Hi Nicolinux,

As long as I can, I don't usually use offset because it adds unwanted control points and complexity to the resulting curves/surfaces.

Here's a little tut. Obviously there are many ways to do it, and this is one of them.
N.B. I used 2 circle with a radius of 15 and 19 and I did a final chamfer of 2, that is (19-15 = 2).


http://take.ms/OOKHX


And here's the 3dm file http://take.ms/nBqlB

Best,

- Marco (mkdm)