What joinery would you use for this?

[Jim Klein]

Friends -

I'm looking at building this chair and wondering how best to join the two curved pieces. I don't have a Domino or mortising machine, although I imagine I will get a Domino at some point. I have built a number of tables and chairs, and have done all of my mortising using a moxon vise and router with shop-made floating tenons. I've done lots of angled mortises, but in this instance, the surfaces to be joined have curved edges with complementary curves. I've been trying to visualize a way to create mortises that will keep the subtle curve, which I think has a lot of visual appeal. The only idea I have is to create two "jigs" (just narrow pieces of BB or MDF) that fit the curve of each mating piece on one side and are flat on the other, and to register my router (or dowel jig) against the flat side of the jig. Anyone have any advice or ideas here? Thanks!! https://www.dropbox.com/s/jrm7xkvbl3vt448/Arc2.jpeg?dl=0

[John Kananis]

How do you feel about working with a chisel and mallet? Cut the tenon on the back piece (the shorter one) with a router and template. Then, you can either go straight to a pig sticker or whatever you use to chop a mortise on the opposing piece. If you aren't that confident then use a slot cutter on the router table for the initial cut to establish a "working area" (either stack some cutters or make multiple passes) and then it'll be pretty cake to chop to depth.

[Kevin Jenness]

The only idea I have is to create two "jigs" (just narrow pieces of BB or MDF) that fit the curve of each mating piece on one side and are flat on the other, and to register my router (or dowel jig) against the flat side of the jig.

Yes, that is the way to go.

[Jamie Buxton]

The mortises (and tenons) don't have to follow the curve. What holds a M&T joint together is the glue on the faces of the tenon. To get that, the faces of the tenon should fit nicely with the walls of the mortise. But there's no need for the ends of the mortise to fit tightly to the ends of the tenon.

[Jim Becker]

Do the joinery with straight edges and then complete the rounded profile. That makes it a heck of a lot easier to use whatever mortise and tenon method you wish to use. Once the pieces are joined, you can saw close to the line with a bandsaw and complete the work with a template and pattern routing. You want no more than 1/16"-1mm left for the router to deal with an use quality router tooling for the deed.

[John TenEyck]

Following up on what Jamie said, just make your jig so you can clamp them perpendicular to the mortise center line. They don't need to be curved. You just need a flat surface (table) on top for the router to slide on. Cut an opening in the "table" for a collet to index into to cut the desired mortise width. Or use a piloted mortising bit.

John

[Tom Bender]

The forces on that joint will be minimal, a couple of dowels should suffice. To understand the forces just imagine deleting the short curved piece and holding up the back of the chair with a stack of books.

The bigger challenge will be the joints to the plywood cushion boards, especially the upper one. Might be good to hide some metal brackets there. Make them by bending flat stock. Cut to final size and drill screw holes in them prior to bending. First make paper patterns.

[Maurice Mcmurry]

I can imagine trying a ball bearing guided router bit combo. Biscuits could work too.

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[Kevin Jenness]

Do the joinery with straight edges and then complete the rounded profile. That makes it a heck of a lot easier to use whatever mortise and tenon method you wish to use. Once the pieces are joined, you can saw close to the line with a bandsaw and complete the work with a template and pattern routing. You want no more than 1/16"-1mm left for the router to deal with an use quality router tooling for the deed.

That's a good idea in principle but would be difficult in this case due to the depth of the curve. I guesstimate the distance from the chord to the inside curve at 4" or more. Even using a router on a platform keyed to the inside curve will space the reference surface well away from the mortise shoulder and limit the mortise depth. You would have to look at an accurate drawing to judge what can be done with that approach.

# 20 biscuits with a slot cutter seem a bit undersized for the scale of the work although if stacked and closely spaced might be adequate. Something similar and stouter could be accomplished using splines and a bearing guided groover on a shaper. The grooving bits for routers I have seen are limited to 1/2" cutting depth but perhaps there are larger ones available.

[Jim Becker]

Kevin, there are no curves until after the jointery is completed. But yes, there would be a need for some hand-work to clean up into the corners where the "rear leg" intersects with the longer curved element.

[Kevin Jenness]

Kevin, there are no curves until after the jointery is completed. But yes, there would be a need for some hand-work to clean up into the corners where the "rear leg" intersects with the longer curved element.

I guess I am misunderstanding your approach. I am envisioning mortising the upper piece to accept either a tenon or spline tenon, which would require a very deep reach if done before the inside curve is cut. Can you explain a little more thoroughly?

[Steve Rozmiarek]

Looks to me like you could even use biscuits on that if you are so inclined, you'd set the machine a little deeper for the concave part to compensate for the curve. That particular approach lends itself to lateral variances in the cuts. You always have a little wiggle room which would be handy for fitting.

You could do the same thing with a domino but you would have to make sure the machine is referenced on the same tangent for both pieces. Dry fit with a reference line extended across the parts, then visually line up the machine like you were sighting a rifle so it stays true to the tangent. Or you could lay a square against the fence of the domino to get more precise against the reference line.

I'd personally use a hybrid approach in my shop, I'd use the witness line and cut a mortise with the slot mortiser. Similar to the domino approach.

That is a low stress joint, pretty much anything would mechanically work I'd think.

[Joe Calhoon]

Agree the large inside curve needs to be cut first. That would require a very deep mortise otherwise. I run into similar joinery in door work. It gets more complicated with profiles and true tenons. I think loose tenon joinery if fine for this application. I would use a shaper but router process the same. Shape the big inside curve first, the leg that meets this cut the long curves last. Cut a angle similar to the curve radius on the end of this piece. Use your router jig or domino to cut the mortise for the loose tenon on the end of this. This will allow for no rocking of the jig or domino. Next. Cut the end grain curve on this. now the 2 long grain curves can be cut. Since these are cut last you have room to adjust so the joint fits perfectly.

Below are pictures of a similar door joint we did while exactly reproducing a bathroom door for a narrow gauge train bathroom for the local railroad museum. The Instagram link shows the curve shaping. Similar processes using shaper and stationary mortising machine.

https://www.instagram.com/p/CDKJc1TjWWS/?igshid=YWJhMjlhZTc=

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[Steve Demuth]

You have lots of wood to work with there. I'd use a slot cutter on a router to cut slots close to the whole length of the joint, and then join with a floating spline. If the pieces are thick enough (can't tell from the photo), I'd use two slots and splines.

[Alex Zeller]

Do you have a large bandsaw? I might try to create the front/ upper piece and then rip it into 3 pieces and then make a slot (the mortise) in the middle piece before gluing them back together. I think the gain pattern should line up and make the glue joints look invisible.