Expert: Jamie Yocono - 11/4/2004

Question
Thanks Jamie for the quick reply.  As a followup, I'd like to understand the reasons for not gluing the 1/4" to the 3/4".  Not to argue the point much.

You pretty well have the design in your mind with the exception of that the table top sits on top of the leg/apron frame----extending over it.  Here's an example at:  http://www.buildeazy.com/coffee_lounge.html

I understand the ideas of the importance of materials being on both sides of a ply for stability and warping.  What I don't understand is if you have a 3/4" ACX plywood which is already stable and you add to that (via glue) a 1/4" MDF veneer both sides that is already stable on top, why you'd have problems.  Of course, I have doubts otherwise I'd just go for it.

If indeed I got buckling or warping then I'd be in deep ***@!.

So, using your ideas I may have to either groove my edge trim to allow the 1/4 MDF to "float" on top of the 3/4" ACX (I hate that idea because I wanted a clean straight flat plane from the top to the edge).

or

Glue another piece of MDF to the bottom side of the ACX.  That makes no sense.  I think part of the reason for same material on two sides of a ply is because different materials absorb moisture from the air differently, as well as temperature--expansion and contraction.  So if I glued MDF on both sides of the ACX it seems you would still have a problem between the two-----but now on two sides.

You know I called a plywood company and they said it probably wouldn't be a problem----but I wasn't talking to an engineer either.  Plus it's not their time and money.

Just trying to think this out.  Thanks again for the ideas.

Mike

P.S.  You might wonder why I just don't get a 3/4" version of the same Lyptus plywood.  Money !   The 3/4" version is more than double the cost of a 4x8 1/4" which sells for around $50.00

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Followup To
Question -
Hi Jamie:

I'm in the process of building a simple coffee table with a wood top.  This top will be attached to a apron/rail/leg frame. It will be 2x4 foot. For the top, I'm planning to use 1/4" MDF Lyptus veneer 2 sided (Lyptus is a hardwood simliar to cherry).  To give the MDF Lyptus veneer support, my hope was to glue it to a 3/4" piece of AC plywood.  Finally, I was going to attach soild Lyptus pieces to the edges.

Now the questions:

1.  Would it be better to use particle board under the MDF Lyptus veneer for dimensional stability----instead of the 3/4" AC plywood?
I was worried about its weight.

2.  What about the process of gluing the MDF Lyptus veneer to the particle board or plywood---can you think of any problems?  I was thinking of just using a construction adhesive like "Hard as Nails...etc.

3.  I know that the MDF Lyptus veneer is stable due to the veneer on two sides, so I just want to make sure that when I glue it to a piece of particle board or plywood, I don't get into trouble.

Thanks for your ideas.


Answer -
Hi Mike,

This turned into a really long answer, so grab a cup of coffee and put your feet up!

Those are all really good questions about table making, and I hope I can shed some light on the process. I consider myself to be somewhat of an expert about tablemaking, having built them nearly exclusively for the last 10 years. I'm a little unclear about your intentions, but it sounds like you're using a veneered 1/4" MDF panel and are wondering about beefing it up so that it will not flex. I hope I understood that right!

The thing about panel goods is that you need to treat both sides of it the same manner so that it does not warp. For example, if you were using a 1/4" panel for a drawer bottom or the back of a cabinet, you wouldn't just finish one side of it, or else it would warp. Now in a drawer bottom or the back of a cabinet, that might not be that big a deal, but in other areas, it might be a disaster. So gluing a sheet of anything, be it MDF or plywood to the 1/4" panel is going to result in some warping. That is going to be a problem.

Since the top is relatively small, the warping might not be very evident, but I do think that warping would be very apparent in a large table top. It's hard to say, but I think in a 2 x 4 top, you might experience it.

I'm trying to come up with some sort of design in which you could use the 1/4" panel for your top, and yet NOT adhere it to another panel. One suggestion would be to beef up the panel around the perimeter edge, gluing a  3/4" by 2" strip of wood all the way the top. So your top would be 1/4" thick in the middle, but at the edges, it would appear to be 1 inch thick.  Now your panel would still be weak in the middle, that's for sure. It would flex and surely crack if there was no support underneath it. One solution would be to add some solid wood stretchers under the top for support.

But that's where you can do some creative designing on your table base. Here is a suggestion: on your finished base (legs and aprons), attach a panel, the same thickness as the edging strips. The size of it would JUST fit in the area of the tabletop that is not beefed up, the area with only 1/4" thickness. (I hope you're able to understand that.) That way, the fragile top could sit right on top of the plywood or MDF that is fastened to the base.

As far as joining them together, I feel like you could use a minimal amount of glue, as gravity will mostly hold the top in place. I wish there was a way to screw it down easily, but there really isn't. So perhaps you could use some flat metal plate that would attach the exterior strips to the plywood base. So the only fastening device would be on the underside, out of view.  

Or.... instead of a metal plate, you could even make it with some sort of a wood plate, so conceal the plywood or MDF entirely.

Another way to use the 1/4" panel is to make a "sandwich" of it, along with a thicker panel to strengthen it and keep it from flexing. No glue, just put the two panels together. Then figure out a way to "house" the entire panel, either in a dado (groove) that is on the inside of the apron/leg structure.  If the panel sandwich is really wide, you might want to put some sort of a stretcher across the aprons connecting the long side, for additional support.

Even another suggestions is to make a tabletop using that same sandwich of the two panels, and then adding a solid wood edging around it, like framework that would keep the 2 pieces together. Sort of like a large Raised Panel Door. Only, it's a tabletop.

Either way, with all of these suggestions, you could use solid wood edging to conceal your MDF edges.

Those are just 3 solutions I can think of off the top of my head, but after reading this, you might come up with one or two of your own. The key thing to remember- gluing 2 panels together isn't a good idea. If you want to strengthen it up, either add some sort of reinforcement by using stretchers or solid panels to keep it flat and unable to flex.

Good luck, I hope this helps. Feel free to write back with any other questions you might have after reading this. This sounds like it's going to be a nice looking table.

Jamie Yocono
Wood It Is! Custom Cabinetry
Las Vegas, NV


Answer
Hi Mike,

You're putting a lot of thought into this table project, which usually leads to having a successful outcome.

Your question about understanding why one shouldn't glue 2 panels is a common one. Most would think that laminating 2 stable panels together would result in one thicker, stable panel. But it really doesn't work that way. The panels are individually pressed and glued, resulting in a stable shape (flat) and adhering anything to only one side results in an imbalance for that panel. Moisture isn't absorbed (or released) at the same rate, and heat affects it, too.

One of the biggest problems with wood- be it solid or dimensionally stable, is that the environment affects it. Too much moisture and it will swell, and too little will make it shrink. Plywood (and MDF) is stable because of the manufacturing process. Plywood, for example, always consists of an odd number of plys, each perpendicular to the one before it. So as one ply wants to shrink one way, the one above and below it impede that process. Add into the mix that it is laminated under a great deal of pressure, and you understand why those plys can't really move anywhere, so it is considered a stable sheet.

All that is to say that if you glued your 1/4" Lyptus plywood to a sheet of 3/4" AC plywood, you're not only changing the moisture balance of the top sheet, but of the bottom one, as well. Now your top is relative small, and there's a good chance you could get away with it, but you would be wise to try to control it in some way. How?

Good question!

I can think of one way that would probably work, but it's not an ideal solution. The only reason I'm mentioning it is because I do understand the design ramifications if you have to use a frame around your panel. That "flat" and sleek look you want is mostly destroyed if you put a frame around it, and I understand why you don't want to do it.

So here's a thought....and I'm only suggesting is because (for the third time)- your top is small and probably not going to warp that much.  You could make your table base as planned, and then attach the 3/4 sheet to it, using whatever fastening method you had planned. Personally, I would want to keep that panel very flat, and since you don't really have major movement issues to deal with, I would probably screw down from the top right into the aprons. That would start you off with a flat tabletop, reinforced to stay flat with all the fasteners.

Then, you could add the top 1/4" Lyptus plywood, gluing it down to the 3/4" sheet. I would try to use some sort of glue that is slightly flexible, allowing for a little movement. I think you mentioned Liquid Nails or something like that, and while that wouldn't be my first choice (because it's so thick) it would probably work. Try to spread it thin, or use a flexible yellow aliphatic glue, like Titebond, which is thinner and flows better.

Normally, you would make your entire top separately, and then attach it to the base. But you're going to attach part of the top first, and then laminate your panels together.  It's a little different from normal construction techniques.

I think the key here is to attach the thicker sheet really well, to discourage movement. The wood on the apron is positioned vertically, which will stabilize the plywood, and the help keep it flat. It would even be helpful if you add a middle stretcher so that you have something to screw the panel into in the center, helping to keep that area flat, as well.

Edge the whole top and keep your fingers crossed. It's not the right way, but it will probably be fine, especially if you can keep it away from the sun, or a furnace/AC vent. Those things facilitate movement.

On a related note, I was building a workbench in my woodshop a few years ago; I needed an oversized worktop to accommodate a desk I was building. And I had this grand idea to put plastic laminate on the top of it, so that when I dripped glue on to the top, they would chip away easily. I built the base, attached a 3/4" sheet of plywood to the top, and then adhered a sheet of Formica to the top, trimming the edges. About a week later, top was so bowed, you could roll marbles off it, as it had a huge crown in the middle, probably an inch higher than the edges. Since it wasn't "fine woodworking", I managed to clamp the offending top down and shoot a few huge nails into the underside bracing, thereby holding the top in place. But I learned a good lesson.

OK, I've gotten chatty again, and I need to get out to the woodshop. Good luck, you'll do fine, and we're probably BOTH thinking about this project too hard. You'll be fine.

Jamie