I’ve been making low benches (both workbenches and general furniture) for a while now and I’d like to talk about the various ways to join the legs to the top. In my experience, some are better or worse than others, depending on several factors, including: (i) the purpose of the finished piece, (ii) the thickness of the benchtop, (iii) the materials used, and (iv) the tools available.
Let’s discuss several choices to join the legs to the top (I swear this is not a clip show):
First, we have cylindrical through tenons. The tenon can be hand carved, turned on a lathe or made with a round tenon cutter (like for rustic joinery). The mortise is bored with a large auger bit or some other boring bit.
Next, we have conical through tenons. The tenon is hand carved with a drawknife or plane and then typically refined with a special tapered tenon cutter. The mortise is bored with a smaller bit and enlarged with a tapered reamer.
In addition, we have rectilinear through tenons, which can be cut with regular edge tools. The tenon is sawed to shape with an angled shoulder. The mortise is chopped out with a chisel. No boring tools needed.
Finally, legs can be joined with notched lap joints. The tenon is sawed at an angle with a birds mouth shoulder. The mortise is just a dado in the side of the top. Also no boring tools needed.
There is also the tapered sliding dovetail used on Roy Underhill’s Timber Bench, but that’s outside the scope of this article.
Let’s take these in order. This is just my opinion based on experience; others may have their own takes.
- Relatively easy to cut: You just need an augur/boring bit of appropriate diameter (and a bit of skill to follow an angle) to cut the tenon. For the tenon, either use a lathe or log joinery tenon cutter or employ a few common hand tools (saws, chisels, spokeshave, rasps, sand paper) to round off the tenon and taper the shoulder.
- Strong: Full thickness tenons, glued and secured with wedges, form a secure and durable joint.
- Hard to correct: If you mess up the angle or wallow out the mortise too much, you’re stuck with it. There is no correcting after the fact.
- Limited sizes available: If you want a 2″ round tenon, you need to find a 2″ boring bit to cut the mortise. And a drill or brace that can use the damn thing without releasing the blue smoke (or tearing a UCL).
- Certain materials work better: Leg stock needs to be bone dry, otherwise the tenon may shrink and need to be re-wedged in the future. Also, ideally, the wedges are made of something even harder than the legs. Finally, 2x dimensional lumber may not be thick enough for a sturdy leg.
- Cosmetics: To me, full size round tenons just look off. If the angles are done right (so the exit holes are proper circles and not wallowed out ovals) and you use a contrasting wood for the wedges, they can be beautiful. But there are too many variables for me.
- Relatively easy to cut: Same as above, except you need a tapered reamer to make the tapered mortise and typically want a matching tapered tenon cutter to refine the tenon (both are generally available from the usual woodworking suppliers).
- Self-tightening: As you put weight on the legs, the tapered tenon will seat even further into the joint. The leg stock should still be bone dry, but as long as your legs are made of something equal to or harder than the top, the joint can sort itself out over time.
- Easy to correct: Unlike the cylindrical tenon, where you are stuck with the hole you bored (including any wonky angles), you can correct the angles using the tapered reamer. Just take it slow and check often.
- Less strong: The tapered tenon has less material making contact with the top and the wedge is not full width, so for a low workbench or a sitting bench for more than one person, you may need to add a cross rail to each pair of legs stabilize things.
- Requires specialized equipment: As noted above, you need at least one piece of special equipment (a tapered reamer) to make the mortise. Refining the tapered tenon without the tapered tenon cutter is doable but takes some practice.
Rectilinear Through Tenons
- Easy to cut: No specialized tools needed. A chisel, a mallet, a bevel gauge (or a block of wood cut to the right angle), and some patience will give you a clean and precise hole (that you can further refine with a file or rasp). I like to bore out most of the waste with an augur bit and pare down to lines, but this is ultimately just a square or rectangular mortise.
- Strong: Of all the joints described, this one has the most material forming the tenon.
- Customizable: You’re only limited by the size of the stock (not the size of the boring tool), so this joint can be used for everything from a footstool to a full size standing workbench. I would imagine this is the joint used in the workbench shown in Plate IX, Figure 68 of the marquetry entry in Diderot’s Encyclopedie.
- Cosmetics: Rectangular tenons wedged at 90 degrees or square tenons wedged at 45 degrees are very pleasing to my eye. I think they look the best of all of the through tenons.
- Complex layout: Unlike cylindrical or conical tenons, you have to actually lay out both sides of the mortise. This requires carrying compound angles around to the other face. You’ll need to not only S4S the top but also square the ends.
- Harder to correct: If you overshoot your lines, the only option is to make the tenon larger (which is easy if you overshoot side to side, but much harder front to back).
Notched Lap Joints
- Simple to cut: This joint can be cut with one saw and one chisel. Even the compound angled version is not particular difficult to work out. There is a reason why saw benches are the quintessential intro woodworking course.
- Any material works: 2x dimensional softwood lumber from the home center is perfectly acceptable for the legs. Using thinner material for the top (such as 2x dimensional softwood lumber) does not materially weaken the joint, either.
- Versatile: When using a narrower than ideal top, this joint can be used on the back legs to extend the footprint to a stable depth.
- Weak without reinforcement: As an external joint, glue alone is unlikely to be enough for a lasting joint. Metal fasteners and gussets are required to keep this joint together long term.
- Legs protrude beyond the top: Another drawback to the external joint, the legs will likely be in the way for some sawing and other operations. You can fix this by laminating on boards after the legs are attached, making a de facto rectilinear through tenon.
- Cosmetics: This is generally not a furniture grade joint. There are certain instances where it can be attractive. But most often, you’ll be using this joint for workshop stuff.
So what do you think? Did I miss anything in the pros and cons lists? Have I ruined everything forever?
I mean, yeah. Of course I have. But maybe not because of this article.