diff --git a/involute_gears.scad b/involute_gears.scad
new file mode 100644
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--- /dev/null
+++ b/involute_gears.scad
@@ -0,0 +1,670 @@
+// Parametric Involute Bevel and Spur Gears by GregFrost
+// It is licensed under the Creative Commons - GNU GPL license.
+// © 2010 by GregFrost
+// http://www.thingiverse.com/thing:3575
+
+// Simple Test:
+//gear (circular_pitch=700,
+//	gear_thickness = 12,
+//	rim_thickness = 15,
+//	hub_thickness = 17,
+//	circles=8);
+
+//Complex Spur Gear Test:
+//test_gears ();
+
+// Meshing Double Helix:
+//meshing_double_helix ();
+
+// Demonstrate the backlash option for Spur gears.
+//test_backlash ();
+
+// Demonstrate how to make meshing bevel gears.
+bevel_gear_pair ();
+
+pi=3.1415926535897932384626433832795;
+
+//==================================================
+// Bevel Gears:
+// Two gears with the same cone distance, circular pitch (measured at the cone distance) 
+// and pressure angle will mesh.
+
+module bevel_gear_pair (
+	gear1_teeth = 41,
+	gear2_teeth = 7,
+	axis_angle = 90,
+	outside_circular_pitch=1000)
+{
+	outside_pitch_radius1 = gear1_teeth * outside_circular_pitch / 360;
+	outside_pitch_radius2 = gear2_teeth * outside_circular_pitch / 360;
+	pitch_apex1=outside_pitch_radius2 * sin (axis_angle) + 
+		(outside_pitch_radius2 * cos (axis_angle) + outside_pitch_radius1) / tan (axis_angle);
+	cone_distance = sqrt (pow (pitch_apex1, 2) + pow (outside_pitch_radius1, 2));
+	pitch_apex2 = sqrt (pow (cone_distance, 2) - pow (outside_pitch_radius2, 2));
+	echo ("cone_distance", cone_distance);
+	pitch_angle1 = asin (outside_pitch_radius1 / cone_distance);
+	pitch_angle2 = asin (outside_pitch_radius2 / cone_distance);
+	echo ("pitch_angle1, pitch_angle2", pitch_angle1, pitch_angle2);
+	echo ("pitch_angle1 + pitch_angle2", pitch_angle1 + pitch_angle2);
+
+	rotate([0,0,90])
+	translate ([0,0,pitch_apex1+20])
+	{
+		translate([0,0,-pitch_apex1])
+		bevel_gear (
+			number_of_teeth=gear1_teeth,
+			cone_distance=cone_distance,
+			pressure_angle=30,
+			outside_circular_pitch=outside_circular_pitch);
+	
+		rotate([0,-(pitch_angle1+pitch_angle2),0])
+		translate([0,0,-pitch_apex2])
+		bevel_gear (
+			number_of_teeth=gear2_teeth,
+			cone_distance=cone_distance,
+			pressure_angle=30,
+			outside_circular_pitch=outside_circular_pitch);
+	}
+}
+
+//Bevel Gear Finishing Options:
+bevel_gear_flat = 0;
+bevel_gear_back_cone = 1;
+
+module bevel_gear (
+	number_of_teeth=11,
+	cone_distance=100,
+	face_width=20,
+	outside_circular_pitch=1000,
+	pressure_angle=30,
+	clearance = 0.2,
+	bore_diameter=5,
+	gear_thickness = 15,
+	backlash = 0,
+	involute_facets=0,
+	finish = -1)
+{
+	echo ("bevel_gear",
+		"teeth", number_of_teeth,
+		"cone distance", cone_distance,
+		face_width,
+		outside_circular_pitch,
+		pressure_angle,
+		clearance,
+		bore_diameter,
+		involute_facets,
+		finish);
+
+	// Pitch diameter: Diameter of pitch circle at the fat end of the gear.
+	outside_pitch_diameter  =  number_of_teeth * outside_circular_pitch / 180;
+	outside_pitch_radius = outside_pitch_diameter / 2;
+
+	// The height of the pitch apex.
+	pitch_apex = sqrt (pow (cone_distance, 2) - pow (outside_pitch_radius, 2));
+	pitch_angle = asin (outside_pitch_radius/cone_distance);
+
+	echo ("Num Teeth:", number_of_teeth, " Pitch Angle:", pitch_angle);
+
+	finish = (finish != -1) ? finish : (pitch_angle < 45) ? bevel_gear_flat : bevel_gear_back_cone;
+
+	apex_to_apex=cone_distance / cos (pitch_angle);
+	back_cone_radius = apex_to_apex * sin (pitch_angle);
+
+	// Calculate and display the pitch angle. This is needed to determine the angle to mount two meshing cone gears.
+
+	// Base Circle for forming the involute teeth shape.
+	base_radius = back_cone_radius * cos (pressure_angle);	
+
+	// Diametrial pitch: Number of teeth per unit length.
+	pitch_diametrial = number_of_teeth / outside_pitch_diameter;
+
+	// Addendum: Radial distance from pitch circle to outside circle.
+	addendum = 1 / pitch_diametrial;
+	// Outer Circle
+	outer_radius = back_cone_radius + addendum;
+
+	// Dedendum: Radial distance from pitch circle to root diameter
+	dedendum = addendum + clearance;
+	dedendum_angle = atan (dedendum / cone_distance);
+	root_angle = pitch_angle - dedendum_angle;
+
+	root_cone_full_radius = tan (root_angle)*apex_to_apex;
+	back_cone_full_radius=apex_to_apex / tan (pitch_angle);
+
+	back_cone_end_radius = 
+		outside_pitch_radius - 
+		dedendum * cos (pitch_angle) - 
+		gear_thickness / tan (pitch_angle);
+	back_cone_descent = dedendum * sin (pitch_angle) + gear_thickness;
+
+	// Root diameter: Diameter of bottom of tooth spaces.
+	root_radius = back_cone_radius - dedendum;
+
+	half_tooth_thickness = outside_pitch_radius * sin (360 / (4 * number_of_teeth)) - backlash / 4;
+	half_thick_angle = asin (half_tooth_thickness / back_cone_radius);
+
+	face_cone_height = apex_to_apex-face_width / cos (pitch_angle);
+	face_cone_full_radius = face_cone_height / tan (pitch_angle);
+	face_cone_descent = dedendum * sin (pitch_angle);
+	face_cone_end_radius = 
+		outside_pitch_radius -
+		face_width / sin (pitch_angle) - 
+		face_cone_descent / tan (pitch_angle);
+
+	// For the bevel_gear_flat finish option, calculate the height of a cube to select the portion of the gear that includes the full pitch face.
+	bevel_gear_flat_height = pitch_apex - (cone_distance - face_width) * cos (pitch_angle);
+
+//	translate([0,0,-pitch_apex])
+	difference ()
+	{
+		intersection ()
+		{
+			union()
+			{
+				rotate (half_thick_angle)
+				translate ([0,0,pitch_apex-apex_to_apex])
+				cylinder ($fn=number_of_teeth*2, r1=root_cone_full_radius,r2=0,h=apex_to_apex);
+				for (i = [1:number_of_teeth])
+//				for (i = [1:1])
+				{
+					rotate ([0,0,i*360/number_of_teeth])
+					{
+						involute_bevel_gear_tooth (
+							back_cone_radius = back_cone_radius,
+							root_radius = root_radius,
+							base_radius = base_radius,
+							outer_radius = outer_radius,
+							pitch_apex = pitch_apex,
+							cone_distance = cone_distance,
+							half_thick_angle = half_thick_angle,
+							involute_facets = involute_facets);
+					}
+				}
+			}
+
+			if (finish == bevel_gear_back_cone)
+			{
+				translate ([0,0,-back_cone_descent])
+				cylinder (
+					$fn=number_of_teeth*2, 
+					r1=back_cone_end_radius,
+					r2=back_cone_full_radius*2,
+					h=apex_to_apex + back_cone_descent);
+			}
+			else
+			{
+				translate ([-1.5*outside_pitch_radius,-1.5*outside_pitch_radius,0])
+				cube ([3*outside_pitch_radius,
+					3*outside_pitch_radius,
+					bevel_gear_flat_height]);
+			}
+		}
+		
+		if (finish == bevel_gear_back_cone)
+		{
+			translate ([0,0,-face_cone_descent])
+			cylinder (
+				r1=face_cone_end_radius,
+				r2=face_cone_full_radius * 2,
+				h=face_cone_height + face_cone_descent+pitch_apex);
+		}
+
+		translate ([0,0,pitch_apex - apex_to_apex])
+		cylinder (r=bore_diameter/2,h=apex_to_apex);
+	}	
+}
+
+module involute_bevel_gear_tooth (
+	back_cone_radius,
+	root_radius,
+	base_radius,
+	outer_radius,
+	pitch_apex,
+	cone_distance,
+	half_thick_angle,
+	involute_facets)
+{
+//	echo ("involute_bevel_gear_tooth",
+//		back_cone_radius,
+//		root_radius,
+//		base_radius,
+//		outer_radius,
+//		pitch_apex,
+//		cone_distance,
+//		half_thick_angle);
+
+	min_radius = max (base_radius*2,root_radius*2);
+
+	pitch_point = 
+		involute (
+			base_radius*2, 
+			involute_intersect_angle (base_radius*2, back_cone_radius*2));
+	pitch_angle = atan2 (pitch_point[1], pitch_point[0]);
+	centre_angle = pitch_angle + half_thick_angle;
+
+	start_angle = involute_intersect_angle (base_radius*2, min_radius);
+	stop_angle = involute_intersect_angle (base_radius*2, outer_radius*2);
+
+	res=(involute_facets!=0)?involute_facets:($fn==0)?5:$fn/4;
+
+	translate ([0,0,pitch_apex])
+	rotate ([0,-atan(back_cone_radius/cone_distance),0])
+	translate ([-back_cone_radius*2,0,-cone_distance*2])
+	union ()
+	{
+		for (i=[1:res])
+		{
+			assign (
+				point1=
+					involute (base_radius*2,start_angle+(stop_angle - start_angle)*(i-1)/res),
+				point2=
+					involute (base_radius*2,start_angle+(stop_angle - start_angle)*(i)/res))
+			{
+				assign (
+					side1_point1 = rotate_point (centre_angle, point1),
+					side1_point2 = rotate_point (centre_angle, point2),
+					side2_point1 = mirror_point (rotate_point (centre_angle, point1)),
+					side2_point2 = mirror_point (rotate_point (centre_angle, point2)))
+				{
+					polyhedron (
+						points=[
+							[back_cone_radius*2+0.1,0,cone_distance*2],
+							[side1_point1[0],side1_point1[1],0],
+							[side1_point2[0],side1_point2[1],0],
+							[side2_point2[0],side2_point2[1],0],
+							[side2_point1[0],side2_point1[1],0],
+							[0.1,0,0]],
+						triangles=[[0,1,2],[0,2,3],[0,3,4],[0,5,1],[1,5,2],[2,5,3],[3,5,4],[0,4,5]]);
+				}
+			}
+		}
+	}
+}
+
+module gear (
+	number_of_teeth=15,
+	circular_pitch=false, diametral_pitch=false,
+	pressure_angle=28,
+	clearance = 0.2,
+	gear_thickness=5,
+	rim_thickness=8,
+	rim_width=5,
+	hub_thickness=10,
+	hub_diameter=15,
+	bore_diameter=5,
+	circles=0,
+	backlash=0,
+	twist=0,
+	involute_facets=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;
+	echo ("Teeth:", number_of_teeth, " Pitch radius:", pitch_radius);
+
+	// Base Circle
+	base_radius = pitch_radius*cos(pressure_angle);
+
+	// 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;
+
+	// 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;
+	backlash_angle = backlash / pitch_radius * 180 / pi;
+	half_thick_angle = (360 / number_of_teeth - backlash_angle) / 4;
+
+	// Variables controlling the rim.
+	rim_radius = root_radius - rim_width;
+
+	// Variables controlling the circular holes in the gear.
+	circle_orbit_diameter=hub_diameter/2+rim_radius;
+	circle_orbit_curcumference=pi*circle_orbit_diameter;
+
+	// Limit the circle size to 90% of the gear face.
+	circle_diameter=
+		min (
+			0.70*circle_orbit_curcumference/circles,
+			(rim_radius-hub_diameter/2)*0.9);
+
+	difference ()
+	{
+		union ()
+		{
+			difference ()
+			{
+				linear_extrude (height=rim_thickness, convexity=10, twist=twist)
+				gear_shape (
+					number_of_teeth,
+					pitch_radius = pitch_radius,
+					root_radius = root_radius,
+					base_radius = base_radius,
+					outer_radius = outer_radius,
+					half_thick_angle = half_thick_angle,
+					involute_facets=involute_facets);
+
+				if (gear_thickness < rim_thickness)
+					translate ([0,0,gear_thickness])
+					cylinder (r=rim_radius,h=rim_thickness-gear_thickness+1);
+			}
+			if (gear_thickness > rim_thickness)
+				cylinder (r=rim_radius,h=gear_thickness);
+			if (hub_thickness > gear_thickness)
+				translate ([0,0,gear_thickness])
+				cylinder (r=hub_diameter/2,h=hub_thickness-gear_thickness);
+		}
+		translate ([0,0,-1])
+		cylinder (
+			r=bore_diameter/2,
+			h=2+max(rim_thickness,hub_thickness,gear_thickness));
+		if (circles>0)
+		{
+			for(i=[0:circles-1])	
+				rotate([0,0,i*360/circles])
+				translate([circle_orbit_diameter/2,0,-1])
+				cylinder(r=circle_diameter/2,h=max(gear_thickness,rim_thickness)+3);
+		}
+	}
+}
+
+module gear_shape (
+	number_of_teeth,
+	pitch_radius,
+	root_radius,
+	base_radius,
+	outer_radius,
+	half_thick_angle,
+	involute_facets)
+{
+	union()
+	{
+		rotate (half_thick_angle) circle ($fn=number_of_teeth*2, r=root_radius);
+
+		for (i = [1:number_of_teeth])
+		{
+			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,
+					involute_facets=involute_facets);
+			}
+		}
+	}
+}
+
+module involute_gear_tooth (
+	pitch_radius,
+	root_radius,
+	base_radius,
+	outer_radius,
+	half_thick_angle,
+	involute_facets)
+{
+	min_radius = max (base_radius,root_radius);
+
+	pitch_point = involute (base_radius, involute_intersect_angle (base_radius, pitch_radius));
+	pitch_angle = atan2 (pitch_point[1], pitch_point[0]);
+	centre_angle = pitch_angle + half_thick_angle;
+
+	start_angle = involute_intersect_angle (base_radius, min_radius);
+	stop_angle = involute_intersect_angle (base_radius, outer_radius);
+
+	res=(involute_facets!=0)?involute_facets:($fn==0)?5:$fn/4;
+
+	union ()
+	{
+		for (i=[1:res])
+		assign (
+			point1=involute (base_radius,start_angle+(stop_angle - start_angle)*(i-1)/res),
+			point2=involute (base_radius,start_angle+(stop_angle - start_angle)*i/res))
+		{
+			assign (
+				side1_point1=rotate_point (centre_angle, point1),
+				side1_point2=rotate_point (centre_angle, point2),
+				side2_point1=mirror_point (rotate_point (centre_angle, point1)),
+				side2_point2=mirror_point (rotate_point (centre_angle, point2)))
+			{
+				polygon (
+					points=[[0,0],side1_point1,side1_point2,side2_point2,side2_point1],
+					paths=[[0,1,2,3,4,0]]);
+			}
+		}
+	}
+}
+
+// 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) * 180 / pi;
+
+// Calculate the involute position for a given base radius and involute angle.
+
+function rotated_involute (rotate, base_radius, involute_angle) = 
+[
+	cos (rotate) * involute (base_radius, involute_angle)[0] + sin (rotate) * involute (base_radius, involute_angle)[1],
+	cos (rotate) * involute (base_radius, involute_angle)[1] - sin (rotate) * involute (base_radius, involute_angle)[0]
+];
+
+function mirror_point (coord) = 
+[
+	coord[0], 
+	-coord[1]
+];
+
+function rotate_point (rotate, coord) =
+[
+	cos (rotate) * coord[0] + sin (rotate) * coord[1],
+	cos (rotate) * coord[1] - sin (rotate) * coord[0]
+];
+
+function involute (base_radius, involute_angle) = 
+[
+	base_radius*(cos (involute_angle) + involute_angle*pi/180*sin (involute_angle)),
+	base_radius*(sin (involute_angle) - involute_angle*pi/180*cos (involute_angle)),
+];
+
+
+// Test Cases
+//===============
+
+module test_gears()
+{
+	translate([17,-15])
+	{
+		gear (number_of_teeth=17,
+			circular_pitch=500,
+			circles=8);
+	
+		rotate ([0,0,360*4/17])
+		translate ([39.088888,0,0])
+		{
+			gear (number_of_teeth=11,
+				circular_pitch=500,
+				hub_diameter=0,
+				rim_width=65);
+			translate ([0,0,8])
+			{
+				gear (number_of_teeth=6,
+					circular_pitch=300,
+					hub_diameter=0,
+					rim_width=5,
+					rim_thickness=6,
+					pressure_angle=31);
+				rotate ([0,0,360*5/6])
+				translate ([22.5,0,1])
+				gear (number_of_teeth=21,
+					circular_pitch=300,
+					bore_diameter=2,
+					hub_diameter=4,
+					rim_width=1,
+					hub_thickness=4,
+					rim_thickness=4,
+					gear_thickness=3,
+					pressure_angle=31);
+			}
+		}
+
+		translate ([-61.1111111,0,0])
+		{
+			gear (number_of_teeth=27,
+				circular_pitch=500,
+				circles=5,
+				hub_diameter=2*8.88888889);
+
+			translate ([0,0,10])
+			{
+				gear (
+					number_of_teeth=14,
+					circular_pitch=200,
+					pressure_angle=5,
+					clearance = 0.2,
+					gear_thickness = 10,
+					rim_thickness = 10,
+					rim_width = 15,
+					bore_diameter=5,
+					circles=0);
+				translate ([13.8888888,0,1])
+				gear (
+					number_of_teeth=11,
+					circular_pitch=200,
+					pressure_angle=5,
+					clearance = 0.2,
+					gear_thickness = 10,
+					rim_thickness = 10,
+					rim_width = 15,
+					hub_thickness = 20,
+					hub_diameter=2*7.222222,
+					bore_diameter=5,
+					circles=0);
+			}
+		}
+	
+		rotate ([0,0,360*-5/17])
+		translate ([44.444444444,0,0])
+		gear (number_of_teeth=15,
+			circular_pitch=500,
+			hub_diameter=10,
+			rim_width=5,
+			rim_thickness=5,
+			gear_thickness=4,
+			hub_thickness=6,
+			circles=9);
+	
+		rotate ([0,0,360*-1/17])
+		translate ([30.5555555,0,-1])
+		gear (number_of_teeth=5,
+			circular_pitch=500,
+			hub_diameter=0,
+			rim_width=5,
+			rim_thickness=10);
+	}
+}
+
+module meshing_double_helix ()
+{
+	test_double_helix_gear ();
+	
+	mirror ([0,1,0])
+	translate ([58.33333333,0,0])
+	test_double_helix_gear (teeth=13,circles=6);
+}
+
+module test_double_helix_gear (
+	teeth=17,
+	circles=8)
+{
+	//double helical gear
+	{
+		twist=200;
+		height=20;
+		pressure_angle=30;
+
+		gear (number_of_teeth=teeth,
+			circular_pitch=700,
+			pressure_angle=pressure_angle,
+			clearance = 0.2,
+			gear_thickness = height/2*0.5,
+			rim_thickness = height/2,
+			rim_width = 5,
+			hub_thickness = height/2*1.2,
+			hub_diameter=15,
+			bore_diameter=5,
+			circles=circles,
+			twist=twist/teeth);
+		mirror([0,0,1])
+		gear (number_of_teeth=teeth,
+			circular_pitch=700,
+			pressure_angle=pressure_angle,
+			clearance = 0.2,
+			gear_thickness = height/2,
+			rim_thickness = height/2,
+			rim_width = 5,
+			hub_thickness = height/2,
+			hub_diameter=15,
+			bore_diameter=5,
+			circles=circles,
+			twist=twist/teeth);
+	}
+}
+
+module test_backlash ()
+{
+	backlash = 2;
+	teeth = 15;
+
+	translate ([-29.166666,0,0])
+	{
+		translate ([58.3333333,0,0])
+		rotate ([0,0,-360/teeth/4])
+		gear (
+			number_of_teeth = teeth,
+			circular_pitch=700,
+			gear_thickness = 12,
+			rim_thickness = 15,
+			rim_width = 5,
+			hub_thickness = 17,
+			hub_diameter=15,
+			bore_diameter=5,
+			backlash = 2,
+			circles=8);
+		
+		rotate ([0,0,360/teeth/4])
+		gear (
+			number_of_teeth = teeth,
+			circular_pitch=700,
+			gear_thickness = 12,
+			rim_thickness = 15,
+			rim_width = 5,
+			hub_thickness = 17,
+			hub_diameter=15,
+			bore_diameter=5,
+			backlash = 2,
+			circles=8);
+	}
+
+	color([0,0,128,0.5])
+	translate([0,0,-5])
+	cylinder ($fn=20,r=backlash / 4,h=25);
+}
+