I’m going to glue up a lamination of four 1″ wide by 1/2″ thick poplar strips to make an arched piece 1″ thick by 2″ wide. The radius of the bend will be 49″. How much should I exaggerate the arch on my form to compensate for springback?
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Replies
Quick',
The individual ply's are too thick, and run the risk of cracking as you bend them into position. Ultimately, each ply should be 1/8" -> 3/16" thick. If you use the appropriate epoxy, at the correct ply thickness, there will be barely, if any, spring back. (Yellow or white glue will not work for this application.)
I do a lot of steam bending and laminate bending in the pieces that I design and create. The enclosed photo is of the base of a table that I recently finished. The legs are 4" x 4" and are bent to a 14" radius. Each ply' is 1/8" thick - meaning 64 plys per leg. The wood is Goncalo Alves.
If you have any questions please feel free to ask.
Dan Kornfeld, Owner/President - Odyssey Wood Design, Inc.
Edited 5/7/2006 7:34 pm ET by jackiechan
Edited 5/7/2006 10:40 pm ET by jackiechan
Thanks for the info. I thought I might be able to keep the laminations thick because the radius is relatively large, but ripping narrower is easy and less spingback is good. This goes into a fireplace surround. I've read that epoxy can fail when subjected to heat, so I'm thinking about using urea-formaldhyde, resorcinal or polyurethane. Any thoughts on the best glue?
The epoxy is not going to fail until the temp is up close to 200 degrees, which sounds like you will be breaking a code if it is close enough to get that hot. However the resorcinal is the best choice if you are sure it needs that kind of heat resistance, but it needs a heck of a lot more clamp pressure than the others to make a good bond.
Quick',
The urea-formaldehyde or resorcinol would both work, for this application, polyurethane shouldn't be used.
I use Unibond 800 (available from Vacupress - link below), or G1 or G2 Epoxy available from Lee Valley (link below) for the majority of my laminate bending. For your project, I'd go with the Unibond - it's a two part system with a translucent liquid resin and a powdered catalyst. The catalyst comes in three shades (white, medium, and dark) to help hide the glue lines of the wood.
Unless the fireplace surround is mounted too close to the actual fire (you'll need to make the judgement call on this one), there should be no problem with glue softening. What I would look out for is what the final coating on the wood will be. Depending on what you use, you may find the surface drying, cracking, or hazing.
http://www.vacupress.com/veneerglue.htm
http://www.leevalley.com/wood/page.aspx?c=2&p=20011&cat=1,110,42965&ap=1
Dan Kornfeld, Owner/President - Odyssey Wood Design, Inc.
Edited 5/7/2006 10:23 pm ET by jackiechan
What a great piece! Congratulations.I agree on the lanination thickness, though I've used Gorilla glue at times. Both are pretty messy. I also agree on the "no springback" statement.Did you glue all 64 at one time or in stages?Did you use clamps or a vacuum bag?Jerry
"Did you glue all 64 at one time or in stages? Did you use clamps or a vacuum bag?"
Jerry,
I glued up the laminations in two steps, 32 ply's at a time. Even though each ply easily bent to the radius of my bending form, 32 layers took some creativity to bend tightly to the form.
I used a vacuum press/bag, so the wood needed to be flush against the form or else as the bag would get sucked between the form and the wood. I used flat tie down straps to cinch the bundle to the form, and then the whole set up was sealed up in the bag.
The piece was left under vacuum pressure for 24 hours.
Dan Kornfeld, Owner/President - Odyssey Wood Design, Inc.
Thanks for the info on "flat straps to the form". I hadn't figured out how to keep the package tight to the form for placement into the bag - so it has alaways been lots-of-clamps. I would prefer the vacuum bag.Jerry
Have you been putting the form in the bag, and using the pressure to draw the material down to the form, or are you putting it in a bag that is bent around a form outside of the bag?For short-run projects, you can build a less substantial form if it is not in the bag. It only has to be strong enough to counter the spring-back forces of the parts being bent, whereas if it is in the bag, it needs to withstand a ton of pressure per sq. ft. without collapsing.
As you commented, there is a need "to prevent the bag being sucked between the lams and the form". I had not figured out how to accomplish that, so I went to using clamps to bend the lams around a form of 3 layers of 3/4 ply. Lots of clamps and very satisfactory results. But, I still would like to try your system. Poly glue eliminates the 'slip' problem but is messy stuff.Jerry
Jackie, Why would I not get springback with epoxy and would get it with yellow glues. I notice that I do get spring back with yellow glue. Will epoxy eliminate spring back? and what type do I use.
Thanx, Lou
Lou, for bending, because there are spring-back forces that are forever present, you need to use a rigid curing glue. Yellow, white; aliphatic and vinylester; and contact, are all too flexible to hold up without creeping. The thicker those plys, the greater the spring-back.I think the general rule is for each time you double the thickness, you quadruple the stiffness. Someone correct me if that is wrong. And when bending, you can never bend any closer to the end than eight times the thickness, and if you plan to go back and cut that extra off on a laminated bend, those forces try to fulfill that rule, and you are likely to get some spring-back on the ends. However if it has been steamed properly, the stresses are gone, and the part will hold the shape right to the end.And finally to Boss Crunk, you don't always get spring-back with steaming, The last project that I steamed, I had the opposite of spring-back. After steaming, and taking them off of the bending form, I monitored them for a day, and determined that they were not going to spring-back so I hung them from a rod in my loft to dry, and went away for a couple of months. When I got back to them, they had gone the other way. I was still able to use them though, because they were S curved arms, and they are to far apart to compare.
Edited 5/9/2006 12:12 am by KeithNewton
"Why would I not get springback with epoxy and would get it with yellow glues"
Lou,
Epoxy, plastic resin (i.e. Unibond 800), and resorcinol all set up (cure) to a much harder state than white or yellow glues. White and yellow glues have a certain amount of flexibility even when they are fully dry (cured).
When you bend each ply in your lamination, they will naturally want to spring back to their straight position. If you use white or yellow glues, their still flexible cured state will allow the plys to move and thus spring back. You'd be asking a lot of these glues to keep the layers held firmly enough to not move.
Epoxy will eliminate spring back if each ply being used in the bend are of the correct thickness. A rule of thumb to follow is that each ply should be no more than 1/8" to 3/16" thick depending on the severity of the bend. The thinner the ply the easier the bend, and in turn - the least amount of spring back.
If the plys are too thin, the epoxy will bleed through the wood, causing splotchy spots and difficulties in applying an even finish.
For the majority of my laminate bending, I use Unibond 800 from Vacupress http://www.vacupress.com/veneerglue.htm or G1 or G2 Epoxy from Lee Valley
http://www.leevalley.com/wood/page.aspx?c=2&p=20011&cat=1,110,42965&ap=1Dan Kornfeld, Owner/President - Odyssey Wood Design, Inc.
Dan, I am having a little trouble with your math. If the plys are 1/8 and 4" thick, then that comes to 32 total. You are loosing me with the two steps getting to 64? That would be 8" thick using old-math.........
Keith,
Each ply (for the table legs) are 56" long, 4" wide, and 1/8" thick - yes, 32 plys per single leg. My explanation was a little off. There are two legs per leg set - two curves joined together to make one leg set. The 64 plys came from the 32 from each side.
I've enclosed some additional views of the leg structures. Dan Kornfeld, Owner/President - Odyssey Wood Design, Inc.
Not sure but I bet some wouldn't mind seeing some pics of the form if you have any. I know it would clear up some of the questions out there. nice base by the way.....
Some requested to see the bending forms that I used to make the legs of the table in the previous photos. Here they are...
There's only one form in the photos, but I built two to speed up production.
Each form was built using Baltic Birch ply' and hard Maple. The bottom of the form is a torsion box so that it could withstand the heavy pressure when under vacuum. The arch itself is built like a bottomless torsion box and is skinned (covered) by two layers of 1/4" bending ply. A thick layer of packing tape was laid over the outside of the form to prevent the work piece from sticking to it.
The wire cable and turnbuckle seen inside the form was put there to counteract a slight twist that had occurred with the form.
The other photo is of a stack of 1/8" thick Goncalo Alves plys' awaiting a sanding to a final uniform thickness. The ply's are straight off of the bandsaw, and the blue tape is there to keep them in the order that they were cut from the board.Dan Kornfeld, Owner/President - Odyssey Wood Design, Inc.
The point behind a glue-lam is to not have any springback (or a neglible amount at least). This is the advantage is brings to the table over steam bending.
As somebody else said, your lams are too thick. Get them down to a scant eighth or so.
In addition to the invaluable information already provided by Dan, I'll offer this:
To find a suitable thickness for laminations I bend a single lamination around my bending form. If I experience problems bending a single lamination, a whole flock of 'em will only compound my problems and I'll slice the rest a little thinner; if I can easily get a single lamination to hug the form with hand pressure, the glue-up will usually work out just fine.
I've had good luck with Unibond-800, Weldwood plastic cement, and epoxy. Unibond is a light brown color, resorcinol in reddish brown.
Good luck,
-Jazzdogg-
"Don't ask yourself what the world needs. Ask yourself what makes you come alive, and go do that, because what the world needs is people who have come alive." Gil Bailie
The other option is to steam bend the strips first, let them set and then laminated them. I made an arched piece 3.5" wide from 4 strips of 1" thick birdseye maple. If your goal is to preserve the integrity of the grain then steam bending works better than a lot of thin laminations.
I laminated 4 runners to make 2 dogsleds out of ash and also a fishnet out of ash also, using regular yellow glue, very little springback, the dogsled runners were 1/4" thick 4 layers, the fishing net thinner, cant remember but this one is like a small radius circle
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