I thought I solved my problem by adding Triangulate modifier. It only worked on one object. I have made a round base plate using a cylinder. I have pushed a smaller cylinder through and created a hole using Boolean modifier in the base plate, applied, hide and add Triangulate.
Everything works, there is a hole in the plate, but after exporting the stl file to the slicer, the two cylinders are on top of each other. What am I doing wrong ?