Issues with solid difference in waffle structure

Could somebody take a quick look to my gh-file and maybe explain why I don’t get the desired cut in the beams of a waffle structure?
Thanks in advance.
Waffle_01.gh (15.0 KB)
Surface_01.3dm (191.8 KB)

extruding a Curve does not make it a solid, you can see the top/bottom are open:

use Cap Holes:

and it works fine:

You can also create a Boundary Surface from the curves and then Extrude these to create a solid.

Six of one, half a dozen of the other!

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Thanks!
I realized this while eating lunch, but you guys were faster to help me :slight_smile

How quick? :roll_eyes: