Bending Large Pieces

Experienced hands offer tips on bending a thick piece of cherry for a curved rail. June 12, 2005

I've read a lot about bending wood. I need to bend a piece of cherry (13' long, 3" wide, and 3/4" thick) to be a bar rail on a curved bar. Will these steaming methods work for this thickness (3/4")? Any help is appreciated.

Forum Responses
(Architectural Woodworking Forum)
From contributor B:
Steam bending will work, but that is a rather large piece for a first time attempt at the process.
Laminating 1/8" strips around a form is another tried and true method. Either method requires calculating in for some spring back.

From contributor G:
Cherry is not an easy wood to bend, and the steaming can make it really dark. My suggestion is to rip down material like you do for a hand rail. Then drill a hole along the front edge, large enough for clamps. Then glue your strips together and starting in middle, clamp them down.

From the original questioner:
Won't the darkness leave the wood as it dries out? Also, if I put Kerf cuts on the inside of the curve, will it help? I would put the Kerf cuts along inside of the curve, but only about 2/3 of the width so they are not visible from the top of the rail. Does anyone have any advice about that?

From contributor J:
To the original questioner: Kerfing will not work unless the kerf is cut the full width of the board. The only way that I see the kerfs working is if this is some sort of apron that will be covered on the top.

As for the steaming - make sure that you aren't using kiln dried stock. The wood should be green for steam bending. The linen in the fibers sets like glue during the kiln process and will cause splitting and splintering when you try to bend it.

From contributor M:
To the original questioner: Anhydrous ammonia could be used for your application, but I doubt this technique is feasible. Have you considered buying 13' of bender rail from one of the many stair parts suppliers?

From contributor M:
What type of radius is it? You might want to try saturating the wood in water for a few days rather than steaming. It is a pain in the butt and dangerous as well. It also offers no advantages for degree of bending as far as I can tell. If you are working a very tight radius you will need to cut in strips and laminate.

Another possibility is to get cherry straight from the mill off a green log and you will find it bends reasonably well if cut into strips. You would need to clamp it to a form and exercise patience as it dries. I have made some pretty tight corners with white oak using this technique. Gluing the strips together works best after some surface drying has occurred.

From contributor L:
I'd make strips and glue them up using a hard setting glue (not PVA) if the bend is very tight. You can get away with yellow glue on very gentle bends with lots of thin strips. Gluing directly to the top will ensure a good fit. Figuring spring back is tough if you are going to install after you’ve made the bent part. Note that hard setting glues generally are not gap filling, and you must use a lot of clamps and a cull to prevent denting the cherry.

I did an experiment to see what kind of spring pack and long term creep various glues gave. PVA (yellow or white glues) had lots of creep, so did epoxy and urethanes. Urea formaldehyde and resorcinol both held with little or no long term creep. Resorcinol is dark reddish brown and water proof. Urea formaldehyde is very brittle when it has any thickness in the glue line, and will fail under impact where there is noticeable glue thickness.

We are making curved cherry base and chair rail in our shop right now and we use the CNC router to make the inner and outer forms for these kinds of curves. The casing is usually glued up on an adjustable steel form, and the longer it can be left on the form, the fewer problems with spring back.

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

Comment from contributor V:
I have had good experience with bending 1 x stock red oak boards to almost 90 degree angles after first soaking in water and then boiling. (This is with straight grain wood with the heartwood of the tree facing away from the curve). A 1" x 2.5" x 72" board usually takes me 2 hours of boiling to produce such a result. Using a jig with a curved radius I will slowly bend the wood, boil it more and bend it again. I repeat this process until the desired result is achieved.(Quick note: you have a window of 30 seconds max to start bending the wood once it is pulled out of boiling water so have clamps and jigs ready).

For an added precaution you can use a metal band applied to the outside face of the curve when bending and clamping. This piece of metal will help keep the grain from separating when using less than desirable kiln dried lumber. It works well for making traditional re-curve, reflex, and de-flex bows where both ends and the middle of the bow stave must be curved at the same time and clamped in a jig until dry. I don't see why this would not work for a bar moulding (especially considering that it will be a stationary piece - a bow has to continue to maintain its shape through many years of use and abuse). It can be a day long project just to do the boiling process.