I have a weird spoilboard problem.

[Steve knight]

Ok I have a 4x8 shopbot and two vac systems one runs on 120v and one 220v. I redid my table a month ago and when I was down to about 1/4" on the ultra light mdf I glued a 1" sheet on top (surfaced both sides) after it dried and I sealed the edges I flattened it with a 2.5" bit with the vac system on and my dc on so no dust is left on the surface. I flattened it a couple of times and all worked as usual. one of my vac motors started screaming and after a bit I found the level of vacuum with nothing on the table was way too high. I get max 7" or so of vac with all of my valves shut. well I was getting 5" with one zone open. I tested them all and they were all the same. all four open I still had way too high of vac. I made my setup so I could take it apard and of course did not find anything. I had to cut part of it apart to check it all and nothing I checked before my check valves and after and after the filter. the next day I redid the pipe to simplify the setup. it worked fine. I figured it must have been the filter box I removed enough though I checked and had airflow through it. then Monday I fire it up and same thing way too much vacuum. the only thing left was a sticky check valve. I removed it and replaced it with a piece of pipe and no change. The pressure is same with both vac boxes so it is not them.
so all that is left is the mdf. I don't know if a change in humidity causes it to change as much shop is pretty stable. I took 1/4" off today and it never changed.
The only solution can think of is to take the sheet off and put another one on.

[Dan Hintz]

I don't know how you laid it out, but I would think a layer of glue would be a serious impediment to pulling air through any material...

[Jerome Stanek]

Did you surface the old spoil board down to the plenum. If not and you glued the new board down to the old spoil board you sealed the table.

[Steve knight]

No I did not spread glue over the whole thing. I glued in the same places as the bottom sheet is glued to the plenum. so the glue does nothing to stop I use the machine to mark were to put the glue down. I learned this from the big guys and i have done it several times. it seems to make a more stable top over removing the spoilboard completely. Plus no chance of cutting int the plenum.
remember the top worked for a couple of weeks till I surfaced it with the vac system on and it started.

[Steve knight]

To throw a monkey wrench into the bad mdf I forgot that I used some of another sheet. I cut the top into oversize 2x4 pieces so it is easier to glue down. I had one come loose and get chewed up so I replaced it with another sheet. all 4 zones show exactly the same resistance. So now I am fully clueless.

[Paul Grothouse]

I think that gap between the two sheets (no matter how tight) is the culprit. It it essentially a break in the air path. For example I can't stack two sheets of ply on my spoilboard and expect the top sheet to be held firmly.

[Jim Underwood]

Just replace the whole thing with a single sheet rather than a glued up sheet. If that's the only thing that changed, then the answer is staring you in the face...
MDF is cheap compared to all the time you've spent...

[Jerome Stanek]

You can not get glue the new spoil board dow to the old one without cutting new channels in.

[Steve knight]

Just replace the whole thing with a single sheet rather than a glued up sheet. If that's the only thing that changed, then the answer is staring you in the face...
MDF is cheap compared to all the time you've spent...

I know if I did it that it would not work life seems to be that way. I shaved the table down to the first layer of mdf and no change when I was down to about 1/16" it had lost 2" of vacuum but that was it. the off and on of it is what has thrown me off. I have been wondering if it is humidity that may be causing it. my shop is pretty dry right now around 38%
I surfaced one side of the 1" ultra light mdf laid it onto the plenum and it shows 5" hopefully lower when I flatten the top but we will see monday.
so it has nothing to do with the two layers or the mdf.
this is one thing that makes life easier is to cut the mdf in 4 oversize pieces seal the inside edges with yellow glue then it is much easier to glue it on 1 at a time.