yet another vacuum table question....

[boomerize]

This is a GREAT site!!!! i wish I found this sooner.

Well, I am about to start making my own MDF zoned vacuum table. I came up with one question though....

I understand on making the seperate zones which can close off by valves as needed, sealing the top and of the plenum, and sealing the sides of the spoilboard to avoid leaks and hold the vacuum. If I have a vacuum table of 4'x8' and the sheet that is being cut is 36"x50" would the rest of the vacuum table need to be covered with a non porous material to not let the air leak in through the spoilboard?

Since the spoilboard is porous wouldn't the air leak in through the surface and travel horizontally into the area that is being used even though the other zones are turned off and there would be a loss in vacuum? Or does the vacuum only travel vertically through the spoilboard?

I would think that the zones would only be seperated and pull a complete vacuum vertically to hold down the item if the spoilboard was cut into pieces the size of each zone and the sides sealed to prevent leaks from other zones. I've looked at many very detailed vacuum table layouts which helped with the construction of the table but all of them use a single sheet of LDF as a spoilboard.

Could someone please help me out with this question? Thanks.

[ger21]

Probably depends on the cfm's your pulling. The more you pull, the less you need to worry about leakage.

[Boots]

(I would think that the zones would only be seperated and pull a complete vacuum vertically to hold down the item if the spoilboard was cut into pieces the size of each zone and the sides sealed to prevent leaks from other zones. I've looked at many very detailed vacuum table layouts which helped with the construction of the table but all of them use a single sheet of LDF as a spoilboard.)

You answered your own question.

[Boots]

If you use a full sheet of MDF or LDF on your table you have to have all the zones on that are covered by the sheet, seal the sides with a primer paint to prevent leakage, it dries quickly. You can also paint the surface then mill a small area of the paint off to hold your part or you could run a gasket into the spoilboard just a tad smaller than your part and drill an air hole through the spoil to give you maximum hold down.
A word of caution on using LDF, some particulates such as Corian dust can be pulled right through LDF and can ruin your vacume pump.

[BobF]

Maybe this will help.

http://shopbotwiki.pbwiki.com/Build-your-own-vac-system

[oldhack]

Originally Posted by BobF Maybe this will help. http://shopbotwiki.pbwiki.com/Build-your-own-vac-system

anybody have a cad file of that shopbot vacum plenum design? I guess it would be easy to make but why do it all over. I think that setup allows for anybody to shut off any section and that would strengthen the other areas suction. Im certainly going to try it when i get that far with my build.

[bill south]

Ok Guys;
I've read the thread, watched slide shows and still feel pretty much like a dumb as_. When placing the third layer in the shopbotwiki, they use trupan but don't show any drilling or slotting operations for the top layer. They mention to make sure it is sealed along the edges. Does this stuff allow air to be pulled through it???? I don't understand how it works or maybe I'm missing another step in the wiki!!! Can someone explain how this works? Just call my dummy!
Thanks.
Bill

[ger21]

Originally Posted by bill south Does this stuff allow air to be pulled through it????

Yes. You can even pull vacuum through regular 3/4" MDF, but it flows through the Trupan much easier.

[bill south]

Wow, that knocks my socks off. I thought MDF was much more dense than that! Oh well, guess I'll give it a try with this new router build!
Thanks again Gerry.
bill

[Boots]

Just a reminder that MDF has a coating on for ware and it is rather hard and somewhat shinny. To get maximum vacuum you have to machine this layer off from both sides. Then cover any un-used portion with some scrap wood or MDF to get maximum vacuum on your parts.