Guidance please for building a curved lamination

[Dick Snyder]

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

[Phisherman]

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.

[Dick Snyder]

Thanks for the tip.

[John Grossbohlin]

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

[Hambone Slim]

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

[Lew Hodgett]

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.

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

[Dick Snyder]

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.

[Lew Hodgett]

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.

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

[Leuf]

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

[Dick Snyder]

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 po> > My daughter has asked me to build a bookcase for her baby's

[Dick Snyder]

[John Grossbohlin]

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

[Leuf]

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

[CW]

35 5/8 radius. Actually 35.641 but close enough.

[Dick Snyder]

[Andy]

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

[J T]

Sat, Jul 22, 2006, 7:10am REMOVE [e-mail] (Dick=A0Snyder) doth claimeth: Here is my question. I know you have to build the lamination on a form

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

[Pat Barber]

Is the top like this ???

similar, I would use their(Woodsmith) method which requires NO bending....

I bought a back copy of Woodsmith for another project and this armoire just happen to be in that issue. Very interesting method of making the curved top.

Use multiple pieces and a router.

Dick Snyder wrote:

[Dick Snyder]

Here is a link to what my daughter wants me to build.

you can see the top is a complex curve but I may just build a simple curve. The top is 12" deep and has to span the book case which is 45" wide. I couldn't guess from the woodsmith link you sent in your posting what they did to build the curved top unless it is only a false front edge and the inside (behind the curved front edge) is really flat.

Dick

[Dick Snyder]

I guess I don't follow you. Here is a link to what my daughter wants me to build.

would I cut solid wood to shape?

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