Three way joint help

[Prashun Patel]

(there's no traffic on the design forum, so I'm posting this here...)

I'm making a chair that requires the leg, seat support, and back support to intersect close to the same point.

I'd like to cut this as 3 pieces per the picture, but the M&T's would intersect. Any ideas?

I thought of making the left two pieces out of a single piece. However, cutting the angles sides perfectly seems trickier than a three-way glue up.

As this joint is the most stressed part of a chair, it needs to be strong.

Any ideas?

[Michael Heffernan]

Prashun, I haven't built a lot of chairs, but enough to understand some of the intricacies of the construction. I've seen chairs that are built with two pieces for the leg and back support, and all of them have had structural failure of varying degrees. I've always crafted my chairs from a solid piece for the back/leg and M/T the seat stretchers into it.
I just finished 10 counter height chairs out of walnut for a client, with a similar profile for the back in your picture. I sorted through a lot of 8/4 and 10/4 boards at my local lumber supplier, looking for good grain direction (slight curves). A lot of waste when cutting them out, but worth it for the look and structural integrity.
I made a template for the back/leg profile out of 1/4" hardboard, rough cut the pieces on the bandsaw, then cleaned them up using the template on my router table with a bearing straight bit. Didn't take long.
Let us know what you've come up with.

[Prashun Patel]

I normally (as if I've built more than a few chairs) build them as you say: with continuous leg/back support. However, this one is an Adirondak style lounge chair. So the rear leg and back are at a sufficiently large angle that I won't be able to find a single piece with grain that won't run out in the leg.

[Peter Kelly]

That rear leg seems like it'd be a good candidate for a bent lamination.

[Loren Woirhaye]

Open mortise & tenon for the leg. A 4" long spline should do. Let the glue cure, then you can machine a blind or not-blind half-lap or dovetail half lap into the inner face of the leg for the side rail. The spline can be towards the outside if you like so the half lap is not machined into the spline, not that I believe that going into the spline would be weaker. Alternately the side rail could be attached with dowels, biscuits or pocket screws penetrating the spline. Corner blocks are often important to a chair's strength when lower stretchers are absent. Corner blocks keep the joints together since racking forces can be considerable and without them the tenon shoulders get bruised and pretty soon the tenon may be working its way out.

Another way to go is half-lap the leg and treat it thereafter as one piece and put a mortise in it.

Looking at the picture of the chair you're imitating, I have the leg going the wrong way in my head, but the stuff above still applies. You'll notice the leg in the chair in the picture is sawn out of straight grained wood. What you may not have noticed is that it's common in production chair making to glue up blanks of 2" wide stock to saw legs out of and it looks to me like there may be a such a glue joint in the chair in the image.

[Dave Richards]

How about a long bridle joint between the seat rail and the rear leg. Then a simple Mortise and tenon between that assembly and the back rest.

[Andrew Hughes]

Bent lamination is the way to go.Nice looking chair design.
Will she be walnut?

[Jamie Buxton]

Prashun, you don't say how thick the three parts are. That has a bit to do with how much tenon you can fit inside the joint.

One approach is to make one tenon -- say the up-down tenon in the pic -- wide, and drive the mortise for the left-right tenon right through the middle of the up-down tenon. The left-right tenon would be thinner than the up-down one, both the tenons have full-length glue faces.

Another possibility is to make a single Y-shaped floating tenon from plywood -- say Baltic Birch. You could even make double y-shaped floating tenons from thinner plywood. That is, there'd be double mortises in each of the three pieces.
Earlier this year, I built a dining table which had a similar joint among three pieces which are about 2"x3" in cross-section. That one got five Y-shaped floating tenons, each cut from 6 mm Finn ply.

[Shawn Pixley]

I really like Jamie's idea of the three way tenon. If everthing is in the same plane, it should be easy and an ideal application for it.

[Pat Barry]

I normally (as if I've built more than a few chairs) build them as you say: with continuous leg/back support. However, this one is an Adirondak style lounge chair. So the rear leg and back are at a sufficiently large angle that I won't be able to find a single piece with grain that won't run out in the leg.
Why not build it just like the picture? It sure looks like a two piece construction with the back being attached to the leg/seat rail. Put your efforts into making this single joint as strong as possible

[lowell holmes]

+1 for Pat.

I would find a piece of wood large enough to make the leg out of and orient the grain with the long part of the leg.

I would glue up a board if I had to to get the piece large enough.

The back could be a mortise and tenon joint with dowel pins in the joint.

I also would make a model out of fir from the local borg.

[Loren Woirhaye]

For sure, always mess around with mockups when fooling with chairs. I don't even really draw chairs... I blunder around with mockups instead. It takes some time but the feedback on how it feels in relation to every part of your body is most useful and you'll avoid wasting costlier hardwoods if you work out the problems with a mockup.

[Jim Matthews]

Brian Boggs makes a chair with a three-way "pinwheel" joint.
Brian is friendly, and accommodating - I would drop him a line about this.

He can likely steer you toward something that you can simply make, without extensive shop tooling.

The legs are pretty beefy, to allow lots of glue area.

I concur that bent laminations would be easier to handle,
if you're only making a few chairs.


http://www.youtube.com/watch?v=ug3Tsn45Ao0

[Prashun Patel]

Thanks for all the replies. Someone emailed me that video. Fun watching, but I'm not that good at that kind of clamping. I've done three-way miters before and they're just so darn difficult to clamp properly and quickly.

I have been playing with the design and I think I'll go back to the leg/back continuous piece. It just feels more right to me. The seat will be solid stock. I'll notch the corner and M&T it into the leg from the front. I'll post a pic when I make some progress.

Thanks everyone. This was very informative.

[Pat Barry]

I think you are putting quite a signifcant angle into play to have the back and the leg made from a common board. Perhaps I misunderstood the similarity of your project to the picture that you posted as reference? I took it literally and therefore my response to combine the leg and seat into a single board with a separate back piece. I think you may be headed for mechanical issues to have the back and leg be combined.

[Kent A Bathurst]

"Three way joint help (http://www.sawmillcreek.org/showthread.php?219545-Three-way-joint-help)"


Oooopsie - wrong thread.

I was reaching back to my long-haired-war-protesting-hippie college days, when the correct answer was 2 frat brothers and a pair of hemostats.


Never mind. Carry on.