//An involute spur gear, with reasonable defaults for all the parameters.
//Normally, you should just choose the first 4 parameters, and let the rest be default values.
//Meshing gears must match in mm_per_tooth, pressure_angle, and twist,
//and be separated by the sum of their pitch radii, which can be found with pitch_radius().
module gear (
mm_per_tooth = 3, //this is the "circular pitch", the circumference of the pitch circle divided by the number of teeth
number_of_teeth = 11, //total number of teeth around the entire perimeter
thickness = 6, //thickness of gear in mm
hole_diameter = 3, //diameter of the hole in the center, in mm
twist = 0, //teeth rotate this many degrees from bottom of gear to top. 360 makes the gear a screw with each thread going around once
teeth_to_hide = 0, //number of teeth to delete to make this only a fraction of a circle
pressure_angle = 28, //Controls how straight or bulged the tooth sides are. In degrees.
clearance = 0.0, //gap between top of a tooth on one gear and bottom of valley on a meshing gear (in millimeters)
backlash = 0.0 //gap between two meshing teeth, in the direction along the circumference of the pitch circle
) {
pi = 3.1415926;
p = mm_per_tooth * number_of_teeth / pi / 2; // radius of pitch circle
c = p + mm_per_tooth / pi - clearance; // radius of outer circle
b = p*cos(pressure_angle); // radius of base circle
r = p-(c-p)-clearance; // radius of root circle
t = mm_per_tooth/2-backlash/2; // tooth thickness at pitch circle
k = -iang(b, p) - t/2/p/pi*180; // angle to where involute meets base circle on each side of tooth
{
difference() {
for (i = [0:number_of_teeth-teeth_to_hide-1] )
rotate([0,0,i*360/number_of_teeth])
linear_extrude(height = thickness, center = true, convexity = 10, twist = twist)
polygon(
points=[
[0, -hole_diameter/10],
polar(r, -181/number_of_teeth),
polar(r, r