add twisted_donut

examples/twisted_donut.scad

@@ -0,0 +1,26 @@
+use <ring_extrude.scad>;
+
+$fn = 96;
+arc_a = 120;
+arc_r = 15;
+inner_r = 12;
+twisted_circles = 1;
+
+twisted_donut(arc_a, arc_r, inner_r, twisted_circles) ;
+
+module twisted_donut(arc_a, arc_r, inner_r, twisted_circles) {
+
+	arc_step = 360 / $fn;
+	for(init_a = [0, 180]) {
+		sh = [
+			for(a = [0:arc_step:arc_a]) 
+				[arc_r * cos(a + init_a) , arc_r * sin(a + init_a)]
+		];
+		ring_extrude(
+			sh, 
+			radius = arc_r + inner_r, 
+			twist = 180 + 360 * twisted_circles, 
+			angle = 360
+		);
+	}
+}