Copy Curve Offset

Hi, 

I´m just starting to work with grasshopper, so maybe my questions are very stupid for somebody here, but I don`t know how to do it. 

I`ve ha drilled beam, a closed volume. For CNC-machining we need some Lines on the surfaces. We have a tool with 100mm widt, so I make a line every 95mm. Now in Rhino its no problem. 

ExtractSrf takes the surface I want to machine, DupFaceBorder gives me the Outline of the Surface. I Explode this, and get 4 lines. Now I take the line along the length an OffsetCrvSrf. Thats all, and it works. 

Yesterday evening I tried this in grasshopper, but I had no success.

Maybe somebody have a idea.

PS: I`m German, so please excuse my English ;)