Here are the initial steps:
- Mount the lower roller to the motor shaft.
- Mount the lower brush assembly to the motor housing.
- Enclose the lower unit.
Do not use wood for the enclosure: Wood is easy to work with, but it does absorb moisture from the air, which can adversely effect the Van de Graaff generator. Make the case from plastic -- Plexiglas from a hardware store works well. Remember to leave access to set the belt to your roller and to leave an opening at the top to route the belt to the top roller.
For the column assembly, I used a 6-inch-diameter, 32-inch-long piece of PVC tube. I mounted one end of the tube to the top of the housing and drilled holes in the other end of the tube. You mount your upper roller to the top of the tube via a bolt or rod through the drilled holes. Depending on how your upper roller mounts to the tube, you may want to put the belt on the roller before you mount it. After the top roller is mounted, you can then attach the other end of the belt to the lower roller and close your housing.
Finally, you are ready to mount the sphere and upper brush assembly. To do this, I cut a hole in the bottom of one of the salad bowls. I then used conductive "metal bond" to secure the braided grounding strap to the inside of the bowls. Next, I mounted the bowls to a 6-inch to 4-inch PVC reducer. I inserted the 4-inch end of the reducer into the hole in the bowl and then coated it with silicon caulk. I then routed the brush end of the grounding strap to the inside of the reducer and mounted it (you may have to play with this a bit in order to get the best separation distance from the upper roller assembly).
At last, all you have to do now is put the reducer over the top of the PVC. Make sure that the brush is facing the belt and on the same side as the lower brush. You have your very own Van de Graaff generator!
It's a good idea to ground a piece of wire to the motor housing, because you can then touch the other end of the wire to the sphere when you turn it off. This will keep you from getting a nasty little pop when you touch the switch. Also, you may want to discharge the sphere without turning it off. Keep in mind, though, that if you do not hold the end of the wire during operation you will get a little pop when you pick it up.