Cut the Sliding Dovetail Tenon Joint for the Roubo Workbench
Those of you who have been following Chris Schwarz's blow-by-blow blog entries regarding how build the Roubo workbench using 5" thick timbers with only hand tools, might be tempted to observe that the most daunting task is creating the Sliding-Dovetail-Tenon Joint, chopping out their associated mortises, and then actually pulling off the mating of your five-inch-thick benchtop with those beautiful, massive posts and their seemingly complex double tenons.
As you're going through this process, bear in mind that if I could pull this off several years ago as a rank amateur, then so can you. You see--when Chris first posted the Roubo Workbench article back in the Fall 2005 issue of WoodWorking Magazine (stimulating a flood of responses on the blog!), I knew I wanted to make this bench in the manner Andre Roubo originally developed. Besides, I had some really nice walnut on hand from a tree that blew down on my acreage, and I thought the double through-tenons would look great piercing through my benchtop.
So here are the steps I recommend:
Practice on 2 x 3 (or 3 x 4, or whatever) junk piece of wood the first time you go about this. Practice the layout, get a feel for the procedure, and work out the kind of rhythm that works best for you. Even while recreating the joint for the purpose of this DIY article, I still had my 'woulda-coulda-shoulda' moments while going through the practice routine, which prepared me better for the real thing. Note the 2 x 3 chunk of black locust I'm sawing up here. I like testing my Bad Axe saws out on black locust, because it's ornery wood with dense grain about as hard as hickory, so if I can succeed with black locust, then the saw is ready to cut just about anything else.
Now for the real thing : Joint and plane your timber to your desired thickness. Square off one end of your timber to make the cut. I wouldn't cut it to 34" length yet (or whatever final height you're shooting for, based on your physique). In fact, I'd start working on a 40" post; that way, if you completely goof up the joint, you get another chance to do it again on the same post by simply cutting off the mistake. I like any excuse I've got to use my Millers Falls Langdon Acme No. 75 miter box, with its honkin' big 30" Simonds back saw, but in these pics I'm demonstrating how, with careful sawing, you can achieve the same square cuts with one of my bench hook sets and the Bad Axe 16" saw I sell.
Mark your Layout: For the 5-x walnut timbers depicted in the following graphics, note that I'm using 1.5" widths for both the dovetail, the tenon, and the airgap in between. This leaves 3/4" for the tenon shoulder on the backside. Given that I've cut this tenon with my 18" saw at 4.5" under the back, I limited depth of cut for a 4.5" benchtop. For those of you who want a beefier saw that will reach a full 5" depth of cut, look at my Roubo Beastmaster, which I have designed specifically for this application, as well as other timber-framing requirements.
Time to start sawing your cuts to depth: Fire up the biggest, baddest backsaw in your inventory (give yourself a good stretch before engaging in the upcoming cardio, by the way, lol), and start sawing away. During this stage, you'll want to saw all tenon cheeks to depth. In this example, I'm using my Bad Axe 18" saw, which has 4.5" under the back, so I'll simply saw as far as I can go.During this stage, you'll make five cuts to depth with your back saw: the tenon shoulder cheek, the inside cheeks of both main tenons, and the 45o angle cuts of the dovetail. Afterwards, make multiple cuts in the waste area between the two tenons with a coarse rip handsaw before chiseling out the waste.
Now it's time to chisel out the waste, clean up the air gap, and cut the back tenon shoulder. I like deepening the scoring marks with a bench chisel, then wail away on the multiple cuts in the waste area with a heavy mortising chisel, like the Ray Iles 3/8" pigsticker I'm using here. Really need to buy the 1/2" chisel he makes for this particular job, LOL. You'll find a shoulder plane handy for cleaning out the floor of the air gap (or mortise, I should say).
And then you're done! All it took was some care in marking out your cut lines, a couple of stout saws to make your cuts, a couple of stout chisels to pound out the waste, and a stout heart capable of rising to the cardio occasion, lol.