I would like to import an stl file and then add a shape.

If I render ONLY the stl file (F6), everything is fine!!!

If I create a base for it, however, it runs into an error:
ERROR: The given mesh is not closed! Unable to convert to CGAL_Nef_Polyhedron.

WHY????

$fn=64;
turret_height = 25;
p0 = 0;
p1 = 50;
p2 = 60;

module base(){
  difference(){
      cylinder(h=15, r1=50, r2=52, center=true);
      
      translate([0,0,5])
        cylinder(h=15, r1=47, r2=49, center=true);
  }//difference
}

module d_base(){
  difference(){
    base();

    translate([0,0,1])
      for (i=[0 : 45 : 360]) {    
        rotate([0, 0, i])
          linear_extrude(height = turret_height) {
            polygon( points=[[0,0],[p2,p1],[p1,p2]] );
          } //linear_extrude
      } //for
  } //difference
} //module


module gorilla(){
    resize([48,0,0], auto=true)
      translate([0,0,3])
    import("buddha_gorilla.stl",convexity=10);  
  }

  gorilla();
  d_base();

/*union(){
  gorilla();
  d_base();

}//union
*/

without the stl file to import, it's just a rough guess on my part, not being particularly skilled with OpenSCAD, but I'd venture a guess that the imported file sits on the x/y plane and your d_base is slightly below that level. Translate one down or the other up and aim for a bit of overlap (i.e., 0.0625 mm) to ensure a clean join/union.

On Friday, March 3, 2023 at 08:03:26 AM EST, gmagyar6@gmail.com <gmagyar6@gmail.com> wrote:  

I would like to import an stl file and then add a shape.

If I render ONLY the stl file (F6), everything is fine!!!

If I create a base for it, however, it runs into an error:
ERROR: The given mesh is not closed! Unable to convert to CGAL_Nef_Polyhedron.

WHY????

$fn=64;
turret_height = 25;
p0 = 0;
p1 = 50;
p2 = 60;

module base(){
difference(){
cylinder(h=15, r1=50, r2=52, center=true);

  translate([0,0,5])
    cylinder(h=15, r1=47, r2=49, center=true);

}//difference
}

module d_base(){
difference(){
base();

translate([0,0,1])
  for (i=[0 : 45 : 360]) {    
    rotate([0, 0, i])
      linear_extrude(height = turret_height) {
        polygon( points=[[0,0],[p2,p1],[p1,p2]] );
      } //linear_extrude
  } //for

} //difference
} //module

module gorilla(){
resize([48,0,0], auto=true)
translate([0,0,3])
import("buddha_gorilla.stl",convexity=10);
}

gorilla();
d_base();

/*union(){
gorilla();
d_base();

}//union
*/

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See also
https://en.wikibooks.org/wiki/OpenSCAD_User_Manual/FAQ#Why_is_my_imported_STL_file_appearing_with_F5_but_not_F6?

... which unfortunately doesn't mention that the geometry calculation
mentioned doesn't really happen until you have a second object.

Thanks!!

“but I'd venture a guess that the imported file sits on the x/y plane and your d_base is slightly below that level“
How can I check this?

The stl file:
https://drive.google.com/file/d/1obJKqO5HVk4BSYG67ADb5i2W3FSeXz0p/view?usp=share_link

I double checked by translating your gorilla below the x/y plane even more than it was. My guess was wrong as the result is the same. I also imported the gorilla into Meshmixer which reports no problem with the mesh.
I changed the code a bit and imported the gorilla into the progam without scaling and it still fails. It doesn't fail when it's the gorilla alone.Bypassing everything except the gorilla and a simple cube creates a failure as well. That code was simple import the gorilla un-scaled, un-translated and add a cube(10) to the scene. Failed.
Perhaps a bug?

On Saturday, March 4, 2023 at 09:40:43 AM EST, gmagyar6@gmail.com <gmagyar6@gmail.com> wrote:  

Thanks!!

“but I'd venture a guess that the imported file sits on the x/y plane and your d_base is slightly below that level“
How can I check this?

The stl file:
https://drive.google.com/file/d/1obJKqO5HVk4BSYG67ADb5i2W3FSeXz0p/view?usp=share_link

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