Hi Wim, and re: first cannot trim attachment, that also looks related to the same kind of "small numeric size around default fitting tolerance" type thing that I was describing above, that one has even smaller features in it, like the thin surface in there has a width of 0.0002 units.

Things will work better for you if you make these things have a larger scale, try to build things around sizes not much smaller than 0.01 units or so and these same cases will probably work better for you.

For example in your case there I switched units from meters to centimeters so the object didn't have such numerically small features and then trimming that same object with a few drawn lines seemed to go ok.

There are some areas of MoI that are less sensitive to small objects than others, I'm on a general path of trying to make more things work better with small objects by using an "adaptive tolerance" which is when fitting functions use a fraction of the object's size as the accuracy for the fitting operation rather than just a hard coded value of 0.001, but there are still areas that use 0.001 accuracy right now so just in general things will work more smoothly if you don't create things with such small numeric values in them. Try building them at 10x or 100x size or better yet use a unit system that will let you put in numbers in a range more like 100 to 0.1 rather than something like 0.0001

- Michael