I have a large STEP file to import into Cadkey 19.  It comes in fine 35+meg, but when I cut it to take out a part to make a STL model, it is thin surfaces.  I converted the whole thing to surfaces, and created surfaces where the cut planes are but it will not reconvert to a real solid.  (Invalid model)
I tried reducing tolerances, but that did not work.  The STEP file weas created out of CATIA.  I need to subtract some cylinders for countersinks and bolt holes. I am stuck.