Hi Danny, well Join in MoI anyway does not really deform surfaces, it only does stuff to the trimming boundaries, and even then all it does is basically throws out one of the 3D edges and keeps the other one as the one for the joined structure.

But if the Join tolerance is too loose the problem can be that things end up getting glued together that should have stayed as distinct individual edges.

If tolerances are set too loose for calculations that involve fitting or iterative refinment, that's when you can get stuff like a sagging result.

- Michael