# looking for joinery suggestions for modern chair



## jdh122 (Sep 8, 2010)

I'm working on making a chair similar to this one: 
https://inabstracto.files.wordpress.com/2011/07/spanner.jpg

It's coming along well, I finished the bent lamination for the back "spring" and carved out the backrest. I'm trying to decide how to attach the backrest to the bent spring or post. I'm not quite sure how the original that you see in the picture is attached - it does have four screws or bolts that are visible from the back. But the backrest is only about 3/8 inches thick and I can't quite imagine that wood screws in this application will hold up.
I've wondered about using bolts through from the front (with nuts in the back). I think that aesthetically it would look OK with the industrial look of the chair, but wonder if it will make the chair uncomfortable to have 4 bolts sticking into your back (even with round-headed carriage bolts). I'm afraid that if I countersunk them there would be too little wood behind them.
I also wonder about using dowels, either split-and-wedged (from the front) or not. But I'm unsure if that will hold over time given how thin the backrest is.

All suggestions are appreciated.


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## whiteshoecovers (Jun 7, 2015)

On the similar, iconic, eames plywood chair I believe there are some pucks glued to the backside of the backrest which receive the bolts from the "post".


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## jdh122 (Sep 8, 2010)

Thanks for the suggestion. I had thought about adding a thicker piece along the back of the backrest to thicken it just on the flat spot where the post attaches, but thought it would look funny and be hard to transition it with the rest of the piece of wood. Better to do what you suggest, making the pucks part of the design instead.


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## a1Jim (Aug 9, 2008)

How about a thicker seat frame all around, then do a 1/2" rabbit on the inside about a 3/4" down from the top to make it look thinner under the webbing. This makes the back and entire frame stronger.


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## JBrow (Nov 18, 2015)

jdh122,

I think chairs are the most challenging of woodworking projects. My hat is off to you. Whatever connection method used should take into account the forces that will stress the connection. The connection can then be designed to handle these forces.

I would think that stresses on the backrest and the spring that could cause the connection to fail would be relatively low. Backward pressure would be most severe when someone leans back in the chair treating it as if it were a rocking chair. This backward force would not stress the connection much if at all. Maybe you would get a little uplift force applied to the connection; otherwise most of that force would be transferred through the backrest to the backspring.

The uplift and twisting forces are more of a concern, since I would think the chair could be lifted from the backrest or the backrest used to scoot the chair along the floor. The uplifting and twisting forces would be resisted by screws or dowels running more or less parallel to the floor. Since these forces are resisted by screws or dowels, I would want to ensure these are well anchored, and like you I would worry about 3/8" being enough. I can offer two alternative ideas, neither of which will be particularly easy but both devoid of metal fasteners.

The first idea is one you have apparently considered but rejected. However, I think you could make it look fairly good. The idea is to construct a pocket into which the backspring sets. Using ¾" stock, hog out the pocket with a router, leaving sides and top of the pocket solid. Taper the top and sides with a wide bevel or large radius cove thus feathering the pocket into the backrest. Alternatively, the pocket could be rounded into an arc across its entire outward face. Then apply the pocket to the backrest after fitting to the contour of the backrest, attaching the pocket to the backrest with glue. Then slip the backspring into the pocket and glue in place. The tapers on the sides and top of the pocket would lighten the look of the pocket and give it an appearance consistent with the look of the chair. My only concern would be wood movement, since the wood grain in the pocket would be perpendicular to the wood grain on the backrest. But if the pocket is minimized in width and height, I doubt there would be a problem.

Whether you go with a pocket type design or not, making a through connection would, in my view be superior to screws. Additionally through mortise and tenon or through dowels enhance the look of craftsmanship. The connection could be made using loose tenons that pass through the pocket, the backspring and the backrest so that the ends of the loose tenons are visible from the front and the back. If the through mortises are not flared, I doubt the wedged tenons would do much to add strength the connection. If the mortises are not flared, then the mortises could be round holes and loose tenons could be dowels.

Wedges inserted into the loose tenons would strengthen the connection by keeping the loose tenons mechanically in place. But to achieve this enhanced mechanical connection, the mortises would have to be flared both on both faces (front and back of the chair). In so doing the flared loose tenons could neither move forward or back.

The second idea is a sliding dovetail connection. Cut a dovetailed groove in one piece and prepare a mating length of stock that fits into the dovetailed groove. Cut the dovetailed grove into either the backrest (probably easier) or the backspring and the mating piece of stock glued in place in a rabbet or surface mounted on the other part. The mating piece of stock would be sized in thickness so that the backrest has full bearing across the width of the backspring. The dovetailed connection is glued and made flush.

But if you are really really good and brave to boot, a wide dovetail grove could be cut in the backrest. Then dovetailed mating edges cut into the backspring. The backrest slides onto the backspring and is glued in place. Unfortunately, this requires exacting setup and perfect execution and get it wrong, you have a couple of complex parts to re-make.

Good luck with your project!


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## jdh122 (Sep 8, 2010)

Thanks for the advice. 
a1Jim, Your comment is interesting. I think that the frame is probably strong enough as is (and already glued together with the box joints). I am planning to add corner blocks just to keep it all square, but as it's already made with 3/4 inch yellow birch (stronger than oak or maple) it should hold up. If I make another one, though, I might consider using 5/4 stock and doing as you indicate.

JBrow, thanks for your detailed comment. You're right that the joint will not be subjected to much stress when in use, since the sitter will actually be pushing the backrest into place, and also that the chair will certainly be moved by lifting from the back. I think the sliding dovetail is probably a bit too complicated and risky (I'd hate to waste the time I have invested in carving out the backs by hand by screwing up and having to start over). I'm intrigued by the idea of the doweled and glued "pocket" and will have to think about it a little bit. 
Normally I wouldn't consider using screws in this kind of application, but given that the legs and arm support are all joined to the frame with carriage bolts, I'm less concerned about the aesthetics of screws than I'd normally be. But I'm just as concerned about strength…


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## bearkatwood (Aug 19, 2015)

A sliding dovetail would do the trick. You can make very strong chairs from very thin pieces if you do it right. In these videos they are making chaivari chairs. Cue the first one up to 6 min and watch how the headrest is put on.




In the second video cue it up to 3:40 and see how the headrest slides down onto the legs. 




This could be done with your backrest and spring post and it would hold well. As for putting the spring post into the seat rails bolts would work, but it would be strongest if it were somehow mortised into a support. Think about a guitar neck and how it is mortised into the body. If you bolt it without a mortise, laminate another strip to the bottom back of the post to give it some more beef. Hope this helps.


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## jdh122 (Sep 8, 2010)

Thanks, Brian. Those chaivari chairs sure are elegant, and the way the headrest attaches is really neat - most of that type of chair attach with simple round mortise and tenon, this is more interesting.


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## JBrow (Nov 18, 2015)

jdh122,

Yeah, I would not try the sliding dovetail joinery where the back slides directly onto the backspring. But I thought I would mention every mechanism I could think short of a metal fastener connection; even those beyond my level of craftsmanship.

Since you already have carriage bolts in place and therefore could easily use screws or bolts to make the back to backspring connection while being true to your design, I have additional ideas using bolts. I agree that simply installing carriage bolts as you describe could be uncomfortable. One idea uses a closet bolt and the other is a shop modified carriage bolt.

1) The easiest option is to make a trip to the hardware store and look for closet bolts, used typically to secure a toilet to the closet flange. The advantage of the closet bolt is that it has a thin flat head. However, the head of the bolt is an oval and you may not find it in a finish to your liking.

2) Alternatively a carriage bolt could be modified to create a thin flat head. I have succeeded modifying the carriage bolt by clamping it in a vice and cutting off the dome with a hack saw. It takes some patience to get the cut started and some care to get an even cut all way the through, but since I have done this, I am sure you could also. Scoring marks are left behind, which you could leave or remove with a little file work.

In both case, cut the bolts to length and use an acorn nut and washer and maybe a lock washer on the back side. Also, if you think it necessary, you could even pair out a very shallow pocket on the front side of the back to recess the flat bolt heads. About an 1/8" would be the most you need to make these flat heads set flush; less to simply recess the bolt head just a little.

You can test whether your concern about bolt head comfort is well founded by making a mock up. You could get more elaborate with the mock up, but this is the simplest and fastest one I can think of. Install the bolts in a piece of plywood cut to the approximate size of the back. Lay the plywood on the floor and then position yourself on the plywood as if you were sitting in the chair.  I know this sounds a little goofy, but I would want to be confident that whatever connection method I made would not detract from the comfort of the chair. If someone finds you trying to lay on a piece of plywood in the middle of the shop floor, please do not dime me out.


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