The issue is related to, but not only, z-fighting <https://en.wikipedia.org/wiki/Z-fighting> . With CSG <https://en.wikipedia.org/wiki/Constructive_solid_geometry> , and a difference, or union, you cannot have two object sharing the same place/surface. The software doesn't know which should or shouldn't be subtracted or added. So you need to, for difference - make the subtracted object extend a 'little' beyond the surface of the base object, for union - make the joined object embed a 'little' into the other object. See the wiki <https://en.wikibooks.org/wiki/OpenSCAD_User_Manual/FAQ#Display> . And this change: s=0.1; s2=s*2; prec=90; difference() { hull(){ translate ([-47,2,5]) rotate ([90,0,0]) cylinder (2,r=5, $fn=prec); translate ([47,2,5]) rotate ([90,0,0]) cylinder (2,r=5, $fn=prec); translate ([-52,2,22]) rotate ([90,0,0]) cube ([10,10,2], center=false); translate ([42,2,22]) rotate ([90,0,0]) cube ([10,10,2], center=false); } // évidement hull(){ translate ([-20,2+s,17]) rotate ([90,0,0]) cylinder (2+s2,r=5, $fn=prec); translate ([20,2+s,17]) rotate ([90,0,0]) cylinder (2+s2,r=5, $fn=prec); translate ([-25,2+s,22+s]) rotate ([90,0,0]) cube ([10,10,2+s2], center=false); translate ([15,2+s,22+s]) rotate ([90,0,0]) cube ([10,10,2+s2], center=false); } } ----- Admin - PM me if you need anything, or if I've done something stupid... Unless specifically shown otherwise above, my contribution is in the Public Domain; to the extent possible under law, I have waived all copyright and related or neighbouring rights to this work. Obviously inclusion of works of previous authors is not included in the above. The TPP is no simple “trade agreement.” Fight it! http://www.ourfairdeal.org/ time is running out! -- View this message in context: http://forum.openscad.org/i-am-crazy-of-openscad-but-openscad-makes-me-crazy-tp21571p21572.html Sent from the OpenSCAD mailing list archive at Nabble.com.