Thought the other thread was getting too long. So start anew.
I took a fairly complex and large assembly file from Alibre. It is a model of a lower part of an engine (block, pistons, rods, etc.). The parts and the assembly were part of Alibre's tutorial. I exported it to a 214 STEP file and imported it into OneCNC.
I am quite impressed. It loaded up without a hitch. I then took each individual and unique solid one at a time (block, one rod, one bracket, and one piston) and extracted the surfaces and re-merged it back into a solid. All of the solids re-merged except the Piston.
There is some commonality with the surfaces in the piston that are similar to the other parts that I have modeled, tried to import and modify, and crashed also.
Maybe someone (from OneCNC or Alibre) can pinpoint what those surfaces are that are causing the problem.
Anyway, I will try and include the zip file of the OneCNC XFA file of this engine. It is nice to look at if not for any other reason.
Performed some more work on the model. I took the Piston copied and pasted it into a new file. I extracted all the surfaces. Now, if I re-merge the surfaces a few at a time, in groups, and then re-merge all the groups, I am able to re-merge all the surfaces back into a solid.
Once re-merged, I can then extract all the surfaces and re-merge them all in one command.
Question for OneCNC (perhaps), does OneCNC fix mating edges of surfaces during the merge function? And if there are some slight miss alignments in some surface edges, it is just too much for OneCNC to fix all in one shot, and that is why it will re-merge pieces at a time instead?