I would like to design a gear with a very large number of teeth, about 1000.
The problem is that the rendering time quickly becomes unpractical, even
using simple triangular teeth (and I would like to go involute).
Is there any smart trick to avoid the huge union?

Thanks in advance for any hint!

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I had considered originally that the request was beyond my ability to contribute, but A. Craig West's answer caused my neurons to fire in a different direction. Inkscape has an extension (path to OpenSCAD) which generates points/faces from an SVG image. There are many gear generators which will output to SVG or similar.

I've found the version I have reduces the size of the created model, requiring to change the scale factor from 90/25.4 (or 25.4/90) to 1, but that's trivial.
If there are cavities in the original SVG, a polyhedron is created for each item, but not automatically subtracted from the primary mass. That's easily addressed. Identify the primary, move it to the top of the generated list and difference out all the rest.

On Sunday, June 23, 2019, 11:59:11 AM EDT, A. Craig West <acraigwest@gmail.com> wrote:  

If you can write code to generate the shape of the gear as a polyhedron directly, it would probably decrease the rendering time significantly. It may also work to generate the shape in 2 dimensions using Union, and then use linear_extrude to form the gear
On Sun, 23 Jun 2019, 11:47 DarioPellegrini, pellegrini.dario@gmail.com wrote:

I would like to design a gear with a very large number of teeth, about 1000.
The problem is that the rendering time quickly becomes unpractical, even
using simple triangular teeth (and I would like to go involute).
Is there any smart trick to avoid the huge union?

Thanks in advance for any hint!

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Sent from: http://forum.openscad.org/

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Unfortunately this gear is not supposed to have cylindrical symmetry as it is
actually carved on a 3d surface. But thinking about this made me remember
about the toothed_belt example of list_comprehension_demos. I am going to
try that as it is potentially pretty quick!

I am also familiar with the procedure of pre-computing the data to feed to a
polyhedron, although this is my last option since it will basically be:
/"code it yourself and then just use OpenSCAD to render"/

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I am "toothing" a polygonal-section-moebius ring (that's why it has so many
teeth).

I have found the list-comprehension-demos/toothed-belt.scad perfectly suited
for the case, handling thousands of teeth along whatever path in a matter of
seconds both in preview and rendering.

Thanks to all for the discussion!

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And here is the result:

http://forum.openscad.org/file/t2340/Screenshot_from_2019-06-25_18-42-47.png

And the unbelievably simple and compact code which renders in just 2 seconds
on my machine:

use <scad-utils/transformations.scad>
use <scad-utils/shapes.scad>
use <list-comprehension-demos/skin.scad>

// Moebius Parameters
p = 3;
q = 1;
R = 80;
r = 10;

// Belt parameters
teeth = 800;
belt_width = r*sqrt(3);
tooth_height = 1;
belt_thickness = 3;

path_moeb = [for(t = linspace(0,360,teeth4))
rotation(axis=[0,0,tp])translation(v=[R,0,0])
rotation(axis=[0,t,0])*translation(v=[-r/2,0,0])
];

belt_moeb = [ for (i=[0:len(path_moeb)-1]) let(tooth=floor(i/2)%2)
transform(path_moeb[i] *
rotation(axis=[0,0,180]) *

scaling([tooth?((belt_thickness-tooth_height)/belt_thickness):1,1,1]) *
translation([belt_thickness/2,0,0])*
rotation(axis=[90,0,0]),
rectangle_profile([belt_thickness,belt_width])
)
];

skin(belt_moeb);

////////////////

function linspace(start,stop, n) = let (step=(stop-start)/n)
concat( [ for (i = [0:1:n-0.1]) start+i*step], stop);

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