Mismatched meshes

[rlkillian]

In the last couple years or so Sim Mechanical has gotten really good at matching meshes of surfaces that I import from CAD even when I don't make sure all the line and arcs where they join are on the edge of a surface. In other words I can take a plate then just stick another plate at right angles to it in the middle of the first plate and Sim Mech is smart enough to know the surfaces meet there and mesh accordingly. I made a CAD model of some plates attaching to a pipe normal to the pipe and it meshed wonderfully. I then pulled the plates apart a bit being careful to move them along the length of the pipe only and not move their curved edge off of the pipe. This time the meshes didn't match. I assumed I must have a gap between the plate and the pipe but I can't see one. And yes, "Create matched mesh between parts" is checked. I've done this tens, if not over a hundred times, and it works wonderfully. This time it failed and I can't see why.

Short of going back to my CAD model and starting all over, is there anything I can do to make those suckers match? Oh, and short of moving each node manually.

[Anonymous]

Couple questions,

What was your CAD geometry you brought in?  Was is solid models that you mid-plane meshed?  Surfaces?

How did you "move" your plates?  If you're using Inventor for your geometry, the move should be pretty straight forward, and you won't lose much/if any of your analysis features to just go back to the CAD.

I cannot seem to duplicate what you are seeing, although I'm currently only running 2016, so perhaps there is a difference?

[rlkillian]

Danny, these are surfaces not extracted from midplane of solids. I used AutoCAD to draw the model. I don't know if you are familiar with AutoCAD but the repad (part of pipe where the plates attach) is and extrudedsurface and the plates that connect to it are regions. An earlier version of this model meshed just fine. After doing the analysis I decided the plate spacing should be further. I went back into the same model in AutoCAD and increased the size of the repad and pulled the plates further apart. I just checked again and the arcs that make up the pipe and the edge of the plate that attaches to the pipe or exactly the same. I even set the precision to the maximum it can be and they matched perfectly.

I then reimported the model into Sim Mech from scratch and got the problem your seeing here. I don't have fancy 3D modeling software like Inventor where I can go back and forth between modeler and FEA. I do have SimStudio Tools. I'm not very good at it and I've use AutoCAD for over 30 years. SimStudio and SimMech used to have a button that would let me go back and forth but for some odd reason it's gone. Oh well, that's a whole different post. I don't really consider SimStudio Tools industrial strength (not that AutoCAD is for this type of stuff but at least I know it well) so I really haven't devoted much time to it.

Anyway, to get back on track, it seems to me that there should be some way to fix this right in Sim Mech other than manually moving every node. Is there not? In the old days most FEA packages had something called node merge. I thought there was something like it in Sim Mech but couldn't find it. Not sure it would work exactly right anyway.

[John_Holtz · Accepted solution]

Hi @rlkillian

Sorry, but I do not have any ideas why it works most of the time but not this time. Some things you can try are these ideas:

1. Try a slightly larger and a slightly smaller mesh size. If the problem is related to the mesh size, a different size might fix the problem.
2. Add Construction Vertices to the nodes on the plate and re-mesh the model. The construction vertices force a node to appear on the parts at that coordinate, so the mesh should match. (See Surface models do not match mesh between surface bodies.)

[rlkillian]

Thanks John. I had tried changing the mesh size but it didn't work. I tried again yesterday and this time it did work. I don't remember what I tried the first time but yesterday I went from 0.25" to 0.24" and it worked. Amazing!

I also did a version where I added construction vertices per the article you linked. That worked too but the mesh wasn't nearly as pretty. Some would even say it was down right ugly.

It is very odd that it didn't match the mesh out right. Maybe there was a hiccup in my model but I sure couldn't find it. Thanks for the suggestions.