When designing gears for FDM fabrication in ArtOfIllusion there are two essential things to bear in mind: The basics of conventional gear design, and the accuracy or lack thereof in the manufacturing process. So, before we get stuck in to the AoI nitty-gritty, here's an Express Guide to Gear Design.

Gear Design Basics

The tip of the gear teeth are not supposed to go all the way down into the opposing trough. If that happens the next incoming tooth tip smacks into the top of the opposing tooth. Contact between teeth is notionally along a point just over half way (typically a 1:1.25 split) up the teeth, and this is called the 'Pitch Line'. The bit above the pitch line is called the 'Addendum', the bit below it the 'Dedendum'. The angle of tilt on the faces of the teeth is called the 'Pressure Angle' and is normally about 20 degrees.

To provide mechanical advantage, contacting gears have different numbers of teeth. A little gear driving a big gear will increase the torque and slow down the speed of rotation. Gears typically have prime or at least odd or non-multiple numbers of teeth. This is so that the same teeth do not always press against one another, so distributing wear, dirt, oil and squashed fingers etc. evenly across all gear teeth involved.

FDM Restrictions

Gears made from ABS or Polymorph need to be sturdily constructed in order to survive. Little pointy bits will soon be broken off when the mechanism (inevitably) jams. Wear & tear is a fact of life, and so plenty of extra material is needed. In short, your gears will be chunky and inaccurate.

Finally, we need to attach some of the gears to a shaft. As the plastic is not that strong, this needs big, chunky collars to hold big grub screws, or in the case of this example a recess to hold a nut.

The gears described here are the ones used to raise and lower the turntable. The larger gears attach to studding, which goes up and down when turned in a captive nut on the RepRap carriage. The smaller gear is slipped on to the shaft of a stepper motor and turns the other 3 gears simultaneously. Because the accuracy provided by the thread is very high, and the amount of slack in the gears produces a relatively small error in displacement of the platform, some play in the gears is acceptable. In other situations this may be a big problem, and cunning solutions known as anti-backlash gears^? are required.

References

Basic Gear Formulas