Hi Guy, and re: booleans - like Pilou if you have a hole in your object making it an open surface rather than a closed volume, then the booleans will behave differently, you are trying to work in this case with a now open surface since you basically "un-solidified" it when you did a trim on it initially instead of a boolean.

See here for some more information on boolean behavior with open surfaces:
http://moi3d.com/forum/index.php?webtag=MOI&msg=5195.14

If you want to use the booleans you usually need your base object to be a solid, and then they will work like you were expecting.

So your initial steps for creating this object would go like this - start with your base solid like so:



Then go to the 2D front view and draw in your curve cutting shape like this:



This looks like this in the 3D view now - it does not matter at what depth level the ellipse happens to be at:



Now, you do NOT need to do any more preparation work - if you're doing stuff like projecting curves on to surfaces and doing trims, that's all a bunch of extra work that you can just skip, instead let the booleans do all that work for you!

To get the final result, just select your solid here, and run Construct > Boolean > Diff, select the 2D ellipse curve as the cutting object and you will then get this result right away:



Check out the attached 3DM file for these objects - it's all set to boolean just run boolean difference and use the ellipse as the cutting object and you're done!

By the way these methods of doing booleans to cut solids with 2D curves is covered in the introductory video tutorials, if you have not watched them before it's probably a good idea to go over those, they will help get you going easier:
http://moi3d.com/2.0/docs/tutorials.htm

Basically if you just want to do simple "drill a hole with a 2D profile", and you have a solid as your base object, just draw in your 2D curve and then do a boolean operation and that handles a bunch of work for you - the booleans handle extruding the 2D shape into a "side wall" cutting object, trimming things and throwing away the right pieces, and then joining those pieces back into a solid so that's a lot of individual steps that you don't have to do when you use the booleans.

It looks like you were previously trying to do a bunch of those steps manually, maybe including even projecting the curve onto the solid before trimming with it? Even that projection step would be extra actually since Trim also has built in projection in it, you usually only need to do projection manually if you want to use the curves for some other purpose than cutting, like if you want to build some sweeps with them as rails or things like that. So there are probably a whole lot of extra steps that you were doing which are not necessary at all.

- Michael