Laminating Stock for Arched Casings

Here's a wide-ranging discussion of gluing and clamping processes for laying up arched casings using multiple thin laminations. November 26, 2007

I have a problem with arches moving. We glue up our arches out of 1/8"-1/2" strips (depending on the radius) for the grain to line up. When the arch comes off the glue rack (after about 6-8 hours of gluing) it fits our radius perfect. But a day or so later, the radius gets smaller by quite a bit. So I started making my arches bigger to compensate for the moving. And then after a couple of days, it fits my radius. This is too much of a guessing game for me. I know there are way to many variables to be able to calculate springback, or in my case shrinking, but I was wondering if there might be something out there to help with this problem.

Forum Responses
(Architectural Woodworking Forum)
From contributor K:
What is the finished arch doing? What is its function?
How many plies?
What is the radius that you are bending?
What kind of wood, and glue?
How do you clamp?

Since you say that your radius gets smaller later, I suspect that you are using a glue with water in it which causes the wood to swell when you apply it, and it is being clamped. After it comes out of the clamps, and looses moisture, the wood and glue both shrink, causing it to draw in. A glue-like epoxy does not change the MC of the wood, and has good properties to resist creep.

From contributor L:

Caution with the epoxy - it will creep big time. If youíre pulling in tighter, you have too many plies. Also going to a hard setting glue will reduce the movement problem. Urea formaldehyde works well but has no gap filling properties at all. If it has any significant thickness of glue line, it will be brittle. Resorcinol for exterior work. Putting a restraining board on the curved molding to hold its shape until it dries completely helps, too. Then there's the humidity thing while in storage.

From contributor R:
Usually most of the arches are casing. The radius is always different - different customer, different radius, different wood. The plies depend on my radius and how wide my casing is. Tight radius, more plies to be able to bend them without breaking, and the glue is TB 2. The water in the glue does make sense. Contributor L, I will try some of your glue. See if I get better results.

From contributor K:
Contributor L, I don't think I agree with this. If you are speaking from your experience, I suspect that you may not have realized how slow the cure takes place, and removed your work from the clamps too soon. Most epoxy alone is quite brittle. You can change the properties somewhat by what you mix into it.

If you are using a vacuum bag, the maximum PSI you can achieve is well below what is recommended for resorcinol, by a factor of about ten.

From contributor L:
My information on different glues was from a series of tests I did quite some time ago, before I had a vacuum bag setup. All parts were tested on the same form letting them set overnight. I let all of them rest for at least a month and then compared their shape to the original form. The clamping was with mechanical clamps put along the form about as close together as I could get them, but not tightened to the point of smashing the wood fibers. (If I were to do the test today, I would use a male/female mold CNC machined but I didnít have that capability then.) The strips were 1/8" thick. We did four samples for each glue using 4 different woods - hard maple, walnut, cherry and red oak, all kiln dried. Radius was 4', width was 1 3/4". The surfaces were off the planer (I wouldnít do it that way if I were to do the tests today; they would be directly off the straight line ripsaw, a better glue surface I think, but at the time I didnít have a SL and my normal practice was either from a jointed or planed edge.) After a couple of months with no observed failure, we decided to put them in a hydraulic press and see what happened. We supported the ends and pushed down in the middle until they broke. Most typically, the maple failed along the glue lines, but not always. The rest of the samples broke randomly.

True, this was not a scientific test; the numbers were too small for statistical verification. The 20 ton press did not have a pressure gage, so there was no real measurement other than observation of the deflection. I should have metered all the wood samples before we started but I didn't. My notes don't say what the grain orientation was for the various samples and it's been too long since I did the tests for my aging memory. We've done other tests over the years for finishes, contact cements, sanding, etc., all flawed in some manner, but better than not doing them at all.

From contributor V:
I am interested in your test results. What 3 adhesives were you using besides epoxy? Did you find more springback with epoxy than the other glues? Did you experience failure at the glue joints on maple only with epoxy, or with the others as well? My own experience has proved epoxy to be pretty rigid in curved laminations, though I haven't done head to head tests. I definitely have seen the results the questioner describes with water based adhesives, not so with epoxy. I don't know if it is true with all formulations of epoxy, but Gougeon Bros., makers of West System, call for sawn or roughly sanded (80#) surfaces for lamination. I have had some failures with epoxy in the past that I attribute to not knowing about this requirement. I wonder if the excessive springback you experienced might have been partially due to using planed surfaces. I believe some formulators, such as System 3, purposely make a relatively soft adhesive, whereas West System is supposed to be relatively rigid. Also, there are some urea formaldehyde glues that are supposed to be able to tolerate thick glue lines - Urac 185, available from Highland Hardware, is one such product.

From contributor D:
There are other ways to make arches that do not involve a stack of bent laminations, and don't experience springback (springforward?).

From contributor Q:
I own a custom homebuilding company. All of our projects are fancy, and finding good millworkers in my area is close to impossible. I have expanded my shop area and now have a full cabinet shop in which I am doing my first radius project. The radius is 44" and I am going to make a recessed panel to fit the radius. What type of glue should I use to laminate poplar? Does it vary depending on wood types or is it basically the same all around?

From contributor R:
The reason we do laminations is for the grain on the arch to line up. We cut our strips out of one piece of wood, so then when you glue it, mould it, stain it, it doesn't look like you pieced it together - looks like one piece of wood that is curved. I know there are other easier and faster ways that don't involve these problems, but it just doesn't look the same. It is the attention to small detail that makes the difference in our shop, I believe.

From contributor K:
To the original questioner: you never answered my question of how you clamp. Are you using a vacuum?

From contributor J:
I'm not understanding how a vacuum bag would work in bending a half round arched casing with a 72" chord and a 36" height? Please explain.

From contributor R:
We use the arch glue rack for our arches. You set your glue rack to the desired radius for your arch, glue our laminations, pull it in around the radius on the rack, and then clamp the strips with C-clamps. Virtually no glue line visible.

I have never used a vacuum bag before. Not quite sure how you would bend an arch in a vacuum. I would like to know if you could explain. Maybe I'm missing something?

From contributor K:
Contributor J, you are probably trying to visualize the form being inside of the bag, which is not very easy for this application. If only the parts being glued are inside of the bag, then things get a lot simpler, because after you spread the glue, and I will state again, I think epoxy is perfect for this. You slip the stack of parts into a bag. In this case, it can be a long narrow tube like packaging for parts, for example; then seal it up with a hose running into it.

Then you bend it around the form, and turn on the pump. It usually only requires a couple of clamps along the bottom, and once the vacuum is drawn, you could even take it off of the form for another run if you have another pump, or a larger pump with a manifold to run multiple hoses.

When the form is outside of the bag, the form can be really quite flimsy. It only needs to be strong enough to hold up to the weak force of bending the stack of lubricated parts being bent around it without collapsing.

For something of this size, if it is a one-off, I may use 1/4" MDF or plywood cover-sheets that come on stacks of sheet-goods, with 1 x 2 blocks stapled across at regular intervals about 6 - 10" apart. When I am done with it, I tear them apart and throw them away.

If the wood is fairly straight-grained, it wants to bend to a fair curve on its own, and once the vacuum is drawn. I normally draw 28" hg with my main pump, that is ~= 14 psi on the outside of the bag, which is ~= to a ton per square foot. Even with a slippery glue, this is enough to hold the shape without the form, so long as there are no leaks. I have tried to be brief, so if you have any questions, feel free to ask.

From contributor Q:
Responding to the questioner's earlier post, what would be an easier alternative to bending rails for a radius face frame without compromising quality?

From contributor L:
Epoxy, urea formaldehyde, resorcinol, Titebond Original were the glues I used for my tests. I don't know the brands other than Titebond. A major flaw in my tests was using planed surfaces.

An interesting side note: we do work for other small shops. Recently one was in to have some widebelt sanding done. They saw us gluing directly from the ripsaw and told me how much better their method of always gluing from a jointed edge was! Each to his own, I guess.

From contributor A:
Another vac bag technique, which is the standard for composite boat construction, but rarely done in woodworking... Instead of making a bag, you can use bagtape (super sticky mastic tape) and tape a bag (disposable vac film) to a melamine surface. This is a great technique for stair stringer lams, large panel/door construction (if you don't own a vac press). This technique is how boatbuilders vac bag 100'+ yachts.

From contributor P:
Is this bagtape similar to duct tape? Can duct tape be used? What is and where do you get this disposable plastic film? Would you think vapour barrier used in house construction would be usable? Does the sheet of melamine act as the surface of the vacuum table, and do you crisscross it with grooves to aid evacuation?

From contributor K:
I have used carpet tape. Normally 1 1/2" wide, with the cloth core. You can get it at any good hardware store. Don't get the plastic - it isn't sticky enough, and the fiberglass reinforced leaks through the fiber. You can get sheet vinyl from any automotive upholstery supply, and lots of other fabric stores, in 54" widths, on rolls.

I don't think the house wrap will not work, but there are plenty of other plastic film materials that will work. The thing to make sure of is that the tape will stick to it. I have used the plastic film like you would put under a slab, but even after washing down the tape area with acetone, it didn't seem to want to stick very good, until after the epoxy run cycle, and by then I could not tear it apart.

I have done quiet a few projects where I had to build the bag around the parts, because managing to lift a large stack of slippery parts maybe 20' long into a bag is just too unmanageable.

If you are using melamine for one face of the bag, and score it, you will need to make sure that the other side is pristine, and then seal all of the edges, because the core is too porous, and will leak like a sieve.

If there are broad areas, I sometime put some fabric, like an old bed-sheet, covering the top platen. For something like the jamb parts like the questioner is asking about, I usually just run something like bale-string or parachute cord up one side and down the other, giving the air a good, easy path to travel.

From contributor L:

We've got several melamine panels we've grooved and banded and we use the plastic mesh that the vacuum press companies sell to provide an air path primarily near the hose connections. We've found more uses for the vacuum bagging system than I ever thought we would. For really big bags the 6 mill poly will work but you need to form the seal by folding it over on itself and clamping. Don't figure on getting many uses out of it since it is easily damaged.