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post #1 of 10 Old 04-26-2013, 01:39 PM Thread Starter
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glue-lam distortion

I'm having trouble with a glue lamination and would appreciate some feedback.

I'm making a custom door with an arched top rail. I decided to create a glue-lam. The form is MDF. The inside radius is 13.5 inches (27-inch diameter) and the outside radius is 18 inches (36-inch diameter) for a total depth of about 4.5 inches. Strips are poplar, 1/16-inch on the inside graduated to 1/8 inch on the outside. The glue is Titebond III.

After 1.5 days on the form, I took it off to thickness the glue-lam. Afterward, it sat on the shop floor for a week. In preparing the final joinery, I noticed that the ends of the arch had moved inward about .5 inches (over 27 inches). That was yesterday. I used my universal woodworking tool (the beltsander) to rough in the proper diameter and then made a shaped sanding block to finish the arch. I marked out the joint and called it a day. Today, I cut the final joints and found that the arch has crept in another 1/16-inch at the ends and "pinched out" about the same amount at the top.

Anyone seen this before? Is this a drying or humidity problem? Is the shrinking (with luck) done?

I don't want to try to restrain the arch from moving within the door frame—seems like a setup for joint failure.

Advice appreciated.

—Mark

Last edited by Mark Riffe; 04-26-2013 at 03:31 PM.
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post #2 of 10 Old 04-26-2013, 03:36 PM
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Quote:
Originally Posted by Mark Riffe View Post
I'm having trouble with a glue lamination and would appreciate some feedback.

I'm making a custom door with an arched top rail. I decided to create a glue-lam. The form is MDF. The inside radius is 13.5 inches (27-inch diameter) and the outside radius is 18 inches (36-inch diameter) for a total depth of about 4.5 inches. Strips are poplar, 1/16-inch on the inside graduated to 1/8 inch on the outside. The glue is Titebond III.

After 1.5 days on the form, I took it off to thickness the glue-lam. Afterward, it sat on the shop floor for a week. In preparing the final joinery, I noticed that the ends of the arch had moved inward about .5 inches (over 27 inches). That was yesterday. I used my universal woodworking tool (the beltsander) to rough in the proper diameter and then made a shaped sanding block to finish the arch. I marked out the joint and called it a day. Today, I cut the final joints and found that the arch has crept in another 1/16-inch at the ends and "pinched out" about the same amount at the top.

Anyone seen this before? Is this a drying or humidity problem? Is the shrinking (with luck) done?

I don't want to try to restrain the arch from moving within the door frameóseems like a setup for joint failure.

Advice appreciated.

óMark
How are you assembling the door frame? For the ends to close in sounds odd. If anything, spring back should open it up.





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post #3 of 10 Old 04-26-2013, 07:03 PM
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Do you mean it warped out of planar(won't lie flat) or is the radius changing? Maybe a drawing??
Are all your laminating strips the same thickness and width? I didn't quit understand the 1/16 and 1/8 inch references. A simple arch could just be glued up with 1/4 inch or so strips. With TB III the spring back would be expected to be nil.
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post #4 of 10 Old 04-26-2013, 09:36 PM Thread Starter
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The arch didn't warp, but changed diameter.

The door has three cross-rails, tenoned into the stiles. The arch rail sits above the top cross-rail, with stub-tenons riding in grooves in the stiles.

The strips were ripped on a bandsaw, and the first ones were only 1/16-inch thick to take the bend of the form (thicker ones seemed ready to break when bent to that diameter). I didn't have a metal strap over the last strip.

I bent the strips in groups, given a limited amount of time to smear glue on them, glueing up about 1 to 1.5 inches of depth at a time. As the arc became wider with the laminations, I could use thicker strips.

The weather here has gone from very wet to dry, and I don't know if that is a factor. (I didn't keep windows open in the shop, so the ambient humidity should have been relatively stable.)

I've made glue-lam stems and frames for canoes and a rowboat but haven't had this issue come up before. (Those glue-lams were mahogany.)
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post #5 of 10 Old 04-26-2013, 09:48 PM Thread Starter
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Here are some pictures of the glue-lam process. After taking the arch off of the form, I thicknessed it with a router on a bed and then used the same bed to cut the stub tenons (if that's what they're called) from both sides. The last shot shows the thicknessed/trimmed/tenoned arch being trimmed to fit the stile grooves.

Let me know if the images didn't download. This is my first thread, and I don't know if I've pushed the right buttons.
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post #6 of 10 Old 04-26-2013, 09:53 PM Thread Starter
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Hm. Now?
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post #7 of 10 Old 04-27-2013, 11:56 AM
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I don't know about others, but I can't see any photos.

A thought though. PVA adhesive like TB II are subject to "creep" if the joint is under sheer tension. In other words, the parts of the joint will slide by each other. For making laminations, we always used urea formaldehyde adhesive like DAP Weldwood Plastic Resin. These adhesives have virtually no propensity to creep and are the adhesive of choice for many lamination applications.

Howie..........
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post #8 of 10 Old 04-27-2013, 01:52 PM
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I teach furniture design to college students, and over the years have seen laminated arches both open up and close in after coming out of formwork. The closing in may be due to drying of the wood as it sat out of the form. Imagine it being an entire circle of wood... when drying the circle would get smaller. Glue between layers of dense wood can stay malleable for far longer than you expect, which allows the layer to slip as the arc closes. The thin layers meant considerable glue in the mass as well. You didn't say how you formed it, so we can't know how well the layers were compressed or how much glue should have squeezed out in the process.

Try again with fewer, thicker strips. Less glue will be needed. Keep it in the form for a week or so, but be sure air can get to both sides. Good luck. Laminated curves are always an adventure!

Last edited by 4DThinker; 04-28-2013 at 12:46 AM.
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post #9 of 10 Old 04-27-2013, 09:26 PM
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I hope this is a link to an answer to this problem from FWW questions.

http://shar.es/l0ccW

If not I will try to paraphrase.
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post #10 of 10 Old 04-27-2013, 09:39 PM
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I tried the link and it seems it is only available if you have subscription. May you can get temporary access. Sorry

With respect to the author I emailed it to myself and copied it without the pictures. I hope it is helpful. The resource is very valuable and well worth acquiring . That's the advert that makesmecomfortablesharing.
I think the answer was from Michael Fortune probably one of the most talented and experienced wood benders and workers anywhere.





QA: My Laminated Curve is Shrinking

Question: Two months ago, I laminated a U-shaped curve from approximately 40 strips of 1/16-in.-thick African mahogany using yellow glue. The finished piece has a 16-in. radius and 16-in. legs, and is 2-1/2 in. square in cross section. The problem is that the curve has tightened so that the ends of the legs are now more than 2 in. closer together. The lamination shows no sign of slippage. What happened?
-- Mark Laub , Minneapolis , MN
Answer:
The problem was caused by two related factors: too many laminations and the wrong glue. This combination introduced excessive moisture into the wood via the water-based yellow glue. As the water evaporated, it caused the wood to contract. Then the adhesive itself contracted as it continued to harden. To see this happen, leave a few ounces of yellow glue in an old cup and watch it pull away from the sides as it dries.


Close-in. If a water-based glue introduces excessive moisture into the lamination, the wood can shrink as the glue cures, drawing the workpiece into a tighter curve.

Springback. The most common problem with laminations, it is caused by the strength of the individual plies overcoming the rigidity of the gluelines.
It should take firm hand pressure to bend an individual lamination around the form. If the laminations are thin and too easily bent, then there may be too many and thus too much glue.
Alternatively, if the laminations are too thick, there will be a lot of stress along each glueline. Some springback will occur immediately when removed from the form, with continued movement caused by cold creep (the glue stretching). For your curve, I recommend about 26 laminations, each 3/32 in. thick.
A better adhesive than a water-based glue is Unibond 800 (www.vacupress.com), which is alcohol-based and eliminates the moisture shrinkage problem.

Use the best glue for laminating. A two-part urea-formaldehyde glue will not introduce excessive moisture into a laminated part, reducing the chances of shrinkage as the glue cures.
I apply it with a notched metal spreader made by Hyde, model No. 19120 (www.acehardwareoutlet.com). The very small-notches leave the perfect amount of adhesive for laminating or veneer work.

Apply a thin, even coat of glue. Use a notched metal spreader to apply an even coat of glue to each ply.
Finally, the clamping pressure has to be adequate and consistent. Otherwise, the uneven glueline will promote unpredictable movement.

Drawings: Kelly J. Dunton

From Fine Woodworking #199 July 1, 2008

From Bob's mobile desktop
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