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How is a curved wooden seat/back made?

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Forum topic by JohnEbinezer posted 04-22-2016 12:16 PM 556 views 0 times favorited 6 replies Add to Favorites Watch
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JohnEbinezer

3 posts in 232 days


04-22-2016 12:16 PM

Hello Everyone.
I’m pretty new to the art of wood working. I’d like to know how curved wooden seat/ back for chairs are made. Is there a machine that cuts wood in curved pattern? Or, it has to be completely hand made? Either way, I’d be happy if someone can explain how it is done. A video would be even better.
BTW, I have seen how plywood is bent and used as seat and back. My question is only about solid wood.
Thanks.


6 replies so far

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bondogaposis

4037 posts in 1819 days


#1 posted 04-22-2016 01:06 PM

It is often steamed and bent on a form, or laminated from thin strips, or sawed on a band saw from thick stock.

Is there a machine that cuts wood in curved pattern?
Yes, the band saw.

-- Bondo Gaposis

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Waldo88

188 posts in 764 days


#2 posted 04-22-2016 04:01 PM

You can also join pieces together to make a curve.

“The Chair” by Wegner for example is one curve between the two arms and the back, the 3 pieces are joined toether and then shaped.

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JohnEbinezer

3 posts in 232 days


#3 posted 04-23-2016 06:14 AM

Thank you Bondo and Waldo.

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JBrow

822 posts in 388 days


#4 posted 04-23-2016 05:02 PM

JohnEbinezer,

I have not done steam bending nor have I sculpted a wood seat, but have done bent laminations and simply cut the workpiece into a curved form.

Bent laminations and steam bending both require bending forms at least as tall at the work piece is wide. There are two parts to the form, which can be made from MDF or plywood. One part is used to form the wood to the shape desired. The other part is the complementary caul that is used to pull the wood into place against the bending form.

The plywood or MDF is laminated together to get the needed thickness. The lamination is cut to desired bending shape and the clamping surfaces faired and smoothed. The mating forms are butted against one another and the caul is scribed to account for the thickness of the workpiece to be bent. After cutting the caul to shape, it too is faired and smoothed. Both half of the forms are drilled to accommodate the million clamps need to draw the caul tight to the bending part of the form. The bending form is firmly mounted to a sub straight such as ¾” plywood. At this point things are ready for steam bending. However if the workpiece curve will be a lamination with glue, the bending form, bending caul and the substrate are given a really good coat of wax to make releasing the bent work piece from the form easier. Packing tape on the bending form and caul can be an alternative to waxing.

If steam bending, and with clamps at the ready, the work piece is heated until bendable and immediately and quickly forced into shape. Once the wood cools, forcing it around curves is difficult and could cause the work piece to crack. Straight grain lumber is best since the wood fibers are all parallel. Otherwise there is a greater chance that the workpiece could crack as it is formed to shape.

If a bent lamination, thin strips of wood are cut. The tighter the radius of the bends, the thinner the strips should be. An 1/8” thick is generally a good thickness. The strips should be wider that the final dimension. The strips are labeled for reassembly in the order in which they were cut. A few extra strips are good to have on hand in case one cracks when bending. A good amount of glue is applied to the surfaces of the strips, stacked together and placed in the bending form. The bending caul is butted against the stack and starting at the center of the curved form, clamps are applied to draw the laminations together. As the clamps are applied, a dead blow hammer can be used to keep the stack of strips flush. Once in the form, the glue must be fully cured before removing from the bending form. Then workpiece cleanup and dimensioning to final width is done.

Whether a bent lamination or steam bent workpiece, once it is removed from the bending form, some spring back will occur, changing the radius of the curves a bit. After the initial spring back, the curved form in the workpiece is stable. I have read that Resorcinol glue is more rigid than carpenters glue, and therefore reduces the extent of spring back. I have not used it, but believe this 2 part glue is probably difficult to work with.

Cutting wood to shape is easy, but can waste wood and is generally not as strong as bending techniques. Gluing up shorter pieces to form the approximate shape before cutting adds strength to the curved workpiece since grain direction can be engineered. If cutting multiple identical curved parts, a making a router temple and using a flush trim bit is very useful.

Sculpting wood for a seat, something I have not done, is performed with a few different tools. I have seen a right angle grinder hog out the bulk of the material followed by a draw knife and then a lot of scrapping and sanding.

I have not checked, but I am sure there are YouTube videos that show the various wood bending techniques. Also, this season, a bent lamination chair was made on the PBS show “Rough Cut”. I am not sure whether this episode can be viewed on the “Rough Cut” web site but may be worth a look.

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JohnEbinezer

3 posts in 232 days


#5 posted 04-23-2016 05:50 PM

Woah! that’s a lot of information JBrow. Thanks.
I’ve seen the videos of bending techniques. I’m more interested cutting wood to shape.
Especially, cutting the curved back of a chair. I’ll check Rough cut.
Thanks again.

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JBrow

822 posts in 388 days


#6 posted 04-24-2016 02:36 PM

JohnEbinezer,

Sorry for the info overload. You were discussing curves for a chair, a piece of furniture that is subject to a lot of stress. It seems that somewhere down the road, someone always tries to turn a 4 legged chair into a rocking chair, putting a lot of stress on the curved back at the center of the curve and the joinery at the back. When a piece of wood is cut to shape, it tends to be a little weaker since wood fibers exit the surface of the inside and outside of the curve especially at the center of the curve. Therefore thicker the workpiece the stronger the curved piece can be. Also the gentler the curve, the stronger the piece will be since fewer surface fibers are cut.

Waldo88’s suggestion for a segmented curve is one way to add strength to the cut curved piece especially when the curves are severe, since fewer wood fibers will exit along the curved surfaces. The key to this method is strong joints where the segments meet.

There is no doubt that cutting the curves is the fastest and easiest way to obtain a curve. It is the way I go for workpieces that will not see a lot of stress.

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