook type arrangement - the points do not go in an incremental order, the ending control point of the curve is "back tracking" back along the reverse direction:







- Michael

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