I am having issues 3D printing assemblies.  Whenever I combine two parts into an assembly and I want them to connect, they 3D print with a hole between them.  I know that it is because of non-manifold overlapping surface, but how do you create an assembly that doesn't overlap?  I read the forums and I've tried saving the assembly as a part and it doesn't work.  The only way that I've found is to use makeprintable.com, but I shouldn't have to pay for an extra service to accomplish this.  What am I missing?  See the attachment for one example of many.  It does this every time I combine parts into an assembly.