In this set I'll show the "gearboxes" I cast and machined from aluminum for the drive motors. (I put "gearboxes" in quotes because there aren't any actual gears, just toothed belts and pulleys.)
The gearboxes were cast using the "lost foam" method, so the first step is to make the patterns. This is one of those times a CNC router comes in handy:
Patterns for two gearboxes (right and left hand):
The patterns then get sprues added and are coated in drywall mud, dried, and buried in dry sand. Then, just add molten aluminum:
After the aluminum has had a chance to cool and solidify, the raw castings are pulled from the sand (the burned drywall mud flakes off as the part is pulled):
The raw castings:
Obviously, one of the castings didn't fill completely (aluminum wasn't hot enough). Fortunately, all I had to do was put a new chunk of foam on the router and have it make me another pattern to cast.
The castings were then finish machined on my CNC Bridgeport:
The finish doesn't look that great, but it feels smoother than it looks.
Here's the other sides:
Part of the reason the finish looks so bad is because the aluminum I used just isn't very good, especially for machining. I call the alloy I used "mongrelloy" because its a mongrel alloy of just about any type of aluminum I can get my hands on and can fit into the furnace (i.e. lawnmower engines, lawn chairs, scrap extruded shapes, plate/sheet scraps, those razr scooters that used to be popular, outboard boat motor parts, aluminum wheels, old cookware).
But, I think, the main reason the finish is so bad is because I have no meaningful machinist training (I'm, a weldor by trade and training), so all of my feeds and speeds are just guesses based on various machinist calculators I've found online.
Top view of an open gearbox with motor (right), belt, and pulleys installed:
The pulleys provide a 4:1 ratio.