Hi Ruud, the top surface of that center button has a very small folded-over area in the corners (shown here with surface control points turned on):
It's a little hard to see, I'll give you a simplified example here - say you start with a surface with evenly distributed points like this:
Now imagine that one corner point is moved inwards a bit:
The above is still ok, but if we continue moving it more in that direction, you will get this:
That's now going to be a problem, the surface is folded over on top of itself there, which means the surface normal has a sudden huge jump from one side to the other side. This will cause a lot of problems with more difficult calculations such as booleans, offsets, and fillets. This is the type of thing that is happening in your button's top surface.
And actually this situation here where the surface's corner control points are exactly in line with its neighbor points:
is also not good - in this case the surface is not folded, but having the directions coming off of the corner of a surface be exactly tangent to each other will also create problems with the surface normal there.
So if you want to fillet or boolean your object, you've generally got to avoid creating surfaces that have this kind of bunching / overlapping in their corners. That usually means creating a bigger surface and then trimming it back.
So for this case, I drew 2 slightly bent curves crossing each other above your bottom button outline like this:
Now use Sweep, pick the other one as the rail path, to get this slightly curved surface:
Notice that this surface doesn't have any bunching in those corner areas.
Now you can extrude the bottom outline up so it punches through that curved surface:
Now these can be trimmed with each other and then joined into a single piece. To trim, select both pieces, run Edit/Trim, and then when prompted to select cutting objects, just push Done (or right-click) to indicate that you will cut each with the other. Then click on outside extra pieces to throw away. After trimming, use Edit/Join to glue them together.
If your bottom outline was planar, then when it extruded it would be a solid, and it is a little easier to use Construct / Boolean / Difference to cut a solid by a crossing surface like this, it is just basically fewer steps because you don't have to manually join afterwards.
You can now fillet with this kind of cleaner top surface:
The corners are still a little funky because the bottom outline still kind of comes to a sharp corner pointing inwards, so you may want to re-do that outline as well to make it smooth at the corner.
Hope this helps! Let me know if you need more information on any part.
- Michael
