Solid wood on Plywood

[DavidF]

Maybe a question for Howard A, but anyone can step in...

"If" one had to glue solid wood to 1/2" ply for a project what would be the maximum thickness of solid that would have a sufficiently small amount seasonal movement to be acceptable? 1/8" seems pretty common, but engineered flooring products often talk about a 6mm (1/4") solid wood layer on top of the ply. Any real/practical experience out there? The web often just recycles information so if anybody has actually done it and either not had any problems or did have problems?

TIA

 

[Joe Scharle]

I've put 1/4" walnut over 1/2" ply on a box top. Haven't heard any complaints yet. I felt that for a small area I wouldn't have a problem. But for a table top, I used veneer using the same logic.
I posted mainly to be kept in the loop.

 

[DavidF]

Thanks Joe. BTW, the piece is 70" x 19"

 

[fergy]

It would also depend on the glue being used...some are better for this than others.

Also tagging along...

 

[timf67]

I've never laminated more than ~1/8 thick to plywood. The stress in the solid wood layer increases as a function of the thickness of the solid wood layer. I saw a finite element analysis model that showed a 1/4" thick piece of flat-sawn oak can generate ~3500 psi of stress when constrained on one face and subjected to outdoor humidity variation. That is at or above the holding force of most glues and it is typically more force than the wood can handle. The engineered flooring folks can get away with 1/4" of solid wood since the planks are reletively small and they are well sealed on both faces. If you want to do this lamination, you need to seal the exposed face as best you can, and keep the solid wood layer as thin as you can. Also, It is important to remember that flat-sawn lumber will "move" about twice as much as quarter-sawn lumber in response to humidity.

 

[Dutchman]

70 x 19 is big. Make sure you take into account the size of the lamination with any replies you get. I have always tried to use the rule "balanced construction". I have no practical experience with gluing a solid wood piece that size (70 x 19) to plywood on only one side, but I would bet it will not stay flat. I think if you glued a piece to each side, you would stand a better chance of it staying flat. (balanced construction)

 

[DavidF]

Yep, it is big, hence the question. It is not a free panel so to speak. The ply is captured in a 1" deep frame that lips over the back face of the play and is glued to the underside of the solid wood so any bending forces are restricted. The question lies in the width wise expansion of the solid top causing fractures within its self or in the bond to the plywood or if shrinkage occurs, in any buckling of the solid wood. I can make it what ever I want at this stage, but I would like to keep it as thick as possible. By necessity for the design the frame also breaks all the conventional rules of cross grain construction by being glued to the solid wood across the ends. This is a fairly unconventional dining table design so is pushing the normal limits of what can be done. There is no apron to support the top so there has to be built in rigidity. That means a thick top. There is another design element that comes through the top also potentially restricting cross grain movement, but still I would like to keep the solid wood as thick as possible. If that is 1/8" then so be it, but if a 1/4" will work, even better. I don't really want to be the first to try going bigger than 1/8" on this particular project so if somebody comes back and says that "yes, they did that and it's still fine 2 years later" then I might do it. If they say "I did that and it failed within a year" then I won't. If everybody says "don't know" then I'll stay safe and go to 1/8" But then make a test panel for my own future reference in case I come across it again.

There is enough expertise on the forum to give me a direction I think.....

TIA

 

[Bigdog72]

Sorry, David, have to do it!

:worthless:

 

[FredP]

I have not done it but...... I had an old school teachers desk untill not long ago. the top was 30" x 60" and was constructed as you describe. 1/2" walnut glued to 3/4" plywood. It seemed to be fine for it's age. there were some stress cracks but I can't say for sure if they developed over time or if they were that way from the start. the wood looked like #2 common. It had knots and worm holes in it. If you need thickness for the top you could double layer the plywood and then use thinner veneer for the finish layer. I would think that is a better option.

 

[DavidF]

Sorry, David, have to do it!

:worthless:

These pictures do not show it but the angled short legs have through tenons that come right through the table top - the other source of potential cross grain construction

 

[fergy]

Is that a photo or a rendering? This monitor isn't the best for that and I'm too lazy to walk over to my design station.

If it's a rendering, what did you do it in?

 

[DavidF]

Is that a photo or a rendering? This monitor isn't the best for that and I'm too lazy to walk over to my design station.

If it's a rendering, what did you do it in?

Yes, it's a rendering from Solidworks with Photoworks add in - good isn't it....

 

[jlwest]

I agree with Fred. Use thicker plywood.

Jeff :icon_thum

 

[timf67]

After seeing the "pictures," I am going to change my mind and say you shouldn't use plywood at all. Something that nice looking deserves a solid wood top :icon_thum

 

[scsmith42]

David,

Several years ago I did some research on wide plank flooring for internal applications, and what I discovered is that a 3/8" hardwood board glued to a 3/4" or thicker plywood subfloor would adequately survive most internal RH% variations.

I think that Tim's comments are right on (and he did a great job explaining them) regarding external RH% variations (40% - 100% RH seasonally), but inside a modern home you're not going to see 100% RH, or even close to it (unless you're in a bathroom and like long, hot showers!).

Unfortunately, I don't have any personal experience to back this up - just some research a few years ago of people that had "been there -done that" so to speak with ultra-wide plank flooring.

Scott

 

[DavidF]

Thanks Scott, the engineered flooring world was where I gathered most of my info too and came to the conclusion that 3/8 would probably work, but subject to anybody actually having done it I wasn't prepared to experiment on this project. The whole issue started from a rogue piece of maple that would not surface correctly until it went too thin, that initiated the requirement for the plywood. I had a scheme all set to deal with the cross grain construction until warpage set in while sanding the plywood/solid combination before I had the lipping on which was the key to the whole thing. That meant I had no thickness left in the solid to screw into from below to deal with cross grain movement, and that led to the question... So I already had the ply and the solid glued together at the middle 5" when I asked the question!! Long story short, 3/8" worth of sawdust is in Amy's dust collector!!! The major downside of this episode is that an edge detail I wanted to have has to be left off because of the new construction. I also lost a small amount of the rigidity by losing the thickness in the solid so will retrospectivly add another layer of 1/2" ply to the bottom. For my own education I will be making up some test panels just to see what IS possible with a plywood solid sandwich, I'll publish the results in a year or so...

 

[Herebrooks]

Great Piece David! I love it! I think I understand why you would want a veneered top. It allows you to control the top surface pattern better and it can give you a more stable and thinner plank. Besides, it saves alot of premium lumber being used in one table top.
I've been vacuum bagging since the early 80's, and as far as I know have had no failures. There are a couple of things to keep in mind. You are probably aware of them, but I'll list them anyway. The thinner the veneer, the less force it has to change dimension with climatic chances. I always saw my veneer to a little over 1/8" and sand it(wide belt) to about 3/32 or just under an eighth. I only use plastic resin glue because of its static nature when it dries(very brittle,thus allowing no movement). I learned my lesson early with a flexible glue and commercially thin veneer, It crazed when the heat came on in winter. The substrate for your veneer should be something that resists movement. I've used plywood, veneercore(what the old guys used to use, probably made it themselves), and mdf(which all the young woodworkers at the college of the redwoods were using in the late 80's.If it got past Krenov......
Probably the most important thing is that you put a balancing veneer on the bottom of your table. It doesn't necessarily have to be the same wood, but something similar and in the same grain direction.
Did I say that I really liked the piece!!!!
Hope this helped.
Bill

 

[fergy]

Yes, it's a rendering from Solidworks with Photoworks add in - good isn't it....

What the heck are you using solidworks for? I'm assuming not just for woodworking projects. That's some pretty beefy software for a normal person.

 

[DavidF]

Great Piece David! I love it! I think I understand why you would want a veneered top. It allows you to control the top surface pattern better and it can give you a more stable and thinner plank. Besides, it saves alot of premium lumber being used in one table top.
I've been vacuum bagging since the early 80's, and as far as I know have had no failures. There are a couple of things to keep in mind. You are probably aware of them, but I'll list them anyway. The thinner the veneer, the less force it has to change dimension with climatic chances. I always saw my veneer to a little over 1/8" and sand it(wide belt) to about 3/32 or just under an eighth. I only use plastic resin glue because of its static nature when it dries(very brittle,thus allowing no movement). I learned my lesson early with a flexible glue and commercially thin veneer, It crazed when the heat came on in winter. The substrate for your veneer should be something that resists movement. I've used plywood, veneercore(what the old guys used to use, probably made it themselves), and mdf(which all the young woodworkers at the college of the redwoods were using in the late 80's.If it got past Krenov......
Probably the most important thing is that you put a balancing veneer on the bottom of your table. It doesn't necessarily have to be the same wood, but something similar and in the same grain direction.
Did I say that I really liked the piece!!!!
Hope this helped.
Bill

Thanks Bill. The top currently is sitting at 1/8 give or take and will no doubt stay there except for the final sanding from the current 120g to the final finish. It is on top of two thickness's of 1/2" maple ply with a lipping. There will not be another veneer layer on the bottom so I hope it stays flat. The lipping does turn over the face of the ply for extra resistance against bending... we shall see..

 

[DavidF]

What the heck are you using solidworks for? I'm assuming not just for woodworking projects. That's some pretty beefy software for a normal person.

Yep, just for furniture design. I have been using a variety of 3D programs for a very long time for woodwork and got the opportunity to get Solidworks for a very reasonable price so use it exclusively now. The renderings are a real bonus and I can really study the assembly before I start to build and never build a prototype other than a virtual one.