Another vac hold down question

[jeffp]

We just purchased a 2001 PRT60144 to run cabinet parts on (it has two Z's). The setup came with a 5hp 3 phase vac hold down. The vac has a release valve set at 8". It does not seem like we are getting the kind of hold down that I expected. We are using a 4 zone plenum sealed on all sides and edges with a clear finish (by others), and a heavy coat on the top surface with an oil based paint. The spoilboard is a 3/8 mdf the same sixe of each vac zone, edged on all 4 sides with plastic laminate. I ran a small bead of silicone around the zone on the plenum, and screwed down the perimeter every 4 inches (I pre-drilled pilot holes) just outside of the silicone seal. When I shut down all but one zone, the vac will suck down rubber mats, but will not hold down a piece of mdf (6"x18"). The 8" relief valve pretty much pops after a few second of the vac being turned on. I cannot find any leaks in the pvc piping under the unit. When I put a piece of rubber over the release valve, the most it goes to is about 10", and it still does not hold my mdf down. Even if I lay mats to cover up all of the exposed spoilboard, it will still not hold the mdf tight.
While I am dissappointed in it, I am assuming that I am doing something wrong. Anybody have any ideas?
Thanks,
Jeff

[elcruisr]

MDF can be a challenge because it is as porous as your bleeder board. You are probably pulling a good vacuum but it is sucking right through your stock as well resulting in a vacuum loss. Have you tried adding tabs to your toolpaths and then trimming the sheet with a laminate trimmer afterwards? If these are long runs you could also use a vacuum table gasket as well. Don't cover that relief valve! You will overheat your blower and cook the bearings.

Eric

[Brady Watson]

Jeff,
Normally, that 5HP system is adequate for a 4X4 or 4X8 table. Your 60/144 is nearly twice the surface area of a 4X8' table...and consequently your zones are also larger than a 4X8. You will need more CFM from your vacuum OR you will need to employ some gasketing to seal off areas where you don't need the vacuum. Eric has a point with the tabs. You may be able to get by with just bridging the start point if you don't run too aggressively (taking too much off on the last pass or too fast). Check out the AllStar stuff...

-Brady

[jeffp]

I appreciate the input. Even when I close off 3 of the 4 zones, basically making the table
2 1/2'x5', I still don't get the kind of suck that I was expecting, even when putting down the rubber mats for filler. My thought was that the next time we cut the plenum, that I would make one zone 4x8 (95% of our work is a 4x8 sheet), and the others the remaining area, but I am not sure that this vac would do that. We will be cutting many different size parts on a daily basis, so gasketing would be a real challenge.
My other challenge is that I will not be running the machine myself, it will be one of our shop guys. I am attempting to make it as simple as possible for them. Our plan is to cut down to the melamine on the first pas, and cut the rest on the second. Hopefully, the parts will hold.
Thanks again for any advice.

[Brady Watson]

I have been processing 4X8' sheets of Trupan all week and just as a test, I took a sheet and milled a 1-zone grid into it (about 46 X 94). I then bored a 2.26" hole in my table base to accept the Fein hose. I put some 3/8" AllStar Grabber tape all around the perimeter of the grid and a strip in the middle so that it didn't buckle in the center by the thickness of the tape. I then flipped it over, turned on the Fein and surfaced the top flat like a spoilboard. It holds 4X8 sheets VERY well even when you cut all the way thru. The smallest part so far was 17" square and it cut it no problem using just the Fein. A few times I masked off unused areas with coroplast. If you paid attn to what I wrote...the Trupan is BOTH the grid AND the bleeder. If I can do this with a Fein and Trupan, you could definately do something like this with your setup. (meaning that you have enough pump...something else is going on) I made this setup so that I could take the entire grid off with no fuss and pop it in the sheet rack. I still screw things down to the spoilboard and chasing leaks from punctures in a bleeder isn't something that I want to do in the future!

You can make as many or as little as you wish. The default setup is 4-zones as it was purchased from SB. Nowhere does it say that 4-zones fit everyone's needs...So, you might want to try 1 or 2 zones with only a 4X8 area.

One problem that many people have is getting good MDF that is not all full of resin. The resin content varies from state to state and brand to brand (even batch to batch in some cases). Trupan is hands down the ultimate bleeder. I was at a customer's shop who had a 25HP Cascade and they milled 3/4" MDF down to 1/8" (!) and it still wouldn't bleed thru!

Check your vac when you turn it on...If it is above 2 or 3" Hg with all zones open and the bleeder clear of material, then this is indicating that either A) Your bleeder not porous enough or B) your filter is clogged and needs cleaning. Check for both.

-Brady

[harold_weber]

I'm surprised you can't hold an 8 inch by 18 inch piece of MDF with 8 inches of vacuum. We are talking about 8 inches of mercury aren't we?

Where is the gage connected that reads 8 inches of mercury - hopefully its in the vacuum line between the vacuum table and the filter?

If the gage is in the line between the filter and the vacuum pump then the filter could be plugged and the vacuum at the table will be considerably less..........

[jeffp]

good point, I will check the filter

[richards]

Jeff,

Eight inches of mercury corresponds roughly to 4-lbs of pressure per square inch. A 6x18 piece of sheet goods would have about 400-lbs of atmosheric pressure holding it down (except as Eric mentioned about the porousity of MDF). I like to have at least 200-lbs of atmospheric pressure pressing down on the things that I cut.

[beacon14]

I... milled a 1-zone grid into it... I then flipped it over, turned on the Fein and surfaced the top flat like a spoilboard...the Trupan is BOTH the grid AND the bleeder...

Dang it, Brady, just when I thought I had figured out how to make my new table, you go changing the rules again. I'm going to use that, if you don't mind.

[Brady Watson]

Go for it!

I found that my original base board and spoilboard was allowing vacuum to suck in between the layers...so I tool some AL tape and lined the inside of the vac port to seal it off. That made a big difference...as did the gasketing around the perimeter of the grid. The gasketing is 3/8" wide by the way...NOT 3/8" thick!


-Brady

[les_linton]

Brady,

lite or ultralite on the Trupan?

Thanks,

Les

[Brady Watson]

I only use Ultralite...for just about everything. It seems to work best as a bleeder compared to the other densities.

-B

[richards]

Brady,

You posted the info about your combined vacuum plenum/spoil board just in time. I was waiting for the polyurathane to dry on my new baltic-birch suport board/vacuum plenum when you posted. My plan had been to cut the vacuum plenum in the baltic-birch, seal the plenum with several coats of polyurathane, and then use a normal porous spoil board. My problem is that I have a 120-60 machine and I wanted to use the machine for both 60x60 baltic birch and 97x49 sheet goods. Your method seemed to be ideal, since I could fabricate any size of plenum/spoil board that was needed for any project.

Well, it works very well. I used standard MDF becuase Trupan is not available in Utah. I also used 3/8-inch wide Al*Star gasketing material around the perimeter and between the zones. My two Fein III vacuums are able to hold smaller parts than they were able to do with the 'standard' method (but they're still not strong enough to allow me to make one-pass cuts on cabinet parts.)

[Brady Watson]

Cool!

I've been cutting 3D all week in Trupan using mine. I keep a few sheets of Coroplast around to mask off any open areas. I'm sure that the fact that you sealed your baseboard gives better performance than my setup.

-B

[waynelocke]

Brady,
Did I understand your approach right? You milled the grid in the spoilboard, gasketed it and then surfaced the top. Your tabletop is just flat with the vacuum hole in it and you didn't seal the edges of the spoilbaord. How thick is your spoilboard?

It sounds like a great and flexible system which I plan to try.

Wayne

[Brady Watson]

1) Machine grid into 4X8. Stick gasketing around perimeter and in center to avoid buckling.
2) Cut hole in base board for vac hose
3) Flip over grid (grid side down) & turn on vac to hold down while machining top
4) Machine top portion flat to expose core


-B