Join workbench top to legs

[Billy McCarthy]

After many many moons with my benchtop on a pair of sawhorses, I'm finally ready to finish this project. The top isn't super thick, about 1 3/4" to 1 7/8" once I flatten it. I'm planning on using 4x4 lumber for the legs and many of the things I've seen and read have a single tenon and a dovetail to join the legs to the top. Does anyone know why a dovetail is used instead of a second tenon? I presume it's because it allows you to have the joint flush with the front of the bench, but why is that needed?

For reference, look at at the front right leg on this one built by Chris Schwarz http://blog.lostartpress.com/2010/05/28/another-roubo-workbench-fin/

[Jim Koepke]

Does anyone know why a dovetail is used instead of a second tenon? I presume it's because it allows you to have the joint flush with the front of the bench, but why is that needed?

I think the dovetail is used just to show the makers mastery of the joint. Often it is a "double" dovetail joint. Google > impossible dovetail joint < and you will see a few images and some sites show how to make it.

A leg flush with the front of the bench is handy when clamping large pieces to the front of the bench. My work is often held in the face vise and clamped to the closest leg to hold bigger pieces steady. Of course with a leg vise this only works for big pieces that span as wide as the bench.

Sometimes if a board's end grain is being worked a block is clamped to the leg to keep it from slipping out of vertical.

jtk

[Curt Putnam]

I'm going to guess that, with a leg vise, if the chop bears on the top instead of the leg, the top is likely to move by at least the slop in the tenon + compressibility of the wood. I'm going to do a split top and the leg will be the bearing surface for the top. Hopefully someone who really know of what they speak will chime in and give us the straight scoop.

[Jessica Pierce-LaRose]

having the front plane of the bench flush is helpful for some clamping tasks, as Jim mentions. I'd guess that another big reason that the dovetailed part of the double tenon shows up so much in vintage benches is that it's a heck of lot easier (particularly when you're working with giant slab top benches) to saw out a dovetail than making another big mortise. Saw the angled sides, and then some relief cuts, and you can whack away the waste across the grain pretty quickly. Chopping a through mortise of that size, particularly in the era of Roubo, before screw-tipped augers, was probably more work.

[ryan carlino]

The dovetail also is supposed to force the wood movement in the top to head in the other direction. If the wood expands, the dovetail maintains the flush front and forces the expansion to the back.

I built my split-top Roubo with regular, non-through tenons on the 4 legs. They had a little slop (not intentionally) and when I put the first piece of wood in my leg vise, it pushed the top back (even though I had it screwed to the upper runner in several places). The vise force is way stronger than my wimpy, loose fitting screws. I shimmed the tenon and now the front stays flush with parts in the vise.