Added a utility for making involute spur gears

tests/gears.scad

@@ -0,0 +1,58 @@
+//
+// NopSCADlib Copyright Chris Palmer 2020
+// nop.head@gmail.com
+// hydraraptor.blogspot.com
+//
+// This file is part of NopSCADlib.
+//
+// NopSCADlib is free software: you can redistribute it and/or modify it under the terms of the
+// GNU General Public License as published by the Free Software Foundation, either version 3 of
+// the License, or (at your option) any later version.
+//
+// NopSCADlib is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY;
+// without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+// See the GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License along with NopSCADlib.
+// If not, see <https://www.gnu.org/licenses/>.
+//
+include <../utils/core/core.scad>
+use <../utils/gears.scad>
+
+// left gear teeth
+z1 = 39; // [7 : 1 : 99]
+
+// Right gear teeth
+z2 = 7; // [7 : 1 : 99]
+
+//  Modulus
+m = 2.0; // [0.1 : 0.1 : 5.0]
+
+// Pressure angle
+pa = 20; // [14.5, 20, 22.5, 25]
+
+$show_numbers = false;
+
+module gears() {
+    color(pp1_colour)
+        rotate($t * 360)
+            linear_extrude(eps, center = true, convexity = z1)
+                difference() {
+                    involute_gear_profile(m, z1, pa);
+
+                    circle(r = m * z1 / 10);
+                }
+
+    color(pp2_colour)
+        translate([centre_distance(m, z1, z2, pa), 0])
+            rotate(180 + 180 / z2 + -$t * 360 * z1 / z2)
+               linear_extrude(eps, center = true, convexity = z2)
+                    difference() {
+                         involute_gear_profile(m, z2, pa);
+
+                         circle(r = m * z2 / 10);
+                    }
+}
+
+rotate(is_undef($bom) ? 0 : [70, 0, 315])
+    gears();