I changed the delta into a variable one:

/module calyx(buiten)
{
delta = 2;
delta2 = delta/2;
delta3 = delta2/2;
delta4 = delta3/2;
for (height = [0:delta:maxheight-4delta])
{
calyx_segment(height, delta, buiten);
}
for (height = [maxheight-4delta:delta2:maxheight-4delta2])
{
calyx_segment(height, delta2, buiten);
}
for (height = [maxheight-4delta2:delta3:maxheight-4delta3])
{
calyx_segment(height, delta3, buiten);
}
for (height = [maxheight-4delta3:delta4:maxheight-delta4])
{
calyx_segment(height, delta4, buiten);
}
}/

And also corrected the culculations for the thickness. It is a dy/dx of a
power to 4: (y = B -x^4*A)

--
View this message in context: http://forum.openscad.org/Tips-to-keep-the-speed-acceptable-tp14952p15016.html
Sent from the OpenSCAD mailing list archive at Nabble.com.

@ parkinbot: Interesting this sweep function. And it is indeed fast.
But it's a little to complicate for a beginner like me to implement now.

@neon22. The delta error is corrected nog. This is the result at this
moment.
Rendering the inside and the outside with $fn= 100 took about 20 minutes.
http://forum.openscad.org/file/n15019/kelk.jpg

--
View this message in context: http://forum.openscad.org/Tips-to-keep-the-speed-acceptable-tp14952p15019.html
Sent from the OpenSCAD mailing list archive at Nabble.com.

For anyone who likes the end result.
I generate the file twice and then substract them in an other scad file.
It is also in this file but takes too long to render (even F5).

// object: bass clarinet bell
// version: 4
// design: J.G.Jonker
// website: sax.jonker.co
// date: 7-12-2015
$fn=100;
maxheight = 150;
d_btm_inside = 41;
d_btm_wall =17.5;
d_top_inside = 138;
d_top_wall = 4;
t_top = 60;
rotatie_top = 45;
mildfactor = 0.5;
h_tenon = 5;
//---------------
astraal = maxheight/sin(rotatie_top);

    difference()
    {
        // translate([0,0,-150]) calyx(buiten=true); 
        translate([0,0,-150]) calyx(buiten=false); 
    }

module calyx(buiten)
{
delta = 2;
delta2 = delta/2;
delta3 = delta2/2;
delta4 = delta3/2;
for (height = [0:delta:maxheight-4delta])
{
calyx_segment(height, delta, buiten,0);
}
for (height = [maxheight-7delta2:delta2:maxheight-4delta2])
{
calyx_segment(height, delta2, buiten,0);
}
for (height = [maxheight-7delta3:delta3:maxheight-4delta3])
{
calyx_segment(height, delta3, buiten,0);
}
for (height = [maxheight-7delta4:delta4:maxheight-2*delta4])
{
calyx_segment(height, delta4, buiten,0);
}
for (height = [maxheight-delta4:delta4:maxheight-delta4])
{
calyx_segment(height, delta4, buiten,d_top_wall);
}
}

module calyx_segment(height,delta,buiten,top)
{
percentage = height/maxheight;
percentage2 = (height+delta)/maxheight;
//
d_inside    = d_top_inside
- pow((1-percentage),0.25)* (d_top_inside -
d_btm_inside);
d_inside2  = d_top_inside
- pow((1-percentage2),0.25)* (d_top_inside -
d_btm_inside);
rc          = atan(4maxheightpow((d_top_inside -
d_btm_inside),-4)* pow(d_top_inside - d_inside,3));
rc2        = atan(4maxheightpow((d_top_inside -
d_btm_inside),-4)* pow(d_top_inside-d_inside2,3));
//-------------------------------------------------------------
// determine de wall thickness
// keep the outside straight until d_top_wall is reached
//------------------------------------------------------------
d_wall      = (d_inside + d_top_wall) < (d_btm_inside+d_btm_wall) ?
(d_btm_inside+d_btm_wall - d_inside):
d_top_wall;
d_wall2    = (d_inside2 + d_top_wall) < (d_btm_inside+d_btm_wall) ?
(d_btm_inside+d_btm_wall - d_inside2):
d_top_wall;

    dx_raw      = (d_inside2-d_inside)/2;
    s_raw       = sqrt(dx_raw*dx_raw+delta*delta);
    dx1          = d_wall * sin(rc);
    dx2          = d_wall * sin(rc2);
    dy1           = d_wall * cos(rc);
    dy2          = d_wall * cos(rc2);
    //echo("delta's", rc,rc2,dx1,dx2,dy1,dy2);
    //echo("y x", d_inside, d_inside2, height, height+delta);
    //echo("hoek", hoek, hoek2);
    hoek = rotatie_top*percentage;
    hoek2 = rotatie_top*percentage2;
        
    if (buiten == true)
        {                      
            cylinder_pie(hoek, hoek2, 
                    -astraal*(1-cos(hoek*percentage))*mildfactor,
                    -astraal*(1-cos(hoek2*percentage2))*mildfactor,
                    height-dy1,

height+delta-dy2,d_inside/2+dx1,d_inside2/2+dx2,top,0);
}
else
{
cylinder_pie(hoek, hoek2,
-astraal*(1-cos(hoekpercentage))mildfactor,
-astraal(1-cos(hoek2percentage2))*mildfactor,
height, height+delta,d_inside/2,d_inside2/2,top,4);
}
}

module cylinder_pie(angle1, angle2, dx1,dx2,dy1,dy2,r1,r2,top,dr)
{
// draw top and bottom cylinders
// rotate the cylinder
// hull the cylinders
// trick: draw an extra thick and large cylinder when the top is
reached and
d=0.01;
//echo("top", top,dr);
hull()
{

             translate([ 0,dx1,dy1])
             rotate( angle1, ([1,0,0]))
             translate([0,0,-d])
             cylinder(d,r1, r1, center=false);
           
             translate([ 0,dx2,dy2])
             rotate( angle2, ([1,0,0]))
             translate([0,0,d])  
             cylinder(d+top,r2, r2, center=false);

      }
      if (top > 0)
      {
             translate([ 0,dx2,dy2])
             rotate( angle2, ([1,0,0]))
             translate([0,0,d])  
             cylinder(d+top,r2+dr, r2+dr, center=false);
          
}

}

--
View this message in context: http://forum.openscad.org/Tips-to-keep-the-speed-acceptable-tp14952p15025.html
Sent from the OpenSCAD mailing list archive at Nabble.com.

With the code as-is I get a nice horn shape.
E.g. set $fn to 12 on first line for faster results.

But I get a bad object if I change "buiten=true" over in the difference loop
and get the other surface.
I.e.
difference()  {
translate([0,0,-150]) calyx(buiten=true);
// translate([0,0,-150]) calyx(buiten=false);
}
The nice bell shape is gone. I've tracked it to the first region in calcyx:
for (height = [0:delta:maxheight-4*delta])
calyx_segment(height, delta, buiten,0);
http://forum.openscad.org/file/n15026/bell-2.png

But I can't see the bug.
(For me, at low $fn the difference goes pretty fast, and only 20 sec for
$fn=100.
but the resulting object is super slow in camera due to large number of
surfaces.
The Render() takes a very long time (48sec for $fn=12)  even for single
surface.)

--
View this message in context: http://forum.openscad.org/Tips-to-keep-the-speed-acceptable-tp14952p15026.html
Sent from the OpenSCAD mailing list archive at Nabble.com.

This will fix the bug:

    d_wall      = ((d_inside + d_top_wall) < (d_btm_inside+d_btm_wall) ? 
                    (d_btm_inside+d_btm_wall - d_inside): 
                    d_top_wall)/2;
    d_wall2     = ((d_inside2 + d_top_wall) < (d_btm_inside+d_btm_wall)

?
(d_btm_inside+d_btm_wall - d_inside2):
d_top_wall)/2;

--
View this message in context: http://forum.openscad.org/Tips-to-keep-the-speed-acceptable-tp14952p15028.html
Sent from the OpenSCAD mailing list archive at Nabble.com.

Found it - it was this:
dx1    = d_wall * sin(rc);
dx2    = d_wall2 * sin(rc2);
dy1    = d_wall * cos(rc);
dy2    = d_wall2 * cos(rc2);
They were d_wall and not d_wall2

--
View this message in context: http://forum.openscad.org/Tips-to-keep-the-speed-acceptable-tp14952p15029.html
Sent from the OpenSCAD mailing list archive at Nabble.com.

Yes are very right, that was also wrong.

Another thing is that I see small white dots in the object. I thougth that
everything is done to make it watertight. Also when I import all SLT files
sometime a large hole appears in the object depending on the angle that the
object is viewed. Is it in bug in OpenScad or in my design?

http://forum.openscad.org/file/n15037/waterdicht.jpg
http://forum.openscad.org/file/n15037/waterdicht2.jpg

--
View this message in context: http://forum.openscad.org/Tips-to-keep-the-speed-acceptable-tp14952p15037.html
Sent from the OpenSCAD mailing list archive at Nabble.com.