Gluing and Clamping a Curved Laminated Stair Rail

      A discussion of the fine points of glue choice, glue mixing, clamping methods, and allowing for springback. March 18, 2006

I'm bending a curved rail for the first time and think it wise to use a plastic resin glue for increased working time. I've read that there is less spring back with plastic resin glues than with white glues. How much spring back can I expect? I'm bending an oak, 6010, 7-ply rail to approximate 52 inch radius.

Forum Responses
(Architectural Woodworking Forum)
From contributor R:
In my limited experience, the article below is accurate. I agree that a hard glue is best (plastic resin or epoxy, etc), not so much because of the initial spring back, but because of creep.

Calculating Springback

From contributor H:
We bend rail all the time. 6010 being the smallest style we use. We have always used Titebond yellow glue. Here's what we have found to work: Bend your rail dry first to get the rake and twist right and leave it overnight. The next day, remove and glue, then re-bend (it will bend easier now). Then leave it on the form for 3 days. This allows the glue to dry completely everywhere. We have virtually no spring back. Keep in mind that when you install your rail, it will be held in place by the balustrade and newel posts. Creep after it's installed will be eliminated for all practical purposes. If you want to rush, go with the plastic resin.

From the original questioner:
Based on the above link, the spring back I calculate is .3", which is about 5/16". I just spoke with a rail installer who recommends a 1/4" allowance. I currently have my brackets set up for 1/2" spring back and am thinking of moving them in a bit. I like the above pre-bend procedure. Three day wait is not a problem. I'm in no hurry to screw this up. Contributor H, would you recommend no spring back allowance or, say, 1/4"?

From contributor H:
1/4" allowance is fine if that's what you want to do. In a 52" radius over 13 risers, the rail must be around 12' long +-, so a 1/4" to pull back into shape is, in my opinion, basically nothing. A tip to make seeing things as you bend easier: Don't use full lengths of bending moulding. Instead, use only small sections where the rail will be clamped to the form. This allows you to see what's happening in between the studs of the form. Then spring clamp/c clamp/whatever clamp you use between them. The rail may also split lengthwise after it has been clamped due to the twisting force that's being applied to it. The rail is not ruined. Just fill and sand.

From contributor J:
If you are using standard fittings (gooseneck, volute) you will also need to compensate for them. When we bend a rail we do not compensate for the radius, but we do over bend and over rotate the top and bottom. This is because you are using flat single curvature fittings joining to a double curvature rail. Estimate the location of where the fitting will attach to the rail and over bend the radius by 1/2", then over rotate the rail by placing a 1/4" shim at the bottom of the rail for the gooseneck, and the top of the rail for the volute. This will help the profiles line up properly. We also use plastic resin glue, but be sure it's fresh (less than 6 months old from date of manufacture - National Casein does date their glue).

From the original questioner:
A brief bit of history, then some questions. I'm a homeowner who is redoing a rail job done by a subcontractor. The guy built in 1/2" for spring back and over-bent the end of the rail where it transitions to the gooseneck. At the volute end, he did not over-bend. In the process of trying to fix his work, I became quite familiar with the challenge of trying to get standard fittings to align to both the curved rail and the vertical of the gooseneck.

Do you over-bend each end? Can you explain the twist compensation a bit more? Are you saying I should 1) put a 1/4" shim between the rail and the tread bracket so that the rail bends up at that gooseneck/volute locations, or 2) put the shim under the rail, and at the inside edge, and clamp from the top to impart a twist at the gooseneck/volute locations? Are we talking about a twist or a bend? Do you overcompensate the twist/bend for spring back?

From contributor J:
The shim is to over rotate the rail so that it will line up with standard fittings. We also over bend the ends, not for spring back but for fitting alignment. I have tried some of the formulas in the past, and they are pretty good, but I do so much of this now it's mostly just experience. It does help to draw out the fitting in relation to the rail tangents. This will give you an idea of how much you need to over bend. The over rotation is more for a tighter radius, but with a 52" radius a 1/4" shim should be fine. If you don't shim it, it will just mean more sanding/scraping to get the profiles to line up. Now all of this is assuming that all treads are on the radius and not straight, starting or ending the flight, in which case the over bend and over rotation are not needed.

From the original questioner:
So the rotation is twist around an axis perpendicular to the cross section of the rail?

From contributor J:

From contributor O:
I've generally used Titebond II. Unibond (urea) is also a good choice, and allows more time, and they dry fairly clear. From the center clamping bracket, which is set on the money, set each successive one in both directions 1/16" tighter, which allows for any spring back. The first from center is 1/16, next is 1/8, next is 3/16 and so on.

I've not used the idea of pre-bending for 24 hours, but I like it. And I do as suggested, using small sections of bending mould about 1 1/4 to 1 1/2' long. You generally put about 2-3 between brackets. One thing that's important is that the rail will have a tendency to twist as you get to each end when bending. This is because you're bending in two directions, so as you get to the end, generally you need to put a hand screw clamp around rail and use a brace or kicker to slightly over untwist, and the brace or kicker to hold in position. I leave in brackets for three days, then it's cleanup time. Be prepared - you're going to need a lot of C-clamps and some hand screw clamps (Jorgenson's). Allow at least 4 C-clamps per tread and end needs a few more. Before applying glue, pre-adjust clamps and have them close to where you'll need them. A test run is good if this is your first. Once you put the glue on and set the laminations together, wipe down as much squeeze out as you can and put a couple turns of blue tape around, every couple feet, to help hold together as you assemble to brackets. Then it gets to be an intense rush to get it on within your glue set time window.

From the original questioner:
Can you give me the specifics on the glue you use from National Casein? Product type/name. Where do you get it? Also, what happens if the stuff is too old?

From contributor J:
We buy direct from National Casein. The resin glue is DR regular. We mix 5 parts powder to 2 parts water. I don't know what the smallest size container they sell is, but you would only need 10 lbs. We have also used Weldwood plastic resin, which is usually sold in hardware stores, but as I stated, the age must be less than a year. What happens if it's too old? The catalyst has started to kick and the glue will not set properly. The mix is also lumpy. By the way, put only half of the water into the powder and mix very well. Then keep adding until all of the water is added. The consistency should be a little thinner than yellow glue. Use a 4" roller to apply. Also try Highland Hardware in Atl. if you are unable to get it locally.

From the original questioner:
Ten pounds sounds like a lot, but I don't have a good feel for how much volume that creates. If I mix 10 pounds, will I end up using most of it? By the way, thanks so much for all your advice. The tips I've received have been tremendously helpful.

From contributor J:
10# should be much more than you need (if you are gluing up one 16' rail). But the glue is cheap and it would be better to have extra. I believe that it is sold in a 10# pail (about 1 gallon). 5 cups of powder to 2 cups of water is enough for a 30'+ rail.

From contributor S:
All this info is good. I've bent a few hundred rails. Plastic resin glue is the way to go. Coat both sides of wood. Mix the water in very slowly, till you get peanut butter (not crunchy), then add more till you get close to normal glue. Let stand for about 20 minutes before use. I use duct tape wrapped inside out to hold laminations together till I get it in clamps. Use plenty of clamps. Have plenty of water and rags handy for cleanup of rail and yourself.

From the original questioner:
Good advice, but a little too late. The first rail I glued up I only coated one side and the rail split at the gooseneck. Not severe, but enough to cause concern. So I'm gluing up a new one tomorrow. The reason I only glued one side was because of the working time issue and dealing with the buttered-side-down problem. This time I'm going to have multiple people spreading glue.

I mixed the glue pretty much as you described. But I ended up with small clumps that I could not mix out. They were enough of a concern that I strained the glue with my wife's kitchen strainer. Have you had problems with very small clumps? They were not big and were more akin to sawdust in the glue.

Also, there was spring back on the order of 3/8" and the over twist advice contributor J gave (1/4" shims) was dead on.

From contributor S:
Yeah, I've ruined a few of my wife's kitchen tools too. Straining the fine stuff out never seemed to make a difference in the glue up. I think the temp or age of glue affects this. I got smart and started going to goodwill for strainers and mixers (work great).

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