Hi,
Here is my "improved" version of the Spur Gear model posted by Oldbelt. This version has tried to "englishise" the wording plus has 4 "CONTROL" values in the EquationEditor to control the layout.
Idea is by changing the values for "CONTROL_Module", "CONTROL_NumberOfTeeth", "CONTROL_PressureAngle", "CONTROL_ShaftDiameter", "CONTROL_ToothFaceWidth" the model will update to suit.
In the project tree there are a couple of "suppressed" sketches; these (when enabled) change the model by trimming the teeth above the pitch circle; and/or adding in a pitch circle solid ring; both of which I found useful in assembly placement. That said, once I gained confidence the model was doing the right thing, I found all it was way better to add the "PitchCircleRadius" values (from the EquationEditor) for a pair of gears and applying the total to an AssemblyConstraint that set the distance between the shaft centers; this worked well and allowed movement of the likes of bearing blocks around in the Assemby while retaining the correct spacing.
[Edit] I had another look at an assembly of mine that had a gear pair; my finding was that the spacing between the gears (in my projects anyways) was controlled by applying the distance (attained from adding the pair of "PitchCircleRadius" values) as a spacing of the shaft holes on the backing plate in the BackingPlate PRT file. The shaft holes in the gears are then aligned to the shaft holes in the backing plate within the assembly.
[Edit] If the suppressed "PitchCircleFace" objects in the Gear PRT files are enabled then the Gears can be mated together using an OutsideTangent contraint in the Assembly. The gears will then happily roll around each other if dragged within the assembly.
Cheers,
Norman