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Curved Staircase and Rail Discussion

The Furniture-Making Forum takes on an architectural question: bending and laminating wood for curved stair projects. July 25, 2005

I have a curved freestanding staircase project underway for a somewhat high profile client. I am wondering for tight radii, what is the best way to bend the cherry for small thin stock vs. larger cherry handrail and such? I have been considering an anhydrous ammonia technique, or perhaps trying to shape in place.

Also, on the closed curb stringers, I am planning on using paperbacked veneer and Heat-Lock Iron activated adhesive. A sample and some experiments proved this to be a viable option, however I suspect there may be better techniques. Does anyone have any advice about this?

Forum Responses
(Furniture Making Forum)
From contributor J:
Make a mockup of the stairway core. Use laminating bags for multiply layup using urea resin adhesive. (I would not use pva or pre-cat adhesives).

From contributor R:
Where did you hear about the anhydrous ammonia bending? That requires an elaborate expensive lab set up, not something to be set up in any workshop. It抯 more an MIT thing for a trained scientist. Laminated is the way to go.

There's a place that offers the service of a frame saw to do the laminates. The kerf is .035 and it comes out of the saw ready to glue. No sanding is necessary, and because the kerf is so thin you can barely tell the board is re-sawn.

From contributor K:

I would recommend that you use epoxy for your adhesive for this project. It has all of the properties for this kind of project and some significant advantages over the others:

1. It has a long open time before the clamps or vacuum have to be set.
2. It has the best lubricity. It is like butter between the plys.
3. You can thicken it to improve the properties to your needs, and it likes a thick glue joint, and doesn't require a tremendous clamp pressure psi.
4. It does not have any water, so the wood and glue are not shrinking after the drying process starts, and forever after.
5. It is rigid glue which does not creep, which reduces spring-back.

It may be helpful to hear what you are using under the cherry veneer, and to know what dimensions the handrail will be etc. There has been more discussion of this in the architectural forum. You should try to access the knowledge base there. Also, I was just wondering what kind of floor you are starting up from and what are you tying into?

From contributor J:
I would caution against using paperback veneer for a stair stringer due to durability concerns - the wood layer is typically only about 1/2 the .020" thickness. One good shoe biff and you are into the paper. I would use a drum form, a large vacuum bag, and epoxy to form the stringers. 1/16" sliced veneer should be available from certainly wood, although I have found that the thicker sliced veneers tend to have significant face checking, which would argue for using a shop made thick veneer for the face layer.

From contributor A:
I have been building geometrical stairs for over 15 years, and I have had good results with using a method used by boat builders called strip-built. I have cutters from one quarter inch to five quarters. These strips are glued up with west system epoxy, and I have had very good results. You can make very tight bends and you can also make them an inch or more thick and I use them for structural components. I was wondering if anyone knows a method for profiling the hand rails of an elliptical stair. The rails are the one thing I feel I need some improvement with.

From contributor K:
To contributor A: I have done elliptical stairs before, and I抦 not sure that doubling what you would charge for a smooth arc circular is quite enough. Here are some things to consider.

First, if you do as I did, lay out the treads so that they all have the same rise and run and are given distance from either the inside or outside rail. When you project up from your floor plan to your target marks on you form studs and pick your pattern off of that, you will find that the inside stringers and rails will rise much faster through the tight curve of the elliptical ends, while the outside be rising more slowly, which means that these parts will need a dog-leg up in the former, and a longer sweep down in the later.

If this doesn't sound like a big deal, consider that if you don't have a splice in those parts to compensate for this, then you may have enough grain run-out, that your laminate parts may break instead of making a tight twisting bend
This is only the beginning of the tricky stuff. Each baluster will have a different pitch from the one next to it, and through the tight bend, mine had a two degree difference between the outside and inside of each baluster, that is a twisted bevel. I was using closed end stringers.

Another tricky thing was that the banister rail has to be positioned exactly plumb over where it was bent for. There is no margin for error here. You can not make an arbitrary cut somewhere along the arc, and expect to be able to scoot the rail down from there. Each end and the middle have to go up the same distance and exactly plumb over where it was bent. You can not begin to imagine how hard it is to find this point in space while trying to splice into other finishing touches.

If you are up for this kind of challenge, you may as well go all out and make it free-flying to maximize the visual impact. It would be way too easy to have it supported inside and out with walls.

As for hand rail shaping, I used a big router with a sub-base, and fence. I went with a simple elliptical shape, about 2" X 3" which was somewhat more forgiving than something with a lot of fillets and beads etc. when it came to fairing the curves, and it felt good to the hand.

I know this sound like I took the easy route, but when they came to the elliptical newell post, the outer half of the rails wound around them with a changing pitch like a vine growing around a tree, and came off the other sides at the new height in a very smooth transition.

The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Comment from contributor A:
The best thing to use to bend your stringers would be 1/4" luan. Rip it down to the width of the 2x10 or 2x12 according the run of the stair, which is the width you want the step to be. Use a strip at a time, until you get the right thickness. I would suggest using an epoxy glue, but it is also known as resin glue. You mix it with water in order to activate the bonding chemicals.

For your question on bending your rail, rip your rail down to 1/4" strips which is easier for bending, then use vinegar on your rail to get the best bend. I know it sounds weird, but I have learned it is the best way to do it. Water works well, but if the bend is deep then I would suggest the vinegar.