Hi Burr, so I was experimenting some with different internal options for surface joining, and by turning on an option for squeezing together small edges I was actually able to get your wheel thing there joined into a solid (just 1 surface needed to be trimmed), it is attached here.

But unfortunately I have also seen that "squeeze small edges" option cause some kind of orientation problems in some other cases, that's why it is not turned on currently in MoI.

That seems to be the actual problem with the model though, not that the endpoints have that little overlap, but that there are some little super tiny micro edges in the trimming boundaries, actually at a 90 degree turn from their surrounding edges making for a really messy situation.

Here I have tracked down just one of them:

In this area here:



If you zoom in you can see the 2 larger edges have a kind of criss-cross arrangement (like I was showing earlier, this is another angle on the same kind of thing):



which is not really great compared to touching nicely, but actually the ends of the criss-cross are within 0.0007 units of each other which is within tolerance so that is actually by itself not going to interfere with joining.

But notice the circled area, there is a little spec of yellow in there - that's actually another edge in there. Here i have zoomed in more closely on it:




And here it is unselected so you can see it more clearly:




So that's a terrible super tiny little edge hanging off one of those larger edges, and it is at some kind of 90 degree angle to the overall boundary shape, just really nasty to deal with trying to join that to something else.

That little micro edge is only 0.0000007 units long - this becomes bad when the entire edge is way smaller than the tolerance, it becomes difficult for the joiner to figure out how its endpoints should relate to other way larger edges that it is trying to join it with.

So anyway, that's what's going on with your example - it's not sags, but rather badly formed trimming boundaries with tiny micro edges in them.

At some point I could probably cook up some kind of "edge repair tool" that would do things like try and just remove little tiny micro edges like that to get rid of them.


- Michael