Balancing Veneers
How big a thickness variation front to back might cause warping problems in a veneered panel? Well... maybe it's better not to risk it. July 28, 2006
Question
I plan on using some nice E. maple veneer (1/42") on the front of a project I'm doing and for the backer I found a very good deal on plain E. maple (1/32"). I'm concerned about warping potential because in all my previous vacuum bagging work, I've used veneers of exactly the same thickness. Am I right to be concerned or do you all expect there to be minimum problems? Any suggestions? These are doors that I'm making and I'll be using 3/4" lite MDF as the core.
Forum Responses
(Veneer Forum)
From contributor F:
Hard to say. If I had the time and enough veneer, it might be worthwhile to glue up a test and see what happens.
From contributor J:
I think that variables like species, cut, etc. are more important than the eight-thousandths difference in thickness you're talking about.
From contributor C:
It will warp. 8 thousandths is 1/3 of the thickness of the 1/42 thick face. You will have 25 to 33% more wood stress on the back. Also, MDF does not have the stability of flk and is more prone to cup and warp. You could consider laying up a 5-ply (core, crossbands, then face veneer), as this tends to create a more stable panel. A cheat would be to press them and sand 8/1000 off the back, but you would need a very accurate sander. My personal choice would be to get the right material in the first place; for the few bucks you may save, a negative outcome would be far more expensive. Do it right in the first place.
From contributor J:
Contributor C, I wouldn't quarrel with your experiences, but I am curious about them. We all know that things that work in some shops don't work in others. We've never had a stability issue that could be attributed to thickness difference, though I know that (for example) AWI is quite strict about backing with identical veneer. What kinds of conditions did you have problems under (press type, glue type, etc.)? I wonder if certain procedures could accentuate the problem.
From contributor C:
We primarily hot press and use UR glue and for some applications use a PVA. If you look at the basic mechanical aspects of a 3 ply panel, a thicker veneer has more pulling power than a thinner one and also will absorb and/or dry out more moisture than a thinner one. This creates pull and may cause warp. We have seen about every kind of response to mixing of woods and thickness. Sometimes nothing happens. As a professional, I prefer to follow the rules and not have to worry about a product in a year or 5 years, which is sometimes how long it takes to show up.
At one point, we made a line of 12 ft conference tables and would use a maple back on a cherry faced top (the grain ran the short direction); the reason was to create a crown in the center of the table so that when we put it on the bases, the table would flatten out and not sag under its own weight. In that case, we counted on warp and it was a successful specification. But to answer your question, I have seen many layups move, especially things like doors where there is no structure to hold them flat.
From the original questioner:
Thanks to everyone for the advice. I'm going to reallocate my veneer and save the 1/32" maple backer for a future project and I'll be sure to use 1/32" for the front. I bought some 1/42" maple for the backer on this project, which matches the 1/42" on the front of the doors.
