Added 2d ring

regular_shapes.scad

@@ -76,6 +76,13 @@ module dodecagon(radius)
   reg_polygon(12,radius);
 }
 
+module ring(inside_diameter, thickness){
+  difference(){
+    circle(r=(inside_diameter+thickness*2)/2);
+    circle(r=inside_diameter/2);
+  }
+}
+
 module ellipse(width, height) {
   scale([1, height/width, 1]) circle(r=width/2);
 }