Clamping Veneer Core to Plywood
In your opinion, what would be the best way to tackle making the top thicker? Also, I do not have a vacuum press yet. I would be gluing this up with clamps and weights. I do it for small pieces all the time but have not had the opportunity to do this for something large. Can I get away with gluing this top without a press? Any suggestions would be greatly appreciated.
From contributor P:
Contributor D - are you throwing out 100 psi as being required for clamping? I ask this as a vacuum at sea level only allows for atmospheric pressure of 14.7 pounds per square inch, which does well. Your suggestion of sticks to ceiling is excellent but I may add to put a piece of ply that goes from joist to joist to stiffen the ceiling.
From contributor D:
Not looking up the pressures needed, which I am sure were posted here on this site, I threw out 100psi. I just arbitrarily came up with this number. A new RF press we have I know recommends 180psi for mahogany, which is maybe 70% what we work with.
I know this sounds high. We'd purchased an old press years ago which used an air bladder and our compressed air for side pressure (about 110psi). We used it for about six months and I can say it caused us much heartache. Every expert we brought in, from glue industry or press industry, told us to quit using this machine (from the 1970's), as the pressure was not enough. It was great for edge gluing butcher block type glue upís, as the total area was not that high, but when we used it for veneering door stiles (an area normally 96" x 5"), it was not adequate. This was hard learned (and expensive) experience.
Yes a caul at the ceiling as well is a great idea. I never thought of it as I work in a concrete building. Honestly I have no idea how in the world a vacuum bag magically works at 14psi. I have stacked entire bundles of lumber (about 3000 lbs) with a forklift over veneered door stiles years ago to see gaps and bubbles later, so I do not trust weights for applying pressure.
There are lots of folks on here who use vacuum bags all the time that could weigh in. I only have 13 years in a professional environment, but I am challenged every day in a successful company. I feel pressures of 100psi are needed.
From contributor P:
I had a little tunnel vision. A vacuum achieves near 100% equal distribution which is not possible with weights or even a mechanical press. Without any kind of press I suggest to use 3/4 plywood and glue very carefully using 1" screws every 6" in all directions. The screws would pull the pieces together and hold for the glue to dry. This should be done on a very flat assembly table.
From contributor A:
It seems to me that your biggest problem is not how to glue the two panels together, but that the resulting panel will be unbalanced and may warp. Also, you may be disappointed in the end result using veneer core ply for a table top. It often has lots of ripples. MDF core is better.
From contributor M:
If I had the same constraints, I would laminate a foam-core sandwich with 1/2" ply top and bottom and rigid foam insulation in the middle. Use a flat table, and a flat stiff platen, go sticks or weights, and a gap-filling glue that won't melt the foam. But, then again, a wenge dining table should easily pay for vinyl bag and a refrigerant recovery pump.
From contributor C:
Iíve learned that you can exert an enormous amount of pressure on a work piece if you use curved clamping cauls. You can make these with a band saw or jig saw by cutting a slight curve on a 2 x 6. These cauls apply pressure starting in the middle of the work piece so you know youíll have good surface to surface contact when the cauls are pulled straight. Make a sandwich of your work pieces, platens (MDF or ply) and the clamping cauls.
Start with a dry run to make sure everything will work and you have enough clamps. Apply glue to your sub straight; put some blue painters tape on the edges to keep the work piece from sliding around and place it between your platens. Then starting from the middle of the work apply the clamping cauls alternating either side.
I find it good to use MDF with Formica as the platens because glue doesnít adhere to it well and you can wipe it off with a damp rag or knock it off with a putty knife after itís dry. I would make a balanced sheet by gluing half inch ply to the other side as well and edge with solid stock. A nice thick top looks good on a dining table and adds to the heft which is important with plywood being so light.
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From contributor L:
Just an FYI on why vacuum presses work (and don't sometimes). As mentioned above the maximum pressure is 14.7psi (realistic perhaps 10-13 psi) which is not huge. The key is that (as mentioned) itís on every single square it so it adds up if combined and all it has to do is bring very flexible veneer close to the substrate (squeeze out any air and glue and flex the veneer if necessary). With todayís veneers that's hardly more than flexing paper so the main required is getting rid of air and any excess glue. That's also why spreading the glue carefully is so important and having an accurate caul. I would not expect 13 psi to be enough to flex 1/2" ply over small gaps. However I believe it could implode a 1/2" hollow box with a hole in it. For example a 1 foot square top on a box would be required to hold 13psi x 12"x12" = 1872 pounds. The difference is the cumulative effect of the smallish force over the span. This also shows why weights are not very effective.
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