User:LABoyd2/cylinder over mid 150922

When using cylinder() with difference() to place holes in objects, the holes will be undersized. This is because circular paths are approximated with polygons inscribed within in a circle. The points of the polygon are on the circle, but straight lines between are inside. This is discussed in some detail at http://hydraraptor.blogspot.com.au/2011/02/polyholes.html

While the standard cylinder() and circle() generate inner (inscribed) polygons, here are modules to produce outer (circumscribed) and mid ( 1/2 way between) polygons.

cylinder

 module cylinder_outer(height,radius,fn){
   fudge = 1/cos(180/fn);
   cylinder(h=height,r=radius*fudge,$fn=fn);}

 module cylinder_mid(height,radius,fn){
   fudge = (1+1/cos(180/fn))/2;
   cylinder(h=height,r=radius*fudge,$fn=fn);}

Example of outer and mid cylinder

partial view

 //example script to show differences
 cylinder_outer(5,100,6);
 %cylinder_mid(5.5,100,6);
 cylinder(6,r=100,$fn=6);        // standard
 #cylinder(5.75,r=100,$fn=60);   // reference

 //CSG tree for this example
 group() {
   group() {
     cylinder($fn = 6, $fa = 12, $fs = 2, h = 5, r1 = 115.47, r2 = 115.47, center = false);
   }
 % group() {
     cylinder($fn = 6, $fa = 12, $fs = 2, h = 5.5, r1 = 107.735, r2 = 107.735, center = false);
   }
   cylinder($fn = 6, $fa = 12, $fs = 2, h = 6, r1 = 100, r2 = 100, center = false);
 # cylinder($fn = 60, $fa = 12, $fs = 2, h = 5.75, r1 = 100, r2 = 100, center = false);
 }

cone

For cylinders with different top and bottom radii

 module cone_outer(height,radius1,radius2,fn){
   fudge = 1/cos(180/fn);
   cylinder(h=height,r1=radius1*fudge,r2=radius2*fudge,$fn=fn);}

 module cone_mid(height,radius1,radius2,fn){
   fudge = (1+1/cos(180/fn))/2;
   cylinder(h=height,r1=radius1*fudge,r2=radius2*fudge,$fn=fn);}

 cone_outer(5 ,10 ,5 ,6);
 cone_mid(5 ,10 ,5 ,6);

circle

And lastly not to forget the 2D world

   module circle_outer(radius,fn){
     fudge = 1/cos(180/fn);
     circle(r=radius*fudge,$fn=fn);}

   module circle_mid(radius,fn){
     fudge = (1+1/cos(180/fn))/2;
     circle(r=radius*fudge,$fn=fn);}

 circle_outer(20,6);
 circle_mid(20,6);