Hi Paolo, thanks for posting the model data.

I do not believe that there is any true tangent solution possible to the situation you originally wanted between the cylinder end edges, since those 3 end circles of the cylinders are all in different planes.

I can see that Rhino gives you a solution anyway, however it is not really a fully correct solution, if you actually examine the result that you got in your bal7B.3dm file, you can see that those circles have a deviation between their tangents of about 1.5 degrees.

You can examine that by putting in some trimming lines like so:



Then slice your circles into arcs and run Rhino's gcon command:



The gcon command tells you that they are G1 since the error is below the default 3 degree angle tolerance in Rhino. However, that deviation of 1.5 degrees seems to be a pretty significant amount, I'm not really very comfortable with making a "solution" like that where there is no real solution available.


The other method that you showed is different - by framing your problem all in a single plane you have an actual legitimate solution in the other construction that you showed.

- Michael