Using Bending Rail on a Spiral Staircase

Advice on using the steel rail on a steel staircase as a form for clamping flexible rail strips for laminated glue-up. December 24, 2009

I've been asked to glue up a maple handrail using bending rail for a prefab metal spiral staircase. I have a shop full of machines, hand tools, clamps, etc. and some experience with bent laminations, but not with this application. Looking for any advice. Build a form in the shop? Try to use the staircase rail (1 1/4" square steel) to clamp to? I 'm thinking West Systems epoxy for open time and glue line. I don't think this is beyond me and I'd like to give it a go. Am I missing something, before I commit?

Forum Responses
(Architectural Woodworking Forum)
From contributor R:
You can use the metal handrail as a guide to clamp up the bending rail. The bending rail supplier can provide you with two matching profiles (the handrail profile on the inside, flat on the outside) for clamping. Basically, you will need to clamp the rail to the inside or outside of the existing steel rail. I know of no practical way of clamping the rail to the top of the steel, which would be ideal.

Yes, clamping it to the outside is a different diameter, but the finished rail will be flexible enough to fit onto the rail. There will be considerable cleanup of glue squeeze out all over the rail, and you will then need to plow the bottom of the rail to fit over the steel. It is not complex, but there are some tricks to it.

From contributor L:
You're going to have to tiptoe through a real woodworking minefield. First, you'll want to see if the rail will even bend to this radius without breaking. A typical spiral is about a 3' - 4' radius - very tight by bending rail standards. I'd be willing to bet the standard bending rail won't bend without steaming. Are you prepared to steam bend the rail?

Second, what happens with the railing at the top and bottom? That's a minefield in itself.

Third, have you worked with West before? If not, there's another very real learning curve.

Fourth, is contributor R's suggestion workable? Is this job across town from your shop? Is there even enough clearance to clamp to the existing steel railing? (Sometimes these stairs are in a very small well.) If that doesn't work out, are you prepared to build a bending form in your shop?

Fifth, with a couple of hundred railing bends under my belt, I usually pass on bending wood rails for steel stairs, spiral or otherwise. By now, can you guess why?

From contributor N:
You're going to have a very difficult time getting pre-fab bending rail to form to a typical size spiral. Curve and twist laminations (especially a spiral) are a different animal than two dimensional bends. What I do on spirals is spot weld L brackets to the top of the railing with the correct offset for the width of your railing - make sure you put the vertical portion on the inside of the curve. On a spiral I use 1/8" slices, but plan on using close to a hundred clamps.

The problem with prefab bending rail is the slices are too thick to conform to such a tight radius, but if you steam them the keys swell and won't fit together and the outer slices will cup.

Contributor R, I don't know if you've tried this, but it works pretty slick on most railings (just generally not spirals). On prefab bending rail, take the center five slices out and rip off the thickness of the steel. Then after you glue them up and stick them back together, your plow is already made and you can form the rail right over the top of the steel. You have to clamp pretty close together to keep the shape and also clamp from the underside of the steel to the top of the railing to control the twist. Also make sure to rub some paste wax on the steel prior to make it easier to take the rail off to clean it.

From contributor R:
I have indeed removed the plow prior to glue up. Even did one where a deep groove was needed up the center. That one I had to wax an unglued core and used plastic to keep them separate, then pried out the core after the glue up. Was a bear, but worked.

From contributor L:
What I've used very successfully for mould release is heavy cellophane packing tape from 3M.

I like contributor N's notion of welding the brackets on. That ought to work fine.

As for steaming and having the tongue swell on the bending rail pieces, I run them through the planer and remove the tongue. To keep them together I use vertical clamps with contoured blocks. (I cover the blocks with the cellophane tape also.) With a radius this tight, I would steam the rail first, bend it on the form, let it sit there for a few days, then glue it.

What I think we're all saying is the learning curve is pretty steep. I suspect we all learned this stuff the hard way, and at least in my case, I lost my butt on the first few jobs. The good news is I finally have enough clamps.

From contributor R:
Good point! I have seen more than 120 of the heavy Wetzler clamps on a single handrail glue up.

From the original questioner:
Thank you all for the advice! I have since found out that the spiral stairs will be fabricated right near my shop. I should be able to have the brackets welded and bend the rail in my shop prior to installation. The rail is now in cherry. If steam bending is needed to turn the radius (54" diameter), I'm not sure about the West Systems. Would plastic resin be a better choice? If we steam bend to the form (no glue) and wait overnight (or days?) to let the rail dry, what MC% should be considered?

From contributor H:
What is the profile of your rail? You should not have to steam bend at 54" diameter. I have bent 6010 profile on a helical stair to a diameter of 12" without steaming. Spirals are more about twist than bend. Your profile will have a lot to do with how hard or easy it will be.