Dust collection help

Guys,

I am attempting to make a down draft table for my CNC mill. Hopefully my description will make sense. I have started with a standard sub plate, 18"x 48" x 1.5" thick. The plate comes standard with a blind 1/2-13 threaded hole every 1-1/4" on center. I have modified the plate by drilling a 5/16" thru hole, thru the center of the 1/2" threaded hole. In addition to that, I have added a staggered 5/16" thru hole with a 1-1/8" x 60 degree CS...so there isn't a lot of flat surface for dust to lay. Make sense so far? I have made and installed 2" tall x 1-1/4" round risers to space the sub plate off of the bed of the machine. There is a fabricated sheet metal tray that bolts to the back edge of the sub-plate, and tapers to the front (18" dimension)of the sub-plate, where two 6" ducts are connected. Each duct has 1250 CFM draw.

It works relatively well, but there is a noticeable difference in draw from the back of the table to the front. I thought that I could help this out by inducing compressed air to blow across the holes at the back of the plate, inducing a vacuum, and directing the dust towards the collectors...anyone have any thoughts? I will upload a picture when I have service...

Thanks,
Mark

Compressed air won't work with your arrangement. Possible improvements:
1. increse the stand-off height of the subplate thus increasing the plenum volume
2. reposition the exhausts so they oppose one another
3. install a diffuser in the plenum (a perforated sheet metal panel should help
4. experiment with partial height baffles on the bottom of the subplate
5. restrict the hole openings closest to the exhausts

brucecu,

1) I cannot increase the height of the stand-offs; I will loose to much z-height in the machine.
2) I originally had the collection "funnels" on either end of the table but:
A) the table is 48" long x 18" wide..by pulling from the front, it is a shorter distance
B) with the funnels at the ends, it was a PIA to work with larger parts
3) If I didn't mention, there is a sheet metal "ramp" that allows for a 1/2" gap at the back of the table to the 2" opening of the funnels.

Obviously, the closer you can get to the source of the dust, the better chance one has to collect it; but it can't be cumbersome or a PIA to use. This setup allows the dust to be collected at multiple points, and the fact that gravity always works.

Thank you for your input....