Why are these not right-angles?

[lemelman]

This is a very simple model. There is a body with one surface at 12 degrees to the ZX plane on which I have sketched 2 closed figures with some edges at 12 degrees to the vertical. I then extruded these figures to cut away parts of the body, expecting that the cuts would be normal to that surface. But they're not - see the attached screenshot.

Why are they not.

[davebYYPCU · Accepted solution]

Because you cut it that way, you cut a double taper in relation to the face 1, and 

face 1, is not normal to face 2, if it was it would be 90 degrees, for both those angles you questioned.

Might help...

[lemelman]

Thank you, that solves my query completely.

I should have realised that was the case.