A Couple of Veneering Upgrades

[jteneyck]

I use mostly shop sawn veneer for my work. Sometimes the veneer I make is not all exactly the same thickness. They may not be off by much, a few thousandths, but that's enough to give uneven contact between the veneer and substrate if it's on the bottom of the substrate, against the hard, lower caul. I had a couple of seams open up a year or so ago that I suspect was caused by that.

The solution is to put something compressible between the bottom caul and veneer assembly so that the pressure will be uniform regardless of any variations in thickness of the veneer. But you can't just use any old piece of rubber, etc. The material has to have the right compression characteristics to work with the approx. 14 psi of the vacuum. And the material should also be reusable, durable and, of course, not too expensive. After some looking, I found what looks to be a very good material at McMaster Carr. It's a 1/8" thick natural gum foam sheet (PN 8601K41), in medium compression which has 25% compression at 13 psi. That means it will have about 1/32" "give" at full vacuum. It also has quick recovery after the pressure is relieved. It cost about $5/linear foot at 36" width.

Here's a picture of a piece that I cut to fit one of my lower cauls.



I put a sheet of plastic over it to keep any glue squeeze out off of it.



In the bag it looked like this.





Did you notice the heating blanket under the lower caul? I use Weldwood Plastic Resin Glue for veneering shop sawn veneer. It takes 13 hours to cure at 70F, but only 3-1/2 hours at 100F. If I put one electric blanket under the lower caul and another on top the temperature will get to a little over 100F.



I leave it under vacuum for 5 or 6 hours to be assured I have given it enough time at temperature to cure, but now I can do two pressings a day instead of only one.

When I removed the pieces from press you could see where the plastic sheet had stretched as the piece was compressed into the rubber foam.



The foam had completely recovered so it is reusable.



Seems like a winner. If you use shop sawn veneer, or mixed commercial veneer that's not all the exact same thickness, you may want to consider using it, too.

John

[Paul K. Murphy]

Thanks very much for your excellent post John. Whenever I see something like this, something of interest to me, I pay very close attention. I think I probably pay closer attention than anybody guesses.
We've had this discussion on Woodnet before, and I feel sure it was you who brought it up. I commented as best I could, and I'll do so again. As I recall, last time this discussion came up, I mentioned that I had seen Paul Schürch (online) using a pancake of plumber's putty to even out pressure on a difficult, three dimensional lamination. He used it as the core of a sandwich with visqueen plastic (I think) as the bread. I thought at the time and I still think, the idea was brilliant. It's on his blog. Interested members can look it up. His plumber's putty pancake, in some ways, played the role of the old school sandbags. Sandbags were often used to deal with difficult, shaped laminations.
I know your problem is different. You're looking for performance on a flat panel with veneers which may be of different thicknesses. I love your solution, and I'm going to implement it soon. Let me share with you one of the ideas I had regarding this matter before I read this post. For the record, I think your idea is better, but I like having options. I wondered how I might address the problem of dissimilar veneer thicknesses. What I thought I might do was use two pieces of visqueen like an envelope. Inside, I could stack multiple layers of newspaper. With adhesive tape, I could have something that resembled a quilt. I thought something like that might work. That's just food for thought. Your idea is better. Paul Schürch used canvas when I studied with him in Santa Barbara.
Great post! Be assured, I'll use this information. Thanks.
Paul

[Ken Vick]

John let me have a piece of this material this week when I was pressing some shop sawn veneers. Sometimes the most elegant solutions to problems are the simplest and this falls into that category. This is easy-peasy to use and effective. A great inovation, John. Ken

[Bope]

John, What are you using for the bag? The bag in the pics looks big. I will need to press a 48" octagon which will probably be a one-off thing. I am looking for ideas for an inexpensive large bag.

[jteneyck]

Bope, I use a homemade bag. I'm cheap and make nearly anything I can, and a vacuum bag is easy to make. I used 20 mil clear vinyl that I bought from McMaster Carr, but I've since found a local awning fabricator that carries both 20 and 30 mil for even less money. Anyway, the bag in the picture is the only one I've ever made; it's still going strong after 15+ years with only two small holes over that time, which were easily repaired. I think it was this specific product.

The bag is about 54" wide by about 7' long. The two side seams were just flat glued about an inch wide with regular PVC pipe cement (with no primer first). I laid the seam on my bench, applied the glue in between, and then clamped a caul over it for a few hours. The penetration was homemade from two pieces of rigid PVC sandwiched glued to the bag. I had no real Internet access back then and didn't know about Joe Woodworker. Today I would use one of his hose connections.

The seal is really basic. I just roll the bag around a straight piece of wood a couple of times and it seals well enough. I keep saying to myself that I'm going to mill a wood clamp with a V or C shape, but I haven't yet.

Getting a 48" wide part in this bag would be hard as it's only 54" wide material. I'd either look for wider stock (like 60") or modify the seam design if I couldn't find anything wider. You could use a C shaped seam made from stock say 6" wide. That would give you the full 54" width as the opening with plenty of height capacity for your assembly to go inside.

John

[Bope]

John,
That sounds like an easy bag to make. If I can't find some wider material I could use two long pieces and glue them together along the long side then fold in half the long way to make a C and glue that edge. If I use a platen across the top instead of just fabric the bag seam should affect the pressing.

[jteneyck]

Maybe I don't understand, but isn't that going to give you a seam on the bottom, too? If so, your bottom platen might not stay flat under vacuum.

John

[Bope]

The platens I use are 3/4" MDF. I can't see the MDF caring about a 20mil bump. The table that I have been using to do my veneering on sags more than that in the middle. With a bag setup the vacuum pulls the two sides together not just push down on the top. With the seam there might be a slight low pressure zone right along the edge of the seam but the platen should disperse that force.

[jteneyck]

You probably are right.

John