diff --git a/3d/lib/gears.scad b/3d/lib/gears.scad
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+// Copyright 2010 D1plo1d
+// LGPL 2.1
+
+
+//test_involute_curve();
+//test_gears();
+//demo_3d_gears();
+
+// Geometry Sources:
+//  http://www.cartertools.com/involute.html
+//  gears.py (inkscape extension: /usr/share/inkscape/extensions/gears.py)
+// Usage:
+//  Diametral pitch: Number of teeth per unit length.
+//  Circular pitch: Length of the arc from one tooth to the next
+//  Clearance: Radial distance between top of tooth on one gear to bottom of gap on another.
+
+module gear(number_of_teeth,
+        circular_pitch=false, diametral_pitch=false,
+        pressure_angle=20, clearance = 0)
+{
+    if (circular_pitch==false && diametral_pitch==false) echo("MCAD ERROR: gear module needs either a diametral_pitch or circular_pitch");
+
+    //Convert diametrial pitch to our native circular pitch
+    circular_pitch = (circular_pitch!=false?circular_pitch:180/diametral_pitch);
+
+    // Pitch diameter: Diameter of pitch circle.
+    pitch_diameter  =  number_of_teeth * circular_pitch / 180;
+    pitch_radius = pitch_diameter/2;
+
+    // Base Circle
+    base_diameter = pitch_diameter*cos(pressure_angle);
+    base_radius = base_diameter/2;
+
+    // Diametrial pitch: Number of teeth per unit length.
+    pitch_diametrial = number_of_teeth / pitch_diameter;
+
+    // Addendum: Radial distance from pitch circle to outside circle.
+    addendum = 1/pitch_diametrial;
+
+    //Outer Circle
+    outer_radius = pitch_radius+addendum;
+    outer_diameter = outer_radius*2;
+
+    // Dedendum: Radial distance from pitch circle to root diameter
+    dedendum = addendum + clearance;
+
+    // Root diameter: Diameter of bottom of tooth spaces.
+    root_radius = pitch_radius-dedendum;
+    root_diameter = root_radius * 2;
+
+    half_thick_angle = 360 / (4 * number_of_teeth);
+
+    union()
+    {
+        rotate(half_thick_angle) circle($fn=number_of_teeth*2, r=root_radius*1.001);
+
+        for (i= [1:number_of_teeth])
+        //for (i = [0])
+        {
+            rotate([0,0,i*360/number_of_teeth])
+            {
+                involute_gear_tooth(
+                    pitch_radius = pitch_radius,
+                    root_radius = root_radius,
+                    base_radius = base_radius,
+                    outer_radius = outer_radius,
+                    half_thick_angle = half_thick_angle);
+            }
+        }
+    }
+}
+
+
+module involute_gear_tooth(
+                    pitch_radius,
+                    root_radius,
+                    base_radius,
+                    outer_radius,
+                    half_thick_angle
+                    )
+{
+    pitch_to_base_angle  = involute_intersect_angle( base_radius, pitch_radius );
+
+    outer_to_base_angle = involute_intersect_angle( base_radius, outer_radius );
+
+    base1 = 0 - pitch_to_base_angle - half_thick_angle;
+    pitch1 = 0 - half_thick_angle;
+    outer1 = outer_to_base_angle - pitch_to_base_angle - half_thick_angle;
+
+    b1 = polar_to_cartesian([ base1, base_radius ]);
+    p1 = polar_to_cartesian([ pitch1, pitch_radius ]);
+    o1 = polar_to_cartesian([ outer1, outer_radius ]);
+
+    b2 = polar_to_cartesian([ -base1, base_radius ]);
+    p2 = polar_to_cartesian([ -pitch1, pitch_radius ]);
+    o2 = polar_to_cartesian([ -outer1, outer_radius ]);
+
+    // ( root_radius > base_radius variables )
+        pitch_to_root_angle = pitch_to_base_angle - involute_intersect_angle(base_radius, root_radius );
+        root1 = pitch1 - pitch_to_root_angle;
+        root2 = -pitch1 + pitch_to_root_angle;
+        r1_t =  polar_to_cartesian([ root1, root_radius ]);
+        r2_t =  polar_to_cartesian([ -root1, root_radius ]);
+
+    // ( else )
+        r1_f =  polar_to_cartesian([ base1, root_radius ]);
+        r2_f =  polar_to_cartesian([ -base1, root_radius ]);
+
+    if (root_radius > base_radius)
+    {
+        //echo("true");
+        polygon( points = [
+            r1_t,p1,o1,o2,p2,r2_t
+        ], convexity = 3);
+    }
+    else
+    {
+        polygon( points = [
+            r1_f, b1,p1,o1,o2,p2,b2,r2_f
+        ], convexity = 3);
+    }
+
+}
+
+// Mathematical Functions
+//===============
+
+// Finds the angle of the involute about the base radius at the given distance (radius) from it's center.
+//source: http://www.mathhelpforum.com/math-help/geometry/136011-circle-involute-solving-y-any-given-x.html
+
+function involute_intersect_angle(base_radius, radius) = sqrt( pow(radius/base_radius,2) - 1);
+
+
+
+// Polar coord [angle, radius] to cartesian coord [x,y]
+
+function polar_to_cartesian(polar) = [
+    polar[1]*cos(polar[0]),
+    polar[1]*sin(polar[0])
+];
+
+
+// Test Cases
+//===============
+
+module test_gears()
+{
+    gear(number_of_teeth=51,circular_pitch=200);
+    translate([0, 50])gear(number_of_teeth=17,circular_pitch=200);
+    translate([-50,0]) gear(number_of_teeth=17,diametral_pitch=1);
+}
+
+module demo_3d_gears()
+{
+    //double helical gear
+    translate([50,0])
+    {
+    linear_extrude(height = 10, center = true, convexity = 10, twist = -45)
+     gear(number_of_teeth=17,diametral_pitch=1);
+    translate([0,0,10])
+        rotate([0,180,180/17])
+        linear_extrude(height = 10, center = true, convexity = 10, twist = 45)
+     gear(number_of_teeth=17,diametral_pitch=1);
+    }
+
+    //spur gear
+    translate([0,-50]) linear_extrude(height = 10, center = true, convexity = 10, twist = 0)
+     gear(number_of_teeth=17,diametral_pitch=1);
+
+}
+
+module test_involute_curve()
+{
+    for (i=[0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15])
+    {
+        translate(polar_to_cartesian([involute_intersect_angle( 0.1,i) , i ])) circle($fn=15, r=0.5);
+    }
+}