""""""""" It's just a method I use when I have problems with the trim"""""""""""""

..i do the same..
i think cutting surface must penetrate into other surface,when last one is complex
i suppose that trim using just a curve project a lot of points,and if the target is ondulated or complicated,tool not project enough points to cover all the wavy:so trim fails.
(this is just my deduction,forgive me if is not correct)
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thanks Burr for your tuts,i learned few fundamental tricks in the past
just don't know why you don't speak also...it's your leanguage!!! (i must do lot of effort to speak in English when i record my modeling sessions...damn...)