Plenum

[nat_wheatley]

How deep do the grooves in my plenum need to be to be effective? I'm replacing my spoilboard and may or may not re-route the plenum. The current grooves are 1/2" wide, by a little under 3/8" deep. Plenum is the pattern that comes with the control software for the 4x8 table, and is a grid of 2 1/2" squares.

[Brady Watson]

Between 3/16 and 3/8" is adequate enough. The more CFM you have, the larger the channels should be. When I make a BradyVac setup, I go 3/16" deep so that it lasts longer. Since you are doing a dedicated plenum, I would go deeper, but no more than 3/8" deep since you'll take away some of the sheet's torsional strength.

-B

[nat_wheatley]

Thanks Brady.

I also have 1" bullnose bit that I could re-rout the grid with to get wider grooves. I may not though, unless it would make a noticeable difference.

[Gary Campbell]

Nat...
As Brady says, you dont want to risk cutting much thinnner than 3/8" or the vacuum will suck the bottom of your plenum up.

I just did one that was deeper, but took extra care to saturate the material and completely bond it to my table PVC with a mix of Vinylester Resin & West Systems 404 high density filler. After cutting 2 liberal coats of resin were put over the complete cut area. See this post: http://www.talkshopbot.com/forum/messages/28/35559.html?1229006132
Gary

[nat_wheatley]

Thanks to both of you for saving me from a potential nuisance.

[rb99]

Gary after reading some of the Z height accuracy problems people have are the spaces between the blocks on you table far enough apart to let the spoilboard sag?

Thanks,

RB

[Gary Campbell]

Richard..
As I mentions in the other thread, yes when the spoilboard is 1/4" or less we can make it sag... but since we found that out.. we toss it before it gets to that point. System performance sags as the spoilboard gets thinner, so we have a win/win.

We dont have any Z height problems. Especially caused by anything in the table or vac system. If there is an instability in the table or plenum system.. it is usually caused by using materials that are prone to movement with changes in heat or humidity. These movements usually cause delamination of ply materials or breakdown of adhesives.
Gary

[rb99]

Thanks for the explanation. I would have thought that even a 3/4" sheet would bow under the vacuum pressure but I guess the pressure is not quite that great?

RB

[Gary Campbell]

Richard..
Our vac hold down system works at a max of 9.5 inHg. This equates to a little under -5 psi. When evenly spread, 5 pounds per square inch will not bow many materials. The hold down works due to area. A 12" square has 144 sq. inches of area. This results in the equivalent of 720 lbs. pushing down on the material.

Add this to some friction of a coarse surfaced spoilboard and you can actually apply a good lateral and some lifting force against the workpiece.

Factor this same logic to a 4 by 8 sheet and you have over 23,000 lbs of applied pressure. Since the pressure is applied equally from all sides, top and bottom too, it doesn't manifest as weight on the table. And since it is evenly spread, there should, in normal conditions, not be any bending or bowing of the support structure.
Gary