Michael,

sorry I couldn't be more useful with the file I provided, I know there is nothing wrong with it, at least that's what the Rhino analyze tools say.

But I still find this edge thing a bit funny, so I've made a simple test in the file I've attach. When I did a union on the 2 gray boxes on the left I get the red box where the edges at the intersection of the 2 gray box just disappeared as I would expect. I then copied the red box and made a strait line and a curve that I joined and used this curve to create the green and orange shape using a diff. They're normal I suppose but as you can see I would have expected the black and the red segment to select as one edge just as the black edge on the red box. I think I understand where the blue edge on the green shape comes from but I would have thought I would see no edge there.

I'm not complaining about this, it is just an observation of something I wasn't expecting. I think that a break like this can cause a retract in the g-code and on simple objects like these it doesn't matter but on more complex object with a lot of similar breaks, all these retract can increase machine time significantly. If there is a way to rejoin them, I'd like to know about it.

Thanks Michael and though it my not sound like it, I'm very impressed with MOI and it's quite possible that I follow your advice to the letter and get both MOI and Rhino in the end.

Regards,
Felix