Fabrication» Really large gear teeth can be hand cut. I once made all the teeth for a clock with an angle grinder. Large teeth are easier to make because the distance between the shafts is less critical.
» When getting metal parts laser cut, allow a tiny gap between the teeth of a pair of gears (I use a 0.4mm gap for MOD 2 gears). The output gear will be a bit wobbly (backlash), but this is preferable to the gears being too tight (lots of friction).
Gear alternatives» I often drop the speed of a DC motor by 4 to 1 simply by putting diodes in series with it — the motor loses surprisingly little power.
» Don’t use gears to reduce the speed of an ungeared motor. A motor with a built-in gearbox will be less noisy and more powerful.
» For changes in speed less than 5 to 1, a belt or chain is often easier.
Resources» Pre-made gears of all types are available through various outlets such as McMaster Carr.
» Designing gears and gear trains is easy and fun with the websites Gear Generator and Wood Gears.
» An exhaustive treatment of gears and gear design, including how to choose the right gears and determining gearing calculations, can be found in the book Handbook of Practical Gear Design from CRC Press.
» The website 507movements offers an incredible assortment of gear combinations, along with various other forms of mechanical motion, from pulleys to levers to linkages.