How to design a Spur Gear (with Equations)

Adding Variables

Go to: Tools > Equations, then add these under Global Variables:

Module = 4 mm

Teeth No. = 50

Pitch = Module * pi

Primitive Diameter = Module * Teeth No.

Head Diameter = Primitive Diameter + (2 * Module)

Foot Diameter = Primitive Diameter - (2 * Module)

Remember that any gears in mesh must have the same module.

Drawing the Gear's outline

Draw a circle, then assign its diameter to "Head Diameter".

Adding a Bore (Optional)

If you want, you can add a bore...

Just add another circle - of any dimension - inside the outer one.

Adding a keyway is optional too...

Just draw a rectangle on the smaller circle, trim away its lower side, then add some dimensions.

Don't forget to make the 2 sides equal, so that the keyway is exactly in the middle.

Extruding the Sketch (Base)

Now extrude the selected contour in any dimension you like, but remember to be realistic.

Drawing the Gear's teeth inbetweens

Draw 2 circles, then assign their diameters to "Primitive Diameter" and "Foot Diameter".

Use Convert Entities command to convert the outer circle (from Step 2).

Change the middle circle into a Construction Geometry.

Now draw a vertical centerline from the centre to the outer circle, draw a "3 Point Arc" as in the image, then mirror it about the centerline.

Don't forget to add a dimension to the one of the arcs. I made mine 15 in this case, but I might need to change it if I changed the Module.

Now draw another centerline from the centre to the middle circle, mirror the vertical centerline about it, mirror the first arc about the mirrored centerline, then assign the dimension between the 2 mirrored arcs' upper tips (this one and the one mirrored before) to "Pitch".

And finally, assign the angular dimension between the upper tips of the 2 arcs at the left to 2.5°, then assign the angular dimension between the lower tips of the 2 arcs in the middle to the same degree (by typing "=", then clicking on the intended dimension).

Now your sketch must be fully defined. If not, then check your steps again (I know that this part is hard to follow, but this is what makes it easier to edit afterwards).

Creating the Gear's teeth

Now create a circular pattern, then assign its Number of Instances to "Teeth No.".

Improving the looks (Optional)

Start by drawing this line on a plane perpendicular to the plane used to draw the Gear.

Now draw the sketch in the image.

Make the 2 arcs equal in both radius and length, add a tangent relation between the arc and the vertical line ,then add tangent relations between the 2 arcs and the line between them.

Now draw a centerline (the orange one), mirror the sketch about it, then assign the dimension between the 2 mirrored sketch's upper tips to "15".

Now draw a horizontal centerline (the blue one) to act as our axis, then revolve the sketch to cut the model all the way through.

Now your gear must look like this: