I'm desiging a wing for the back of a car.  I created a wing part, two strut parts, and a base part and put them together in an assembly.  It wouldn't mesh.  I tried everything I could with mesh settings, but nothing worked, so I decided to check each part individually.  I opened the wing part and it wouldn't mesh.  It said most of my extrusions were interfereing.  Well, yeah, I agree- for ease of modeling I just "merged" all the extrusions as I made them.  I would start on the same plane and just extrude through and existing part instead of setting an offest.

So I take it I can't do that?  In other words, I made a mostly solid rib (it has holes in it) that is .1" thick.  I then opened a new sketch from the plane of the part I just extruded and drew the shape of the spar.  I then extruded the spar, which is also .1" thick, but 55 inches long- so .1" of the 55i inches is inside the rib.  I let the extrusion merge with the rib.  When I run Simulation mesh, it says my extrusion collide and stops.  So I have to make every extrusion so it doesn't go through an existing extrusion?  That seems tedius, especially on a complicated part.  My wing is made of composite material, so it will basically be one big piece of plastic once the glue dries, so I don't get why I can merge extrusions. 

Can someone help me out here?  Thanks.