Network and boolean subtraction

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 From:  Lang (LANGLEY)
5195.1 
Hi guys

I've only just got started with MoI and am a novice at the mo but I've ventured into the world of using networks to create some more challenging shapes. The one I've attached is fairly straight forward but what I'm trying to do is subtract a shape using the boole method but when I do it leaves a big hole rather than the shape combined. I reckon its because the shape I created with the network isn't actually a solid shape but if i'm right there is there a way round this? I assume all networked objects would be in theory hollow?

Thanks for any advice guys.

Lang

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 From:  BurrMan
5195.2 In reply to 5195.1 
Select the large object and run construct-planar on it, then do the boolean... Look at the top and bottome of it before and after.
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 From:  bemfarmer
5195.3 In reply to 5195.1 
Planar close top and bottom of bottle.
Select horn and rename horn1.
Select other horn and rename horn2.
Select all, and deselect horns, rename basebottle.
Boolean difference and select basebottle, then horn1.
End up with a hole.

I thought that there would not be a hole?

If you form a solid cylinder and do the boolean difference, with a horn,
there is not a hole, just a cylinder with a scoop shaped portion removed.

So the bottle must have a defect/ not be watertight?

Burrs method works, but why? There are two planes formed at the top,
and two planes formed at the bottom.

EDITED: 16 Jun 2012 by BEMFARMER

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 From:  futagoza (STEFAN)
5195.4 In reply to 5195.1 
Hi Lang,

you could sweep the profiles and would then obtain a solid, like shown on the left in my picture.



Regards
Stefan
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 From:  BurrMan
5195.5 In reply to 5195.3 
""""""""""I thought that there would not be a hole?"""""""

Hey Bem, Check that the bottle is a "solid". After you planar and cap it, select it and look in the upper right where the name is and see if it reads "solid".

Be sure when you are doing your "planar" selection, you are not using the "curves" that are left from construction, but the edges of the surface that was created.. The network command can create a surface that deviates from the curves that created it.
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 From:  bemfarmer
5195.6 In reply to 5195.5 
It is a very interesting model.
Like a glass bottle, the glass has thickness, and is solid, and the bottle is sort of hollow,
and sort of solid...
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 From:  BurrMan
5195.7 In reply to 5195.6 
"""""""It is a very interesting model.""""

You are speaking of a "real glass?" correct? not this model.

Remember that in MoI, a solid is a "closed volume". But you can still go inside of it and look around (In the real world, you couldnt do this)

A "surface" is infinitely thin. it has no volume or thickness matter to it... For that bottle to be "glass", it would have to be "modeled with 2 sides"...

So, create a cube, then run Shell on it. The cube now has walls with thickness and an inner hollow area.
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 From:  Lang (LANGLEY)
5195.8 
Thanks guys
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 From:  Michael Gibson
5195.9 In reply to 5195.3 
Hi Brian,

> Burrs method works, but why? There are two planes formed at the top,
> and two planes formed at the bottom.

It sounds like with your method you created 2 planar cap surfaces individually but if you create them separately like that by selecting the top and bottom curves to planarize then you will need to select all those 3 surface parts and use the Edit > Join command to glue together all those separate surfaces into a fully joined and closed solid.

Burr's method is a shortcut - the Planar command not only allows you to select curves as input to it, you can also select a surface object that has planar openings in it and when you give it that input it will construct planar surfaces for those openings and also automatically do the join for you as well. If you select curves as the input it does not know anything about the other surface piece and does not do any automatic join. So that's the difference.

- Michael
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 From:  Michael Gibson
5195.10 In reply to 5195.1 
Hi Lang, and it sounds like you've got it all worked out now?

Just to clarify - you got your original behavior because your object was not a closed solid, it had openings at the top and bottom.

In order to get the booleans to behave as you were expecting originally you need to close off those openings in order to make your object to be a solid volume.

Booleans are mostly oriented towards working with solids, with it deciding which pieces to keep and which to discard based on which volume they are contained inside of so usually if you plan to use booleans you want to form solids out of your pieces before using them. If you don't want to form solids then usually you will use the Edit > Trim command to cut surfaces up rather than booleans.

Booleans are kind of like a sort of "batch mode" that does a similar job of Trim + deleting things depending on which volume they are inside of + joining. You can do the same thing by manual steps of Trim + delete + join but if it is possible to work with solids the booleans tend to be more convenient and can save some steps.

- Michael
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 From:  Mike K4ICY (MAJIKMIKE)
5195.11 
Hi Lang!


I don't know if this was mentioned above...

Objects disappearing is often an unsatisfactory result of the Boolean commands.

I'm not sure why, but I do know how to quickly work around this problem:


Let's take your bottle Network and your horn shapes.

1) First take the network object and activate the Boolean-Merge command.
2) You should be left with the horn shapes with the network gone. You should actually have the intersection of the horns and the network, plus the remainder of the horns. The network will be gone. - COPY the good stuff.
3) UNDO and select the network again.
4) Run the Boolean-Difference command by selecting the network shape first.
5) You should now have your network bottle shape with holes in it. - PASTE!
6) Now drill-select those outer surfaces and delete them.
7) Join the objects you really wanted into one surface.

Enjoy. :-)
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 From:  Mike K4ICY (MAJIKMIKE)
5195.12 In reply to 5195.1 
Also Lang, a few things I noticed...

Nice shape, by the way!

When I ran a Fillet on the edges of those subtracted ares it looked bad.
I looked at the control points on the network and they seemed fine but were not as consistent as you might would desire.

I rebuilt the shape with Sweep by using the two side profiles as Rails. I first did a Rebuild on those rails and the resultant Sweep shape was very clean looking. Try it out.
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 From:  Michael Gibson
5195.13 In reply to 5195.11 
Hi Mike,

> Objects disappearing is often an unsatisfactory result of the Boolean commands.
>
> I'm not sure why, but I do know how to quickly work around this problem:

It will happen if some of the objects involved in the boolean are open surfaces rather than closed solid volumes.

The boolean commands are mostly oriented around working with closed solids, they decide which pieces of things to keep based on which volumes they are contained inside of.

You can use open surfaces in some ways, like as a cutting object to slice a base solid into 2 pieces if the cutting surface fully divides the base object.

But when your base object in boolean difference (the one that you are trying to cut) is not a solid itself which is the case with this example then it will behave more like a Trim operation.

Usually with the booleans you want your base object that is being booleaned to be a solid, so if there are open ends seal those off by getting caps joined on there before doing the boolean.

If you are not able to make the base object into a solid and you want to work at more of a surface level then usually you will want to be using the Edit > Trim command to work at that level - Trim does not try to do things by volume containment, it just dices surfaces up and lets you pick which fragments you want to keep or discard.

The booleans are kind of convenient though because they work as a kind of batch mode of doing a combination of Trim + automatically deciding which pieces to keep or discard based on which volume they are related to + joining the results up. So they can save some time since they incorporate multiple steps like that into a single operation, but if you want to use that kind of batch mode convenience level you do need to get your base object to be a closed solid first before you start to do booleans on it.

- Michael
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 From:  Michael Gibson
5195.14 
Here's a bit more explanation on why you get this kind of result when doing a boolean with a non-solid open surface as the base object.

Say you try to do a boolean with this plane surface as the base object and the cylinder as the cutting object:



MoI has no way of knowing which piece of the cylinder it is supposed to keep - it doesn't know whether it should keep this part:



Or whether it should keep this other part instead:



Since it has no way of deciding which piece to keep it does not keep either one and instead will make a result like this:



If you do want to cut objects like this with one another and keep one side of the cylinder part, you would usually want to use the Trim command instead of booleans, it will slice objects up into pieces and then you get to pick which pieces to discard, it does not try to automatically decide based on volume containment unlike the booleans.

Because the original network handle type object had open ends at the top and bottom, it was not a solid and so it behaves the exact same way as this example in this post with the plane - I know that it seems like that handle seemed fairly "solid-ish" looking at first glance, but unless it has a totally connected skin that actually forms an enclosed volume MoI is not able to give any special weight to one side of it or the other side since that would require some kind of more artifical intelligence-like judgement about which side is supposed to be the outside region of it, while if you actually have a solid there is a clearly mathematically defined outside and inside region.

- Michael

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