I just completed a downdraft table. I'd like to know what the optimum table hole size is. I took the easy way out and bought a sheet of plastic peg board from Lowes - 1/4" diameter holes on 1" centers. It seems to work okay, but I need some feedback before making the holes larger.
(Dust Collection Forum)
From contributor T:
My homemade downdraft table has 1" holes in 3/4" MDF, chamfered on both sides. Use good filters or you will create a dust pump! I like low micron felt bags and use 8 of them in my table. Furnace filters don't cut it.
Your dust collector may not be pulling enough volume. Dust collectors maximize air speed in the ducts to carry large chips. I use an HVAC type blower with a 1 hp motor, standard mount on top. They are designed for volume. It draws about 2400 cfm. The holes are about 1" and are chamfered on both sides. I have made custom clamps and bench dogs that fit my holes. The table is also used as an assembly table, as my shop is small. Heavy sanding or routing still creates dust, but it is cleared out in a minute or so.
This table is left on all day to continuously filter the shop air. My blower box is in the basement, so exhaust air is not a problem. You will need to think about that - it's a steady blast! Consider trying to create a circular air flow across your shop by the use of large ducts. The more filter area you use, the better the filtration and air flow.
Mine are small dust collector bags of the finest quality. Stay away from cheap bags, as they let the most damaging dust (the super fine stuff) pass through, and in fact make things worse! The bags are stapled and caulked into large holes in a sealed panel under the top.
I could have used a larger blower with a 1 1/2 hp or larger motor but I am trying to be cheap with the electricity! A 2-speed larger motor would be ideal.
I have a cyclone dust collector for all the machines. When the filter bags in the table need cleaning, I use a flex hose attached to the cyclone. This is best done with the table on to keep the bags stretched fully open. My helper cleans it about once a week.
The blower box in the basement.