Sadly no manufacturing process can make a part with a +/- 0mm tolerance. When making mating parts, engineers use some sort of tolerance analysis to make sure the parts can go together. http://en.wikipedia.org/wiki/Tolerance_analysis Several of these parts assumed no manufacturing tolerance so the line to line fit caused them not to go together. I did a worse case analysis and used a +/-0.1mm tolerance to get this to work. I used netfabb to scale the parts. Since these parts all mate in a line, you can see how the tolerances stacked up with how large some of the distal parts got. I also noticed the tolerances changed slightly depending on what material I was using so I included 2 versions of the elbow since that was a troublesome part.