These are tiny little ball-chain gears that work with 3.3mm and 3.5mm diameter ball-chain, and still fit on the NEMA17\'s 5mm output shaft. They are designed for the X and Y axis of a Mendel, though the X axis and deposition bed still need a little more work. Obviously, they can be used elsewhere.
The gears are designed to be \"spiky\", but the vaguearities of the printing and rendering process lop the spikes off and leave enough by accident to produce a functioning gear!
Print two gears and epoxy them on the shaft with an M5 washer sandwiched in between. Make sure the teeth are lined up on both gears. This gives a channel to guide the ball-chain down the centre of the gear, and grips the sides of the balls adequately. As the gears age, it will also stop the balls grinding their way too far through the PLA, though I must admit that my experience with ball-chain Z axis gears suggests this will not be a major issue.
You may wish to print all 4 in one go, to allow the plastic time to cool between layers.
Note: On a Mendel, you will need to install larger pulleys on the X axis corners as 3.3mm ball chain will not make it around a tight corner. The standard Mendel Y Motor Bracket is also thicker than the length of most NEMA17 motor shafts; one solution is to stop printing the bracket a little early to make it thinner or use http://www.thingiverse.com/thing:2011