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radkins
03-25-2016, 08:57 AM
I need to slightly bend/mold some thin plywood for an antique school desk seat and back. The back is only a few inches wide and should bend easily if I can just get it to hold that shape, the seat however will take a bit of effort to form properly but I have an idea for a method of forcing it into shape if I could just figure out now how to get it to hold that shape? Any suggestions? This only requires simple curves, no compound curves, drastic bending or sharp angles.

michigan doug
03-25-2016, 09:19 AM
Heat, moisture and a good fixture. You can steam bend plywood and get a decent result. It will raise the grain, so you will have to sand and so on to get a good final finish.

It's kind of a dark art though. You will have to have a fixture to completely restrain/clamp the plywood for like 24 hours or more for it to set. Then, when you take it out of the form, it will spring back...somewhat. So the form has to have more curvature than the finished product. Well...how much more? Yeah, it depends...

I would start with 20% more curvature and do some test pieces first.

Did I mention, it's a dark art?


There's plenty of youtube about how to do the steaming setup.

JimA
03-25-2016, 09:20 AM
You will need to use water proof plywood, aircraft grade or marine plywood. Look at Aircraft Spruce or Wick's sites. Is thickness 1/4 or 3/8's? Either steam with an iron until pliable or soak in hot water for an hour or more until pliable. I have a steam box for this, but if doing just a couple that would not be necessary. Prior to this you will need to make a mold to shape to clamp plywood in until dry. It will then retain that shape. If seats are 12 - 16 inches wide, take two or three 2x4's and on the 3 1/2 side trace and cut out the shape of the seat and clamp between these. You will want to slightly over exaggerate the curve portions because of what is known as spring back. Even when dry (which I would leave in the mold a minimum of 48 hours) they will want to ever so slightly straighten when you take them out of the mold. Clear as mud?

Carm
03-25-2016, 09:33 AM
If you aren't committed to plywood, consider laying up your own. Thicker veneer stock is available, you can use whatever glue you want, but I'd suggest a two part, can't recall the name! but powder mixes w/water, has a long open time and cures waterproof. Most hardware stores carry it. Weldwood was one popular brand.
You wouldn't have much springback and a suitable species (ash, birch, oak etc.) could be used.

radkins
03-25-2016, 10:14 AM
I don't have access to marine plywood locally unless I buy a full piece and that would be sort of like buying a log to make a toothpick, I was hoping I could use regular 1/4 plywood for this. I was concerned that regular plywood would de-laminate with steam??? Laying up my own is something I hadn't thought of, I will look into that and see if I can find everything locally.

SteveF
03-25-2016, 10:21 AM
If you aren't committed to plywood, consider laying up your own. .......................

Agreed. Plywood is already somewhat committed to its shape. I had to build a curved piece for a mantle and cut 1/8" thick pieces of mahogany and glued them up. Damned jig took more time to build than the piece! Jig was made by screwing together 10 pieces of OSB scrap and cutting the curve on my bandsaw. Multiple clamps held the layers tight while the glue dried.

Steve

J Tiers
03-25-2016, 10:25 AM
I know the desks you mean, spent a good deal of time in those newer ones.... after the ones with the inkwell were replaced.

I think the seats were laid up and formed during the glueing process, and not formed out of already glued-up ply.

flylo
03-25-2016, 10:37 AM
I don't have access to marine plywood locally unless I buy a full piece and that would be sort of like buying a log to make a toothpick, I was hoping I could use regular 1/4 plywood for this. I was concerned that regular plywood would de-laminate with steam??? Laying up my own is something I hadn't thought of, I will look into that and see if I can find everything locally.

AC grade has exterior glue, same as marine but marine has no voids. You can also cut kerfs the back where it will bend easier.

kendall
03-25-2016, 10:48 AM
As Carm and Steve mentioned, you'll want to laminate the seats from at least two layers of thinner ply, preferably three. Only way to keep plywood bent is to lock it in shape with framing.

Stepside
03-25-2016, 10:51 AM
Maybe look for the materials used for making laminated Longboards (The long skateboards) or maybe surfboards. Both items have curved lamination's.

You could saw your own veneer and buy a large piece of veneer for the top layer. Anyway you go, the press/fixtures will take longer than making the veneer.

DR
03-25-2016, 11:47 AM
Yeah, laminate your own.

I recently read in a woodworking book about glues for bent laminations. It said not to use glues like Titebond because they tend to creep over time.

Weldwood plastic resin was a recommended glue type, might not be so easy to find on store shelves anymore since the formaldehyde content freaks everyone out. Mail order it. Long time drying so lots of time to fuss with clamping, etc.

Carm
03-25-2016, 12:09 PM
To bend plywood, more than what you see from absorbing/drying warpage, and keep that shape, consider what's required. Each laminate has to deform, and the glue planes have to slip to allow that and still function. The original glue would need be thermoset if using steam which is the practical method. If you have all the time allotted to the hairs on your head, slow creeping bends can be done. There are Japanese artists who do this with live trees.
Further, for your situation, you don't really want crossplies.
Kerfbending was mentioned by Flylo. One side requires knifed slots, the one that has the face ply towards the radius (material is not removed). The other, Vee-slots. You'll probably saw both at the loss of glue area.
Cut or saw through just short of the face ply. Covering the plies w/ a solid sawn cap hides the uglies and helps w/strength.

SteveF
03-25-2016, 01:40 PM
As Carm and Steve mentioned, you'll want to laminate the seats from at least two layers of thinner ply, preferably three. Only way to keep plywood bent is to lock it in shape with framing.

According to my copy of the Wood Bending Handbook, the amount of springback is directly controlled by the number of plys, more being better. None of the pictures in the book has less than four, I used five. You are right about common grade plywood only staying straight with framing which may be caused by the fact that it only has three plys.

Steve

radkins
03-25-2016, 01:44 PM
What about polyester resin like used for fiberglass layup?


Well this is another fine mess I have gotten myself into!! The story behind this thing is that I was asked by a customer's eight year old daughter if I would "fix" this desk that her Great Grandfather gave her, I was told by her father that it dates to the late 1930's and came from the two room school the old man attended as a kid. I probably have enough shop time in this thing by now to pay for a room full of antique school desks but what do you tell an eight year old girl with a desperate pleading look on her face? It would be a long story but this thing means a great deal to her so I took on the task of rebuilding this 70 or so year old desk shaped pile of scrap metal and rotted wood, after hours of pounding, bending, straightening, welding, sandblasting and painting the darn thing actually looks quite nice with a simi-gloss finished Oak desk top and now I am down to doing the seats. After all this time and effort I want to do this right but woodworking is not one of my strongpoints and I had no real ideas of how to properly mold that wood seat.

BTW, that plywood will be padded and covered, unlike the original hard seat, so the actual appearance of the wood will not matter so long as it is reasonably smooth.

kendall
03-25-2016, 01:58 PM
Agreed, but I was talking about using two or three sheets of thin plywood to build up a curved thicker sheet. A mild curve in plywood, for a chair back etc, can be 'framed' by using a steam bent or sawn batten to keep it in shape.
100% agreed on cheep three layer plywood, only way I'll use it is if it's free then only when I have to.

Carm
03-25-2016, 01:59 PM
"What about polyester resin like used for fiberglass layup?"

Stepside mentioned that, post #10.
Those snowboards/skateboards are built using that and wood veneers. If you're using cloth/wood, guess that works. They use epoxy.

Gary Gill
03-25-2016, 04:40 PM
Consider multiple layers of a material called "bending birch" It is typically two layers at 90 degrees. Cabinetmakers use to form radius fronts. Make a mold and apply several pieces to get to the desired thickness.

J Tiers
03-25-2016, 05:15 PM
There's this kind, big bend in it, but not compound.

http://www.2ogra.com/16698-antique-school-desk-chair/

I seem to recall another type with compound bends, so far no pic found

CarlByrns
03-25-2016, 05:28 PM
If you aren't committed to plywood, consider laying up your own. Thicker veneer stock is available, you can use whatever glue you want, but I'd suggest a two part, can't recall the name! but powder mixes w/water, has a long open time and cures waterproof. Most hardware stores carry it. Weldwood was one popular brand.
You wouldn't have much springback and a suitable species (ash, birch, oak etc.) could be used.

Resorcinol is/was the glue. It has been largely supplanted by two-part epoxy. For a small job, I would head over to a good hobby (model) store and pick up Bob Smith Industries 20 minute epoxy. It may say something else on the front of the bottle, but it will have 'BSI' printed on it somewhere. Good stuff.

Carm
03-25-2016, 05:59 PM
Resorcinol is/was the glue. It has been largely supplanted by two-part epoxy. For a small job, I would head over to a good hobby (model) store and pick up Bob Smith Industries 20 minute epoxy. It may say something else on the front of the bottle, but it will have 'BSI' printed on it somewhere. Good stuff.

No, it was ureaformaldehyde, mixed w/water. But you bumped my memory...resorcinol is the purple stuff, two part,liquid/powder, limited creep and waterproof, long cure time.
Glues have certainly advanced and an epoxy might be cheaper, I figured a water component would increase pliability for the OP.

Mike Burch
03-25-2016, 08:10 PM
Polyester layup resin is NOT a glue, though it is sometimes used as such. In a low-stress situation you'd probably get away with it.
Laminating your own is really the easiest way to go. Whatever glue you do use, it must be gap-filling.
There's a product called something like "Bendy Ply" which boatbuilders use for curved bits if joinery (but like any sheet material,it can be used only for simple, not compound, curvature). Three or four layers of that, glued with epoxy, will hold its curve no trouble.

radkins
03-25-2016, 09:40 PM
Yes I am aware that polyester is not actually a glue but I have used it for glue and it seems to make a very strong and permanent bond, the biggest plus is that I have a gallon of it! :cool:

J Tiers
03-25-2016, 10:06 PM
Polyester layup resin is NOT a glue, though it is sometimes used as such. In a low-stress situation you'd probably get away with it.
Laminating your own is really the easiest way to go. Whatever glue you do use, it must be gap-filling.
There's a product called something like "Bendy Ply" which boatbuilders use for curved bits if joinery (but like any sheet material,it can be used only for simple, not compound, curvature). Three or four layers of that, glued with epoxy, will hold its curve no trouble.

The Eames dining room chair has very significant compound curves. Presumably molded that way when being glued-up. Once you have pre-glued plywood, it is a one dimensional bending material.

http://hivemodern.com/pages/product1435/eames-molded-dining-chair-herman-miller-dcw

At least the originals seemed more compound, that one looks as if it just barely fails to be compound.

http://mobile.designaddict.com/buy-and-sell/Furniture/chairs-stools/DCW-Dining-Chair-Wood-designed-Ray-Charles-Eames

GEP
03-25-2016, 10:19 PM
I have used 1/8 thick plywood, you may be able to mold it,glue it and tack it in place. I had some aggressive bends and worked perfect for me.I did 3 layers

Mike Burch
03-26-2016, 08:30 PM
Thanks, J Tiers, those are lovely chairs. But if you look carefully, only the seat appears to be a compound curve, and even it is actually two simple curves at right angles. Very cunning, and beautiful.
Boat-builders' rule of thumb is that steel plate can be persuaded to take a compound curve without heating, bashing or otherwise distorting only if the lesser curve is less than 1 in 40. Ply is similar, but perhaps a little more compliant (no pun intended!).

J Tiers
03-26-2016, 10:48 PM
Check the seat back of the second one. Agree on the first.

The second one seems to be bent back all around the periphery if you check the light and shadow in the front view. As for the amount of bend, hard to say. Not a lot, but definitely there.

Plywood might be easier than steel, it's hard to do a "furnaced plate" in ply, but if you do it wet, well.......

Mike Burch
03-27-2016, 04:51 AM
Check the seat back of the second one. Agree on the first.

The second one seems to be bent back all around the periphery if you check the light and shadow in the front view. As for the amount of bend, hard to say. Not a lot, but definitely there.

Plywood might be easier than steel, it's hard to do a "furnaced plate" in ply, but if you do it wet, well.......

Yes. you're right! The edges do appear to be bent back a little.
It's interesting that the sales pitch refers to the seat (and presumably the rest) being made in ash. That's reputedly a particularly elastic wood that readily takes steam bending.

The Artful Bodger
03-27-2016, 05:32 AM
Quite complex shapes may be made in plywood using strips of ply laid over a former. Lay the strips close together then smother with glue and lay another layer at about 60 degrees angle and smother that with glue too. Repeat for required thickness but no less than three layers. On the final layer take a router and cut an even gap between the strips of the top layer then glue more strips in the spaces for an complete coverage of the top surface.

I made a small car like that once and when I cut out the bonnet ( aka hood) it barely sprung at all.

Jim Williams
03-27-2016, 10:53 AM
The company I retired from made formed plywood parts. They laid the plys up over a form with a coating of a thermosetting adhesive between sheets. The adhesive was heated for curing by some type of induction equipment. I suggest you fabricate a form with top and bottom to hold the plys together while your adhesive sets.. You can use what you learn for future projects.

Jim

radkins
03-27-2016, 01:52 PM
Well it looks as if my original plans for making these seats was unrealistic, somehow I had mistakenly assumed that thin plywood could be formed into gentle curves but obviously it's not nearly as simple as I first thought! I will take the ideas given here and go shopping for materials tomorrow, what I eventually do will be dependent more so on what I can find to do with but at least now I have a much more realistic understanding of what's involved and what I need to do it with.

The Artful Bodger
03-27-2016, 03:07 PM
Referring again to the opening post..


I need to slightly bend/mold some thin plywood for an antique school desk seat and back. The back is only a few inches wide and should bend easily if I can just get it to hold that shape, the seat however will take a bit of effort to form properly but I have an idea for a method of forcing it into shape if I could just figure out now how to get it to hold that shape?


The general principle when you have a means to force and hold the materials in the required shape is that you build a stack of thin plys, even individual veneers, with plenty of adhesive or resin between the layers then press it into the required shape and allow the adhesive to cure.

John

The Artful Bodger
03-27-2016, 03:14 PM
I will take the ideas given here and go shopping for materials tomorrow, what I eventually do will be dependent more so on what I can find to do with but at least now I have a much more realistic understanding of what's involved and what I need to do it with.

When shopping for ply I suggest you carefully study the edge to check that the outer plys are not paper thin with thicker inner plys, if so the outer plys will too easily crack and the inner plys will not bend evenly. Sometimes cheaper is better!