A question about bending plywood

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Yeah, yeah, I know this is a welding forum and there would be dozens of woodworking forums out there that I could go to but I can't be bothered joining a forum to ask one question and I bet some of you will be able to answer this.So, I've got some 60s bar stools that I needed to recover. They have a back rest that is made from curved plywood.Of course, when I took the old vinyl off the backs, the ply had delaminated.I had some epoxy (used to own an old timber boat) and I relaminated the curved backs.Three of them are fine, but the fourth one is a bit out of shape. (Not much, and I'm the only one who will notice, but it will annoy me.)I had a rash thought and nearly bunged it in the microwave oven. I figured I could heat it up and reclamp it. But then I thought that bunging some epoxy ladden ply into a microwave might be one of those really dumb things to do.Then I thought maybe I could soak the whole thing in boiling water and rebend it. But maybe the epoxy would dissolve?Your thoughts?Scott
Reply:try steam and you have over a 1000 postso ask away
Reply:If you have a MOLD i would try the water, then clamp it in the mold for a few days. won't hurt the epoxy, now i didn't say boiling water, just straight water....Or make a new rest, use several thin sheets of wood and glue and press, again you will need a MOLD.
Reply:A bit depends on -how- the "plywood" was made.It could have been laid up in a form as individual veneer plys and glued-up in the bent shape.  It could have been steam-bent from plain flat plywood, but that is usually used for solid wood as the steam (heat and moisture) could wreak havok with most of the adhesives (the 'glue', but glue is properly only used to refer to somthing like "hide glue" and plain ole 'yellow carpenter's wood glue' isn't really technically glue but an adhesive just like epoxy or polyurethane adhesive ... I digress  ) used to make plywood.Got a picture of the offending piece of laminated wood product?  It might help figure out a bit more exactly what you are dealing with.If it's a 'typical' chair's curved back size/shape, I'd say it was laminated in the curved form as a 'custom' item and was not made by bending a piece of lumber-yard flat plywood.  Which means that if it sprang back or out of shape, you are kind of SOL.And good call listening to that voice in your head.  No, not THAT voice!    The other one saying "Wait a minute.  Are you really supposed to microwave a piece of wood?"  Because most of the time you are NOT supposed to microwave a piece of wood.  Doesn't really do anything to the wood, unless there is moisture (or fat/oil) in the wood in which case the water or fat/oil gets HOT.  But if there isn't enough water/fat/oil in the oven to properly absorb the microwave energy the oven is making when you push START, then you probably damaged the oven.note: yes sometimes microwaves or radiowaves are used in some lumber-producing processes.  Like rapidly curing some special adhesives, or zapping any insects in the lumber, or stuff like that.  But they know (mostly) what they are doing.    The best laid schemes ... Gang oft agley ...
Reply:Thanks all.Steam is tricky, though I could rig something up.I like the sound of soaking it in cold water. I dont have a mold, but could find something with the right curve and clamp it. Glad to know the epoxy should be fine - I had visions of having to reglue it all (which admittedly wouldn't be the end of the world, just annoying).Not sure how it was originally made, nor what sort of glue it was. I took it apart and have reglued each sheet with epoxy.Glad I didn't head down the microwave path. I actually stood in front of the oven in the kitchen when my wife was out with the bit of plywood in my wondering what could go wrong.Last edited by scott brunsdon; 03-05-2009 at 07:42 PM.
Reply:Originally Posted by scott brunsdonI had some epoxy (used to own an old timber boat) and I relaminated the curved backs.Three of them are fine, but the fourth one is a bit out of shape. (Not much, and I'm the only one who will notice, but it will annoy me.)I had a rash thought and nearly bunged it in the microwave oven. I figured I could heat it up and reclamp it. But then I thought that bunging some epoxy ladden ply into a microwave might be one of those really dumb things to do.Then I thought maybe I could soak the whole thing in boiling water and rebend it. But maybe the epoxy would dissolve?Your thoughts?Scott
Reply:See, I knew you blokes would have some good ideas.I've been putting off doing anything with these stools for a few years. I bought them on Ebay from another city and had them shipped.Before I bought them, I got the seller to tell me how high they were (I needed high ones) and assure me that the seats and backs were in good condition.Of course, when they showed up they were lower than he said, so I've had to weld 80mm onto the bottom of each leg. The seats were buggered, so I had to make new seat bases and get them recovered. And the backs fell apart when I pulled the vinyl off them. It would have been cheaper and easier to make new stools from scratch.Last edited by scott brunsdon; 03-05-2009 at 08:13 PM.
Reply:Well they make special 'bendable' plywood just for what this manufacturer does. Good luck on an individual getting just one sheet unless you have a specialty wood supply in your area.The oldest trick, for non viewable pieces, is to run it over your table saw with the depth set to cut through every ply but the last layer. Numerous cuts, as close as 1/8th if the bend is radical. Once that is done flip it over, batter the whole cut side with the glue of your choice, bend to shape and clamp to hold. Be sure and wipe down all excess glue.
Reply:The stair company I deal with does quite a bit of laminated curved stairs. I can pick up 1/8" ply from them specifically for bending. Your post Sandy reminded me of this. I frequently get their scraps as shim material, but have used it to make laminated veneer grade arches on occasion. They even get 1/16" sheet material on occasion.
Reply:How thick is the plywood?Assuming it is 3/4 inch, You could take 3 pieces of luan (cheap flexable ) plywood, glue them together and clamp them before the glue dries.A butterfly without wings, is just an ugly bug
Reply:This brings back some scary methods for glue setting.  I had the pleasure of operating a glue heating device that used what I think was microwaves or at least some sort of high frequency.  The power supply looked like a Lincoln tombstone.  At the end of the cables was a handle and aluminum jaws which could be changed to match the shape of wood.  The Vee jaws were placed on the outside of a 90 degree splined miter loaded with urea formaldehyde based glue and the the trigger was pulled.  You could see in less than ten seconds the glue had dried and set.  The wood was too hot to touch with your bare hands.  Can you microwave wood?? ,, yes with care..  Friend who made woven cedar lampshades did it regularly.
Reply:How thick is the plywood?Assuming it is 3/4 inch, You could take 3 pieces of luan (cheap flexable ) plywood, glue them together and clamp them before the glue dries.
Reply:Originally Posted by MoonRise...microwaves or radiowaves are used in some lumber-producing processes ... zapping any insects in the lumber...
Reply:Originally Posted by scott brunsdonI had a rash thought and nearly bunged it in the microwave oven. I figured I could heat it up and reclamp it. But then I thought that bunging some epoxy ladden ply into a microwave might be one of those really dumb things to do.Then I thought maybe I could soak the whole thing in boiling water and rebend it. But maybe the epoxy would dissolve?Your thoughts?
Reply:You could try soaking the plywood in water for 1-2 days. No matter what thickness you choose, a typical plywood will bend more readily across the grain of the surface plys than with the grain.Midwest Metal ProductsMetal Wire Forming Fan Guards Delta-Larm
Reply:Scott,Your call as to how much time and effort you want to put into it.The 'cut-a-whole-bunch-of saw-slots-and-then-bend-it' is called "kerf bending".  Probably not what you want to do for a chair back, as you would have to trial-and-error a bunch or test pieces in order to get just the right mix of saw kerf width and spacing and bend radius and kerf depth in order to totally 'collapse' the kerfs when you bend the wood .  It's a useful technique sometimes, but I don't think this is the right application for it.  Something like a curved front on a table apron, where the kerfs will be hidden from view, is more of what I think of as a more suitable application of kerf bending.Soaking the already once-delaminated and now reglued-with-epoxy pieces of plywood in water is probably not a good idea.  Most epoxies are at least water resistant (and if you used a 'marine' epoxy it probably is  ) but soaking the wood in water and then trying to rebend it is probably not going to work.  The wood may soften in the water soak alright, but the wood may discolor or stain (probably not an issue as you are covering the wood with some sort of upholstery material) and more importantly YOU ALREADY GLUED IT WITH EPOXY.  If you rebend the water-soaked softened wood, either the wood fibers are going to rip or they will pull out from the epoxy or some other structural oops will probably occur.  Because the epoxy really isn't going to move, it's going to want to remain in the shape/position/place it was when it cured/hardened.At this point, you've pretty much already come to the conclusion that it would be faster and easier to make your own laminated or steam-bent curved chair backs.  I tend to agree.  You can look those techniques up if you don't already know them.  The form can just be some 'plates' of sheetgoods (plywood, particleboard, MDF, etc) bandsawed/jigsawed/whatever into the desired profile and stacked together into a mold form (attach them together with glue  and screws or what-not).  Put clamping notches or otherwise make sure you can clamp the work piece(s) properly.  If you get relatively thin veneer pieces, you can just slather them up with the appropriate adhesive, lay them into/onto the mold/form, and clamp in place until the adhesive sets/dries.  Watch out for glue squeeze-out, it can get messy.  And don't glue the work to the form/mold.    And this is yet another one of those situations where you really should do a dry-run to make sure you have enough clamps and can put them in place.  You can never have too many clamps.  Steam bending a solid piece of wood means you have to rig up some sort of steam chamber and steam the wood until it is pliable.  Then you quickly bend the softened wood on the form and clamp it in place.  No glue/adhesive used.  Still need lots of clamps though.With both methods you have to allow somewhat for "springback" after you take the bent piece out of the mold/form.  The springback could be small enough that you can ignore it for all practical purposes or you may have to over/under bend (depending on whether the mold/form is a positive or a negative mold) to compensate.Or just skip the whole wood thing and bend some 5mm (there, I thought in metric just for you  ) steel in a roller and make the backs out of the steel.  Cover with new vinyl and padding and who's going to know?  lotechman,You probably had some RF (radio frequency) heating going on there.  Microwaves would probably not be used if you were right there (at least they shouldn't!).  And I'll bet your friend with the microwaved cedar was working with either green wood or soaked wood in order to have the water in the wood to be able for the microwaves to heat up the water and thus 'steam' the wood from the inside out.Another use for microwaves and wood is to use the microwaves to dry out some woodworking biscuits if they have absorbed some moisture from the air.  But again, that's not what Scott is dealing with.  The best laid schemes ... Gang oft agley ...
Reply:These are a few of a laminated bridge arch that I made as part of a structural eng class project. I cant find the picts of the completed bridge, but it used 3 of these arches.Originally this started out as a 8" x 2 1/2" 18' long chunks of poplar that we ripped down to 1/2" strips and then reglued in the jig. The finished size was about 14' long 10"h x 1 3/4 wide after we trimmed it all up.Last edited by DSW; 03-06-2009 at 07:42 AM.
Reply:Thanks for that considered response Moonrise. I understand your logic.Alot of clamps you have there, DSW. I like the look of bent ply - thought the backs of these stools will be covered. My wife worked for an American company (Herman Miller) for about 10 years. They still make furniture designed by Charles Eames. We've got a few of his chairs in our house.Scott
Reply:Originally Posted by DSWThese are a few of a laminated bridge arch that I made as part of a structural eng class project. I cant find the picts of the completed bridge, but it used 3 of these arches.Originally this started out as a 8" x 2 1/2" 18' long chunks of poplar that we ripped down to 1/2" strips and then reglued in the jig. The finished size was about 14' long 10"h x 1 3/4 wide after we trimmed it all up.
Reply:Another amazing thing about large wooden beams is that they withstand fire better than a plain steel beam that would support the same load. Seems strange, but the outside of the beam will char but the center of the beam will be insulated by the mass of the wood. The untreated steel beam will soften and buckle long before the wood beam will burn thru to the point of collapse.
Reply:There was a factory in Sydney that was knocked down to make way for the Olympic Games buildings in 2000.All the trusses were laminated timber. They were fantastic. The company that owned the building made plywood. They worked on some of the timber for the Sydney Opera House. This link has some good photos of the factory interior.I bet those trusses ended up in landfill.http://image.sl.nsw.gov.au/cgi-bin/e...22/a270;seq=55
Reply:I don't know about Australia, but around near me they frequently sell those old timbers to guys that will recut and sell the lumber. It's often worth the time it takes to remove then. In several cases I know of the buyer will take the building down to make sure that they keep the baems, floor joists, flooring intact. Kind of a win/win situation. The contractor gets the building demo'd, saves on disposal charges, and gets paid for the beams. Buyer gets lumber that is next to imposible to find nowdays, that will be worth good money for those who are looking for large beams or recycled timber, and usually can get a tax break because they recycle.
Reply:Yeah, there a few places around here that sell second hand timber - old flooring, beams etc. Usually very expensive.