Friendly bet re: plenum table

[myxpykalix]

I have a friendly bet regarding which is the best type of bleeder board/plenum to have. The way mine is set up is a bottom sheet of 3/4 ply, 2nd sheet of ply has the plenum cut in it, and top is a solid sheet of 3/4 mdf. He says its better to have tons of holes drilled in your spoilboard and i say its better to leave it solid, who's right? I can see how having the holes will help with suction, but i can also see how all the holes will have places where vacumn escapes if your material doesn't cover the whole zone.

[richards]

Jack,
It depends. For universal work, I prefer your method, i.e. an MDF spoilboard that has been surfaced both sides, but without through holes. That way, even though you may cut entirely through the material, you might have enough suction to hold the remaining matherial.

On the other hand, when I cut small parts, or when I need to cut all the way through, I cut a template with a hole in the middle, remove the spoil board, lay down some All-Star tape on the template just inside the cutline (to hold vacuum), switch to my GAST vacuum, and cut the small part(s).

Both methods have their place. (However, if I didn't live in the tops of the mountains where the air is thin, and if I had a huge vacuum pump, I would most likely use your method exclusively.)

[patricktoomey]

Jack, I use a solid spoilboard with my 2 shopvac system. I tried the hole method once and it was a disaster. As soon as I cut through my material the air rushed through the holes and I lost pressure and the parts moved. With a solid spoilboard, I cut through plywood all the time with no trouble at all. If you had a big vac pump that could hold pressure even with lots of airflow I guess the holes would work but I don't think they would be necessary.

[jhicks]

We have a base of sealed MDF with the Plenum grids in it, Each grid section is then surrounded by allstar gasket, The spoil board is a sheet of 3/4" trupan on top of it. Then another sheet of Trupan on that.
After significant use we have surfaced the top spoil board on both sides down now to about 3/8" but by repeated surfacing the trupan gets thinner and provides better and better vacuum but still ennough for cut through and never worry about cutting into the base spoil board.
Once we get that 3/8" too thin we will use the 1st sheet and continue with the same process thereby always having one good spoil board progressivly getting thinner with use.
However for light pieces like foam letters or specific shapes we have used thin 1/8" thick plastic sheet like sintra PVC, with holes in it below the piece being cut.
In effect the PVC sheet is sucked down to the trupan in the solid areas , then the top piece of gator foam or whatever is sucked down through the holes below the piece.
No gasket tape or any other hold downs. Just make the holes in the thin PVC sheet as large as possible but offset inside the bit cutting path.

So its best for us to have both.

[patricktoomey]

Jerry, that's a really good idea with the plastic sheet. Do you have different sheets with hole patterns for different parts you normally cut or is it something more generic? I have some production parts that I get orders on regularly that would be worth making sheets for. I'll have to get some and give it a try.

[jhicks]

Patrick, just got back from vacation so sorry for the delay.
We started by using a single 1/8" piece of sintra for a specific project. It had fairly large letters we were making in 3d domed shapes. The holes were set up on a basic grid with 1.1/5" spacing between them. Then we laid the artwork of the letters over that hole grid and removed or relocated those we didnt need or in wrong position. The holes were 1,1/2" diameter and were below both the letters to be cut out and the material to remain as scrap.
Both held in place find but they were fairly large pieces.
After this we had some foam letters to cut and used the same grid interim plenum. It also worked fine even with some holes extending between parts and scrap so i think it would work as a fairly generic grid depending on the detail, torque, and size of the pieces as long as you have as much suction in the surface area as you can get on the part being held. This is a shot of the pilot in poplar but we ended up making 40+ letters for this sign in 1" Medex sheet stock. Height is about 14"
Good Luck.
8534