Question about boolean operations

Coreform Cubit Cleanup/Defeaturing
#1

Hello

I need to clean up in my model and I haven’t been able to find the optimal way to remove overlaps. I’m using the newest Cubit version that has DAGMC support.

I have multiple volumes that have surfaces overlapping and I need to cut out the overlap but keep both volumes. While using ‘subtract’ with the option to keep both A and B volumes, it still registers an overlap. And if I use ‘remove overlap’ option it always deletes one of the volumes.

Is there a way I could keep both volumes but successfully modify one of them? Or am I bound to always loose one of the volumes?

Krislin

#2

Hi,
could you please upload a journal file or geometry so that i can see with what kind of surface overlaps you are dealing with.

#3

Hi

Yes, certainly. Thank you for taking the time to look at it! I will put the model here with a WeTransfer link:

Krislin

#4

How are you currently determining if the volumes actually overlap?

#5

I’m using the find overlap command.

#6

From which data is your model imported?

Looking at the engine, it says facet. This means we got a faced-based geometry.

If you would check for overlaps with the power tools

grafik
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Cubit will print a message with

ERROR: Currently, Cubit is unable to determine overlap for Facet-based geometry.

That doesn’t get us nowhere. so how are we sure that it’s really overlapping?

consider this short example

reset
create brick x 1
create brick x 1
move vol 1 x 1
find overlap

find overlap will tell us that surface 4 and 12 are overlapping. looking at it with the powertools it will say, that there is no problem. doing

intersect vol all

will end up with
grafik

so it is actually not really overlapping, more like it’s already touching but not penetrating.

so how does it look if we really overlap?

reset
create brick x 1
create brick x 1
move vol 1 x 0.9999
find overlap

find overlap will tell us surface 4 and 12 overlap. same as before. looking with the powertools already tells us there is a problem
grafik

that’s unfortunately not an option in your case.

intersect vol all

will in fact create us a new volume. because it’s really overlapping.

grafik
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as you’ve got a pretty complicated model here. there are two options.
• getting an acis model imported in cubit, so we can use the power tools

• sort out the overlapping pairs from find overlap with for example intersect vol 13 2176
intersect vol 13 2176 will produce a null intersection. so it’s not a overlap pair to worry about. when that’s done we can look how to actually solve the real overlaps

#7

i don’t think you’ve done anything wrong with remove overlap

doing it for the brick example from above will endup just like one would wish

grafik
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it’s just that if you will search for overlaps with find overlap it will still tell you, that there are still overlaps. which is really confusing. but the try with intersect volume 1 2 will reveal, that there is nothing to worry about.

#8

Hello

Thank you for the explanation!

Regarding the import, I’m importing the model in as .stl. While trying to load it in as either .step or .sat I get the following error for which I haven’t found a solution for:

You may be trying to read a file that is from an Acis version
newer than you are using.
Geometry engine set to: ACIS Version 33.0.0.0

The human phantom itself comes originally in .obj file and I’m using Fusion360 to convert it.

Krislin

#9

Have you tried to import the .obj directly in cubit?

Import Obj <string> [FEATURE_ANGLE <value>] [SURFACE_FEATURE_ANGLE <value>] [make_elements]

Could you share the .obj too?

#10

Yes, surely.

If imported as the original .obj file it only displays one organ for some reason, thus the reason why I was also using Fusion360. Originally I started off by working on it in it’s .obj format. I had to extract all the volumes individually and build it back up in Cubit. The problem started while creating volumes and surfaces. For some reason some volumes were created as real volumes and some as virtual volumes. I was then advised to use a different file format for importing the model into Cubit.

#11

i guess you’ve been doing the import over the gui.

image
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if you look at the command line it says import asset "/home/user/Downloads/MRCP_AM.obj"

doing import obj "/home/user/Downloads/MRCP_AM.obj"

will actually import the geometry

image
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#12

Oh! And it also already has the volumes and surfaces created. This would have made my life so much easier to begin with :smile:

Thank you for all the help! I will move forward with sorting out what really is an overlap and what isn’t with the find overlap and intersect commands.