Guidance please for building a curved lamination

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My daughter has asked me to build a bookcase for her baby's nursery. She saw something on-line that she likes. It has a curved top. I think I will build the bookcase out of birch as it will be painted and I am thinking of laminating 3 1/4" pieces of birch plywood to do the curved top.
Here is my question. I know you have to build the lamination on a form with a tighter radius than you want to end up with as the wood with spring back a bit when you remove the clamps after gluing up the lamination. Is there some guidance people can give me on how to figure out that tighter radius??
TIA.
Dick Snyder
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On Sat, 22 Jul 2006 07:10:04 -0400, "Dick Snyder"

There are many factors depending on how you go about it, the radius, the number of laminations, and the wood. As a guideline I'd use 4% of the radius.
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Thanks for the tip.
wrote:

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The other option is to take your best guess on the radius, create the bent laminated parts and then, if your resulting dimensions are off a bit, adjust the dimensions of the rest of the bookcase to work with the laminated pieces.
John
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"Dick Snyder" wrote...

with
a
some
Springback is usually a consideration when making bent laminations from solid stock. If you do make the top from 1/4" ply laminations, it will certainly spring back some, so I'd do as John G suggests and take your best shot, then build the casework to match the end result of the lamination.
OTOH, there are some swell products out there made for the use, such as bending birch, wacky wood, and pre-kerfed MDF that will really stay put when constructed as a balanced lamination on a form.
-- Timothy Juvenal www.tjwoodworking.com
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Dick Snyder wrote: > My daughter has asked me to build a bookcase for her baby's nursery. She saw > something on-line that she likes. It has a curved top. I think I will build > the bookcase out of birch as it will be painted and I am thinking of > laminating 3 1/4" pieces of birch plywood to do the curved top.
<snip>
Make sure you use what is known as "bending plywood".
Around here, 1/4" "bending" poplar is readily available.
The amount of "springback" is a function of the lamination thickness and the bending radius, so can't answer your question.
I'd use epoxy with a slow hardner.
It will give you good "open time" and provide a rock solid lamination.
Don't be in a hurry to take it out of the clamps.
Have fun.
Lew
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Hmm. I had planned to wet one surface of each of the three pieces of plywood and then put gorilla glue on the facing dry surface before clamping. I take it you would still go with epoxy and a slow hardener?? If so, any particular brand of epoxy? I have not used it much for anything but repairs and assembling golf clubs so my knowledge is pretty limited.

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Dick Snyder wrote: > Hmm. I had planned to wet one surface of each of the three pieces of plywood > and then put gorilla glue on the facing dry surface before clamping.
> I take > it you would still go with epoxy and a slow hardener??
Absolutely.
The Gougeon Brothers (West System) has a lot of small packages available and good tech service.
Available lots of places, especially those serving the marine industry.
Never checked, but try a Google.
Lew
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I'm not sure how to compute the bending radius. Maybe you can help me. Here is what I know. The curved top piece of the bookcase will span a width of 45". The highest point of the curve above a flat surface for a reference point will be 8".

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Don't compute it... just do it!
Place two nails 45 inches apart and place a mark 8" above the mid point. Then take a thin flexible board, put it up against the nails and bend it up to to the mid-point mark. Then trace the arc of the board with a pencil. You can tinker with this basic approach a bit to get your ending and mid-points just where you want them.
John
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35 5/8 radius. Actually 35.641 but close enough.

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OK. So how did you compute it :-)

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I've had to do this a few times recently - I puzzled over it for quite a while at first, and finally realized it's all a matter of applying the pythagorean theorem. a^2 + b^2 = c^2 (or maybe there's an easier way I haven't found yet, in which case I hope someone will enlighten me...)
You know one side of your triangle with the given width. Let's call that a, and it is 22.5" (half of your total width). You're looking for the hypotenuse, which is the radius, which we'll call c. The third edge of the triangle is the radius minus 8" (your rise), so we'll call that b, or (c-8). Pythagoreus says a^2 + b^2 = c^2, so now as my 8th grade algebra teacher used to say, plug-n-chug.
22.5^2 + (c-8)^2 = c^2 506.25 + c^2 - 16c +64 = c^2 570.25 = 16c c = 35.604625, or just a hair (1/128) under 35 39/64, or practically, I'd use 35 5/8. (It's a lot easier if you can draw a picture...) Andy
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It took me awhile to figure out where the triangle was but as you said, draw a picture. Clever solution. Thanks.

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On Sat, 22 Jul 2006 07:10:04 -0400, "Dick Snyder"

I recently did a test with 3 layers of 1/8" baltic birch ply and it didn't spring back at all. I arranged the grain of the outer plys so they were parallel to the bend. And the 3/8" built up is hard as a rock.
-Leuf
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Did you wet the baltic birch? What glue did you use?
wrote:

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On Sun, 23 Jul 2006 06:15:06 -0400, "Dick Snyder"
Nope, it wasn't that much of a bend, and the 1/8" 3 ply is pretty flexible if you go with the face grain parallel to the bend. I used regular yellow glue.
-Leuf
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Sat, Jul 22, 2006, 7:10am REMOVE snipped-for-privacy@comcast.net (DickSnyder) doth claimeth: <snip> Here is my question. I know you have to build the lamination on a form <snip>
Not if you K.I.S.S. it.
You're gonna paint. So laminate plywood to the proper thickness, or use solid wood, cut to shape, no bending. No prob.
For those of you that don't know, K.I.S.S. - Keep It Simple Stupid.
JOAT Politician \Pol`i*ti"cian\, n. Latin for career criminal
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I guess I don't follow you. Here is a link to what my daughter wants me to build.
http://www.target.com/gp/detail.html/ref=br_1_8/602-0450012-2843051?%5Fencoding=UTF8&frombrowse=1&asin 00CCEV92
How would I cut solid wood to shape?
(Dick Snyder) doth claimeth: <snip> Here is my question. I know you have to build the lamination on a form <snip>
Not if you K.I.S.S. it.
You're gonna paint. So laminate plywood to the proper thickness, or use solid wood, cut to shape, no bending. No prob.
For those of you that don't know, K.I.S.S. - Keep It Simple Stupid.
JOAT Politician \Pol`i*ti"cian\, n. Latin for career criminal
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Tue, Jul 25, 2006, 3:46pm REMOVE snipped-for-privacy@comcast.net (DickSnyder) doth query: I guess I don't follow you. Here is a link to what my daughter wants me to build. http://www.target.com/gp/detail.html/ref=br_1_8/602-0450012-2843051?%5Fencoding=UTF8&frombrowse=1&asin=B000CCEV92 How would I cut solid wood to shape?
From your original post I'd thought just the arch was curbed. Even so, easy enough to make from solit wood. Just cut the curve in multiple pieces and glue together, or cut small pieces, and put together. Or you could use plywood. You plan on painting anyway, so any joints will be covered. Or, just make a curved front arch, then make a flat top behind that. No prob.
Being as you're gonna paint, and no loade will be placed on the top, you could probably even mold it out of sawdust clay.
JOAT Politician \Pol`i*ti"cian\, n. Latin for career criminal
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