Chris Schwarz's Blog

A.J. Roubo's Sliding-Dovetail-Tenon Joint

All week I’ve been itching to saw these joints that connect the legs to the benchtop. I’ve never cut a 5″-deep dovetail joint in a 6×6, so I wasn’t sure what to expect.

It was easy going until my enormous saw suddenly stopped cutting. Had the flesh-detecting technology in my tenon saw kicked in? (Ye Olde Saw Astyntan?) But I’m getting ahead of myself here.

Let’s back up to Tuesday when I was laying out these joints. I spent a long time staring at the original plate from Roubo’s “L’Art du Menuisier,” and it wasn’t making sense to me. Robert Lang and I tried sketching the joint (electronically and on paper) to reconcile the odd perspective of the joint (I believe it’s supposed to be in parallel projection instead of in perspective, but even that doesn’t really explain it).

Oh, and there was the fact that the original text’s dimensions don’t really jibe with the drawings.

So I set forth to create a joint that resembled the drawings of workbenches shown throughout the four volumes of Roubo , and that obeyed some of the basic rules of wood-to-wood joinery set down by Joesph Moxon. And it would split my top like a muffin.

The first question was proportioning the thickness of the sliding dovetail and the tenon. These legs are finishing out a little bigger than 5″ x 5″. So I went for a 1-1/2″-thick dovetail, a 1-1/2″ thick tenon and 1-1/2″ space between the two. The remainder (a bit more than 1/2″) was the shoulder at the back.

About that angle on the dovetail. It looks a lot steeper than is typical in a drawer or carcase. Roy Underhill suggested in “The Woodwright’s Shop” to use a dovetail that has a slope of 2-1/2″ to 1″ when he built his bench with a rising dovetail.

That sloped looked too shallow. After fussing around, we settled on a slope that was 1-3/4″ to 1″. That is one steep slope (about 30Ã?°), but it looks right. So be it.

I laid out the joints last night before I left work and started in on the sawing this morning with a honking enormous 11-point tenon saw that’s 16″ long.

I needed a bigger saw. I couldn’t reach the baseline because the brass back hit the top of the leg. That was a new sensation.

So I got out my full-size ripsaw. And that’s when the fun began. Even with the big saw, it took some time to rip those cheeks. I could have written a couple blog entries while sawing one joint. But it’s going well.

Soon I’ll get to make the female part of the joint and give my mortise chisel and brace a workout.

- Christopher Schwarz

20 thoughts on “A.J. Roubo's Sliding-Dovetail-Tenon Joint

  1. Scott Keith

    I am going to venture a guess as to the utility of this joint. I believe it offers better support against side loading of the bench top than a double tenon would. Imagine standing in front of the bench and pulling on the top toward you. Assuming the leg is not supported by anything other than this joint, it will want to rotate about point where the inside face of the leg meets the bottom side of the bench top. The dovetail tenon resists this type of load much better than a secondary tenon whose outside shoulder would be rather thin in this case. There would not me much material between the outside face of the secondary tenon and the front edge of the bench top.

    Not sure if my explanation makes much since…heck I’m not even sure if I believe myslef…. but it’s at least a guess.

    Scott

  2. Tim Williams

    That back tapered, sliding dovetail is the one I am going to do on the " Royale with Cheese" bench. I have re-watched that episode ten times this week and pretty much have it laid out. Pretty intense joint.

  3. Christopher Schwarz

    David,

    I went and re-watched that show over the weekend. It’s not a traditional bench joint according to Underhill (and my research). And it requires an angled joint at the back, too.

    Very cool, however.

  4. David

    I saw Roy Underhill make a sliding dovetail tenon on his show. It was for the leg of a workbench he was building. The leg ended up flush with the top to allow for a lot of surface contact for items clamped in the vise. It was a bit different from this one, but it was awesome. It was the kind of dovetail that tapers as it slides in, so that nothing really more than gravity and the weight of the top holds it in. But because it tapers and locks it is an extremely rigid connection between the top and the leg. I think he said it was a Roubo bench and joint, but I don’t exactly remember.

  5. fred mcclure

    chris

    i am doing the same with my bench. i am using the sliding dovetail so i can take the bench apart when i move. wish mine looked that good, but the only dovetails i ever cut are on this bench. i will get better at them though–i am trying your 30 day dovetail program

    fred

  6. chris

    Well, you made me look. I checked the dimensions on the legs of my antique Roubo-esque bench…

    http://www.flickr.com/photos/c/72197075/

    A 2:1 slope is actually about right. On mine, actually, it’s 1" to 1/2", but the legs are much smaller at 4-1/4" wide and barely over 3" deep.

    As to why the funky joint… dunno. Looking at mine, the sides of the back full tenon have opened up quite a bit, but the sides on the dovetail are actually fairly tight. Maybe something to do with the dovetail being right on the edge of the board?

    c.

  7. Patrick Lund

    I figure the dovetail was to prevent warping. Being that that leg is flush with the front of the bench, should over time the tenon try to warp away from the front and create a gap in the top and stick out from the front apron. The dovetail prevents that from happening

    Patrick

  8. Christopher Schwarz

    Ah, the middle section of waste was removed with my biggest mortise chisel and my heaviest mallet. At first I tried sawing it out with a bowsaw.

    The chisel was faster for me.

  9. Christopher Schwarz

    Here’s the translated part on the joint:

    "The legs of the workbench are made from hard oak, very stiff, 6” wide by 3” or 4” in thickness; they are assembled through the top with through-tenons and through-dovetails. The custom is to make the tenon flush with the back of the leg, see figure 2. However, I believe that it would be best to leave a shoulder on the back of this same leg so that the top can rest on the shoulder on the back of the legs like on the front. This is so when workbenches get older, they don’t risk sinking in on their legs like it happens sometimes. The assembly of the legs (to the top) must be extremely tight especially along their width. And to make them even sturdier, we widen the mortises on top to make room for wood shims that we insert by force into the tenons, so that they spread in such a way “that they are as a tail” (maintained in a spread) in the mortises and consequently can’t slip back out."

    So no rationale on the dovetail.

    My thoughts: It will be way easier to make than a twin-tenon (I can saw out the front dovetail for the most part. Less bashing with a mortise chisel).

    Just a thought.

  10. Mark

    Chris, can you or anyone else enlighten me as to why this particular joint was used? The tenon is obvious but why the dovetail? Would a simple pegged tenon not perform as well as you’ve done in your previous bench(s)? I suppose the flush dovetail allows one to keep the front edge of the leg flush with the edge of the bench top over time as the two pieces shrink or swell but were 18th century joiners really that fussy with their benches?

  11. Bjenk

    To see this joint being cut is incredibly exciting and satisfying. The only other time I have ever read about anyone doing this was in Roy’s book.

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