A strange problem.
F5 gives a good result (see first image).
F6 seems to forget the main part of the object.

This is my code:
/
difference()
{
union()
{
difference()
{
import("C:\Users\Eigenaar\Documents\Mijn 3D
design\basklarinetbeker buiten v4.stl", convexity = 5);
import("C:\Users\Eigenaar\Documents\Mijn 3D
design\basklarinetbeker binnen v4.stl", convexity = 5);

    }
    translate([0,0,-155]) 
    {
        cylinder(5,(d_btm_inside +d_btm_wall/2)/2, (d_btm_inside

+d_btm_wall/2)/2);
}
}
translate([0,0,-160]) cylinder(11,d_btm_inside/2, d_btm_inside/2);
}/

F5 result:
http://forum.openscad.org/file/n15041/render2.jpg
F6 result:
http://forum.openscad.org/file/n15041/render3.jpg

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When I change convexity into 6 (don't now why, just trying something.

I get the following error messages:

ERROR: CGAL error in CGAL_Nef_polyhedron3(): CGAL ERROR: assertion
violation! Expr: pe_prev->is_border() ||
!internal::Plane_constructor::get_plane(pe_prev->facet(),pe_prev->facet()->plane()).is_degenerate()
File:
/data/OpenSCAD/libraries-mingw64-master/mxe-w64/usr/x86_64-w64-mingw32.static/include/CGAL/Nef_3/polyhedron_3_to_nef_3.h
Line: 251
ERROR: CGAL error in CGAL_Nef_polyhedron3(): CGAL ERROR: assertion
violation! Expr: pe_prev->is_border() ||
!internal::Plane_constructor::get_plane(pe_prev->facet(),pe_prev->facet()->plane()).is_degenerate()
File:
/data/OpenSCAD/libraries-mingw64-master/mxe-w64/usr/x86_64-w64-mingw32.static/include/CGAL/Nef_3/polyhedron_3_to_nef_3.h
Line: 251

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I tried to see if the two imported files need repair.
Meshlab detects some holes and other things, but I can't find and fix them.

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Hi Johan,

This could be related to this FAQ entry, which describes how to repair such things in Meshlab:
https://en.wikibooks.org/wiki/OpenSCAD_User_Manual/FAQ#Why_is_my_imported_STL_file_only_showing_up_with_F5_but_not_F6.3F

-Marius

Hi Kintel,

Thanks for that. It help a lot,. Not I am half way:

One part of the imports is giving an error:

ERROR: CGAL error in CGALUtils::applyBinaryOperator difference: CGAL ERROR:
assertion violation! Expr: itl != it->second.end() File:
/data/OpenSCAD/libraries-mingw64-master/mxe-w64/usr/x86_64-w64-mingw32.static/include/CGAL/Nef_3/SNC_external_structure.h
Line: 1102

http://forum.openscad.org/file/n15057/f5f6.jpg

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Also an attempt with nettfab did not work.
But that tool is completely new to me, maybe I don't use it in the right
way.

I see some coloured parts on the object. It that a problem? What to do with
that?
http://forum.openscad.org/file/n15058/netfabb.jpg

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Hello Kintel,

The files can be found here:
http://sax.jonker.co/techniek/288-ontwerpen-van-de-beker-van-een-bas-klarinet.html

I change fn to 10 and then increased it until the problem occured.
When fn was 60 the problem occured.
I am sorry these files are 6M bytes.

The first file is the Openscad file producing the outside en inside of the
bell.
The last file is the one I use to substract them.

The error I now get is:
ERROR: CGAL error in CGAL_Nef_polyhedron3(): CGAL ERROR: assertion
violation! Expr: pe_prev->is_border() ||
!internal::Plane_constructor::get_plane(pe_prev->facet(),pe_prev->facet()->plane()).is_degenerate()
File:
/data/OpenSCAD/libraries-mingw64-master/mxe-w64/usr/x86_64-w64-mingw32.static/include/CGAL/Nef_3/polyhedron_3_to_nef_3.h
Line: 293

thanks

Johan

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Another thing:

I tried Meshlab.
and used the filter Remove T-vertices by Edge Collapse.
That did not help.

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