Glue for Plywood Chairs

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I'm making plywood chairs pictured here: http://farm1.static.flickr.com/139/373626483_afa215394f.jpg?v=0
I'm having trouble getting the glue joint at the front to be strong enough to support the weight of the person. I'm using Titebond Original glue and I don't believe I put enough enough glue on to bond the 2 surfaces of the oak plywood (one flat surface, and one cut surface).
Anyone have any hints on making this joint stronger? Biscuits? What about the joint types.. is it difficult to glue a cut edge to a surface edge like this? Or do I just need to apply it more liberally?
Also, what's the best way to "break" these joints so I can reglue the weak ones?
Another picture of the joints: http://farm1.static.flickr.com/159/373629961_bfc3548032.jpg?v=0
Thanks for the help!
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As shown, I don't think there's a way, "no-way, no-how" w/ simply a glue joint at that point, even reinforced. To expect a butt joint w/ half end-grain to hold the kind of force that design exposes it to is just ain't gonna' happen. I don't think biscuits would have sufficient bending strength for long term success. And, even if you doweled it w/ a bunch of dowels that were strong enough to hold the joint itself (and that would take either a whole lot of small ones ore metal rather than wood), I would expect the plywood itself would soon delaminate and fail.
I think the only possible chance there is to make the support far lower in the front and far farther back in the rear to provide adequate support against the front riser. That, of course, probably defeats the intent of having the support far enough forward to make use of the "springiness".
Maybe somebody else has a brilliant idea here, but it doesn't look viable to me as is.
If trying anything like this, I'd also make all the joints as housed, not butted.
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At first, your narrow pieces, should be glued using two parts epoxy reinforced with mechanical devices. The mechanical devices could be a Chicago bolt http://www.leevalley.com/hardware/page.aspx?c=1&cat=3,41306,41311,40051&p@051 going thought the narrow piece and larger pieces of plywood. The best solution would to use solid hardwood for the narrow piece between to unite the top and bottom plywood pieces using epoxy and Chicago Bolts. This will be as strong as the weakest part. That is if your plywood has some void or it core is of poor quality the head of the Chicago bolt may go through some of the core material.

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Beyond a doubt you need mechanical reinforcement. I bet finishing nails would do it, if you got them in at an angle so they were being pulled sideways, and put them in parallel to both pieces.
Rumor has it that TB1 can be opened up with vinegar. haven't confirmed that.
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"Also, what's the best way to "break" these joints so I can reglue the weak ones." There are no best way to break these joints. They may go well by using force or it may tear and pull the veneer. You may want to try one as a sacrificial test. If it work all the better. If they look good to you leave them as they are. Then use a piece of solid hardwood to fit behind the narrow piece of plywood epoxy all touching side and secure with Chicago Bolt or better.

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You've basically created two big levers, with the little support pieces of wood as the fulcrums. Guess what's going to happen to those joints?
What dpb said....You're going to have to move those fulcrums.
If this is a design project, could you not have done some laminated curves?
Mike
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I do not know where this design originated. The design may not have been proven. If it is so the chair need to be tested. Before going on a large scale production a prototype should be made and kicked around for several months. The quality of the plywood is unknown at this time. The joints may well be strong but if the larger plywood pieces break under load there is a major problem. Things may look very good on the drafting board but every day use under heavy load may call for design revisions. Safety and legal aspect may become an issue of liability.

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snipped-for-privacy@nb.sympatico.ca wrote:

Looks very similar to the Zig Zag chair designed by Gerrit Rietveld.
Chris
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From looking at this pictures the thickness of the plywood appears to be 3/4" thick, may be 5/8". I wonder what type of plywood is been used, is it construction grade, marine, or premium? Can the average person move these chair with ease. For sure its an interesting design. I am curious to know the final weight of this chair.
wrote:

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Thanks for all the recommendations. I'll look into the mechanical fastners. I guess that won't spoil the look too much.
I built a smaller prototype that is only 10" wide that is holding more than 300lbs.
http://farm1.static.flickr.com/134/377919845_92adfb1c5d_o.jpg
But, the difference is I used biscuits and also cut the 2 angles at 22.5 degrees and glued the end joints together instead of the flat face and 45 degree angle. Given that the real chairs are 16" in width, I thought I'd have enough glue surface to only do the 45 degree angle.
I do think my problem relies on the actual application. I don't think it bonded the end joint to the surface joint as well as the prototype's double 22.5 degree angles. When the one full sized one snapped, the glue only really bonded to the outer 2" of the plywood instead of the full 16" width.
Now I'm a bit worried that the plywood will de-laminate after awhile... but I suppose only time will tell that tale.
On Feb 2, 5:28 pm, upand_at snipped-for-privacy@yahoo.com wrote:

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I bet that's the key difference - more glue area, less end graininess. I also think that splines would help a great deal. Not splines that are parallel to the front of the chair, but after you glue the joint together, run a circ saw through the joint several times, all the way across the joint. Then slip in kerf-thickness pieces of wood with more glue, and you'll have even more glue area. Something like this, but your chair could have one every inch or so: http://www.woodworkingtips.com/etips/etip010511sn.html Could be an interesting design element if done with a contrasting wood. You could probably do this without dissembling your existing chairs. Good luck, Andy
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wrote:

You should be able to hide the fasteners, especially if you're going to pad the seating surface. That's what they make plugs for. :)
As to your other question about taking it apart, once it's glued, your only real option is probably a circular saw along the joint. It should be possible, if you're careful, not to cause much more damage than the 1/8th inch saw blade takes out.
Have you tested to see how much the plywood flexes when someone is sitting on it? It isn't just the joints at the front and rear of the leg that will take stress, but the entire surface of the leg and seat will bow as weight is placed on it. Get someone heavy on the seat and you could get a fracture across either.
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Fri, Feb 2, 2007, 4:15pm (EST-3) snipped-for-privacy@gmail.com (MikePeck) is making some pretty ugly chairs and is at a loss:. I'm making plywood chairs <snip> I'm having trouble getting the glue joint at the front to be strong enough to support the weight of the person. <snip> Anyone have any hints on making this joint stronger? Biscuits? <snip> Also, what's the best way to "break" these joints so I can reglue the weak ones? <snip>
Solid glue block should do it. Or, you could use some triangle shaped bracing pieces on each side, and maybe one or two in the middle. Personally, I'd solve the problem by makng a different type of chair - I don't think those type of chairs are too strong to begin with, probably originally designed by an 'artist', an "artiste", or interior decorator, I'm sure. I use Titebond II myself, but Titebond, or any other wood glue, should be more than strong enough. I 'could' tell you how to 'break' the joints, but instead I'll tell you to learn to use the 1-800 numbers on the cans and bottles - that's what they're there for.
No matter how strong you make the joint I betcha those chairs will still be bouncy. I'd say don't make a chair that won't hold at least 400 lbs - you never know who'll drop in for a visit, better safe than sorry.
JOAT Only those who have the patience to do simple things perfectly will acquire the skill to do difficult things easily. - Johann Von Schiller
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Fri, Feb 2, 2007, 4:15pm (EST-3) snipped-for-privacy@gmail.com (MikePeck) is making some pretty ugly chairs and is at a loss:. I'm making plywood chairs <snip> I'm having trouble getting the glue joint at the front to be strong enough to support the weight of the person. <snip> Anyone have any hints on making this joint stronger? Biscuits? <snip> Also, what's the best way to "break" these joints so I can reglue the weak ones? <snip>
Solid glue block should do it. Or, you could use some triangle shaped bracing pieces on each side, and maybe one or two in the middle. Personally, I'd solve the problem by makng a different type of chair - I don't think those type of chairs are too strong to begin with, probably originally designed by an 'artist', an "artiste", or interior decorator, I'm sure. I use Titebond II myself, but Titebond, or any other wood glue, should be more than strong enough. I 'could' tell you how to 'break' the joints, but instead I'll tell you to learn to use the 1-800 numbers on the cans and bottles - that's what they're there for.
No matter how strong you make the joint I betcha those chairs will still be bouncy. I'd say don't make a chair that won't hold at least 400 lbs - you never know who'll drop in for a visit, better safe than sorry.
Hmm, my system seems to have a hiccup, so this may show up more than once.
JOAT Only those who have the patience to do simple things perfectly will acquire the skill to do difficult things easily. - Johann Von Schiller
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snipped-for-privacy@gmail.com says...

Bisquits won't do it, I think. Dowels, no way. A (hidden?) hardwood spline across the full width might well do the trick. Or you could possibly turn it into a box joint by cutting/routing slots through both members at 90 degrees to the joint and putting in inserts. THAT has the advantage that you can leave the existing glue alone.
As for undoing the glue - I'd try to douse with boiling water with a few timed applications, then finish it off with a heat-gun. No pva-family glue I've ever seen did stand up to that kind of abuse. But I have not tried it with titebond, it's not sold here.
-P.
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I love the spline idea.. as I wouldn't have to unglue the existing joints. And i could really see that adding not only to the strength.. but also the character. Might even run one of the bolts through the small braces as well to really support.. but i'll try the splines first. I was super bummed last night after I "tested" them, but it is nothing like the internet to bring out some wonderful and thoughtful ideas to keep me motivated. I appreciate it greatly and will report back on the process.
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Mike,
As I look at the picture, I have a different take than the others.
You've got two different kind of joints, the back to the seat, and joint of the diagonal support to the seat or base.
On the back to seat joint, when you lean back on the seat, you're trying to pull the back surface away from the edge of the seat. The resistance is the end of the plywood (half of which is endgrain) glued to the outside ply of the plywood. The back wants to pull away from the seat, and that's resisted by a very narrow glue line. Then, if any of the interior glue in the plywood is bad, or there is a void in the plies, the plywood itself will come apart.
I would replace the little plywood brace with a solid glue block (a contrasting wood might look nice) and run some mechanical fasteners in from the back to keep the two pieces plywood together. Could be glued dowels too, I suppose.
I'd also do something about that knife edge at the bottom of the back. It will last about 15 minutes in use, then look shabby for the life of the chair.
On the other joint (support to seat/base) as other posters said, the joint is trying to lever itself apart, again resisted by a very narrow glue line. I don't think you can put enough glue on that joint to keep it from opening. A solid wood glue block to match the back one, along with some vertical splines to tie the joint together should work.
(Another reason for solid glue blocks, if you have kids in the house, you have no idea WHAT you will find in that neat little hidey hole.)
It's a clever design, and takes advantage of the special properties of plywood. I hope you can get the details worked out.
I'd also check tippiness. Other chairs like that I've sat in tended to flip if one leaned forward or back.
Good luck! Old Guy

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That's because the joint just isn't strong enough. It wasn't strong enough when Rietveldt originally designed this chair, which is why almost every museum specimen you see is either brand-new repro or has been "repaired", usually with huge screws.
The _joint_ isn't strong enough, not the glue. Look at the failures you have and I bet it's the timber failing, not the glue.
Several techniques have been used to re-inforce this joint, most of which don't work. Lock mitre joint cutters are dismal. Glue blocks are no use, unless they're the size of logs. Biscuits won't help, as they're no stronger in this kind of shear. A thick dowel in a real timber like ash, rather than the usual flimsywood is strong enough for chairs, but you still need a carcase thick enough to hold the dowel without splitting out, and plywood isn't. Long screws are only useful if the plywood is thick enough to not have the screws act as levers to delaminate it (3/4" birch works, 1/2" is marginal, lower-grade plys haven't a chance). Don't, as a local woodworking mag suggested, try using MDF...
Thin vertical stainless steel rods can work well, but they do lose the silhouette somewhat for stacking. If the chairs are painted a neutral grey colour (as some of Riteveldt's were), then the steel isn't obvious.
My favourite solution is as you have it here, to make the corners into a triangle with a ply buttress. Either finish the buttress flush and make it a triangular tube (you then have to use a 60 angle though) or else make it slightly shorter and chamfer the outer edge in a line with the corner, so as to hide its existence. By doing this, and using modern parallel-thread plywood screws in accurately pre-drilled holes, I've had no trouble with this design, even with kids using them.
IMHO, '20s modernism is vastly over-rated. It looked good, but the engineering just didn't work out. Look at all the concrete flat-roofed houses with leakage problems, or the spalling concrete.
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Sat, Feb 3, 2007, 9:16am (EST-3) snipped-for-privacy@codesmiths.com (AndyDingley) doth elucidate so well with: That's because the joint just isn't strong enough. It wasn't strong enough when Rietveldt originally designed this chair, which is why almost every museum specimen you see is either brand-new repro or has been "repaired", usually with huge screws. <snip>
Ah Andy, that's exactly what I said. But you said it so much better. You silver tongued Devil you. LMAO
I knew that design had been around for awhile, just didn't know how long. I ran across it for the firtt time quite awhile back, on one of those sites with $25 chairs, with about $5 worth of materials, listing for $1500, designed by "artists", "artistes" or interior decorators.
When I said glue blocks I was thinking made out of at least a 4"X4" chunk of wood. You'd need a minimum of two, If you were careful sitting, I think that might work. I also think it'd still be a bouncy chair - so it'd probably be best do keep kids off.
Better would b extend the back down to the bottom. With smaller glue blocks. And fair-sizd triangle pieces, at least one, unter the front. Of course, by the time you did that it would change the basic design. If you're gonna go that far you might's well keep boing, because I'm still not sur just how stong it'd be..
I'd say if you wanted to keep the basic Z design, then laminate one up with pieces at least 2" wide. Personally I think it's be more interesting to stick with the plywood, and use a batch of triangular reinforcement pieces to redo, and make strong, the basic design. Be Hell to dust I bet. All glue of courcxe, no mechanical fasteners, 'cept maybe some brads, just till the glue sets. I can picture at least two versions as I type - probably make the "artistes" drool.. I wouldn't want anything like that in my house, but might be OK for a porch chair. Be a good use for scrap plywood, and could be fund to make one or two. Now that I think on it, I may make one, just for the Hell of it. Anyone wanna buy a chiar, marked down to half-price for a quick sale, only $750. LMAO
I think the design can be made to work - and without any mechanical fasteners. I think the best way woould probably be with triangular shaped sections glued in under the bottom front, and under the seat - that or some type of "legs" as bracing. I also think you'd be well advised to glue some type of reinforcement in for where the back meets the seat, just in case someone desides to lean back. It'll be changed enough by then that I don't know as you'd still be able to call it the same design, but that's what plans are for, to look at and then come up with your own ideas.
And I still think it's ugly.
Ah, sitting here typing and listening to some honking good bluegrass. Life is basically good.
JOAT Only those who have the patience to do simple things perfectly will acquire the skill to do difficult things easily. - Johann Von Schiller
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On Feb 3, 3:39 pm, snipped-for-privacy@webtv.net (J T) wrote:

Except Andy said it with much more tact and inspiration and for that I thank him.
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