alternative for Extrude or Offset?!

Replies to This Discussion

Permalink Reply by Hannes Löschke on January 26, 2014 at 10:33am

It's a problem of how things work.

Extrude moves every point on the surface the same distance in the same direction. So parts that are perpendicular to the direction, everything is fine. For all other parts, the resulting thickness will be smaller than extrusion distance

Offset moves every point on the surface the same distance but in the normal direction of each point. That will ensure equal thickness but will likely distort the boundary.

Look at your original surface. You will see that angle of the surface edge is not horizontal and not constant. so the X in your top sketch is the thickness only at the corners. Right in the middle, you would need to measure thickness at a slight angle. So moving the border curve up by X will actually create a thickness variation along the edge.

The easiest way to ensure thickness while maintaining footprint is to use offset then trim the result with the footprint. Be aware that this will result in a curved edge in side view.

You could try and make your own offset that would move points on the edge in vertical and every other point in normal direction. But that's going to give you a thickness distortion where you see the overhang now.

Permalink Reply by falc on air on January 26, 2014 at 12:08pm

Hi Hannes,

thank you for your quick reaction and your accurate analysis of the problem!
While I tried to find a solution I realized that the angles of the (edge)vectors are not horizontal and that it is a problem for me to find a "hybrid" solution between Extrude and Offset to get the best result.

Maybe you or another professional can give me an "impulse example" for the right way to move points on the edge in vertical and every other point in normal direction.
I just started to use GH + Rhino since 2 weeks..  sorry!!

I would appreciate very much!!

:-)