Seeing some of the rolling beds makes it seem possible to have both downdraft and water separately with one sitting over the other and just roll out which ever one you don't use. I think theres probably a better solution in a combo bed where you can drain the water and turn on the fan. I'm thinking of evenly spaced ducts coming in the bottom of the table. You could just flood the ducts to keep it simple but valves would be good for having a Zoned table anyways.
Maybe we should have the argument first of why would anyone want downdraft instead of water:
Water seems great but its not for everything. There does seem to be two types of water table users users(the work piece is completely submerged or touching it users and the water is just below or way below my work piece users) While there is no doubt that touching or a hair below is best it brings up the point that water still works pretty good as long as you can just get the dust particles to touch the water before they get out into open shop air. Same principle as an oil bath air filter on an engine. A *nonflamable* oiltable would probably actually be pretty good. Anyways, the point is, maybe air circulation should be engineered(fans) to circulate the air down toward the water for times when the workpiece can't or shouldn't touch the water. Nonetheless I think a downdraft table would sometimes just be better. Here are the scenarios:
Tall work pieces: If i want to cut holes in the top of an ibeam or side of a box tube that is deeper then my water table. This is where i think removable slats would be good, but that just means you can cut even bigger pieces that will still stick above the top of the watertable.
Pipe Cutter: I was thinking of building a partially submerged rotating axis, but downdraft would probably be smarter.
Router:
Most people run those topside bristled vacuum dust collectors(see pic). But that is only good for 3axis work and you can't see what the heck is going on under there.
Aluminum and Hydef gases:
Alot of people say a water table is dangerous for aluminum. As a childhood pyro I'm very skeptical of being able to build up enough explosive gases of any kind under a flat work piece with water barely or almost touching it to cause any body or machine damage-theres just not enough volume if your water level is correct. When i'm bored in the shop I blow things up with my oxyfuel torch and it doesn't take much, a bottle cap is deafening, a spray paint cap will take off a finger and a 2 liter bottle could take off an arm but thats with an optimum & concentrated mixture. My only concern would be moving the sheet or distorting it during cutting. I'd love to see proof otherwise. That being said, I'm not so sure i'm going to have an enclosed water chamber. "Oh cool, I can fill up my bathtub with my air compressor" just isn't cool enough to me.. I'll probably use an open top with a pump. Is anybody filtering their water?
So what do you think?