This will be the fourth year that Lie-Nielsen has brought their wares to the shop for a Hand-Tool Event ®. Kevin Drake of Glen-Drake Tools will also be there demonstrating tools and some very cool techniques. I’ll be showing off some of my new methods for hand cut mortise and tenon joinery, and there will be a guest appearance by the new rocking chair. Friday, April 1st, from 10 until 6, and Saturday the 2nd from 10 until 5.
If you can cut a straight tenon, cutting an angled tenon should be no problem. For the most part, all you need is a wedge of the correct angle. OK, it’s a little more complicated than that, but not by much.
Most people cut tenons on the tablesaw, and the question I hear most is ‘why not just angle the blade on the saw?’ The big problem when cutting angled tenons is the shoulders. And when you simply angle the blade, you’re starting out with a handicap because the cuts don’t even end up where the shoulders need to be.
Using a wedge takes care of that, and also means that you don’t have to worry about setting the saw up each time you need to cut the same angle – just save and mark the wedge for a particular project. Not only that, but the angle for all four of the chair’s tenons is usually the same (the size and location may not be, however). There area few chairs where this isn’t true, but the vast majority work this way. One more bonus of using a wedge is that you can use it to cut the end of the workpiece at the correct angle if you choose.
So what about the shoulders? Let’s talk about the ‘longer’ shoulders first (there’s no good terminology for this is there?). I try to avoid cutting these as a separate step. It’s really a challenge to cut these angled shoulders with the workpiece flat on the tablesaw the way you might for a regular tenon; it would take at least two different set-ups, and the alignment is difficult. The answer is to use a dado blade right from the start to cut both cheeks and shoulders at the same time.
You’ll need flat-bottomed dado set to avoid the scored marks left by the outer blades. Your sharpening service can grind your blades that way. Or use one of the finger joint blade sets oriented for a 3/8 in. wide cut. These are specifically designed for the type of cut you’re doing here. In any event, be sure you have something backing up the workpiece to avoid blowing out the workpiece where the blade exits.
I also like router tenoning (on end) for the same reason; you’re cutting the shoulder and the cheek at the same time. More on that later.
There are two things that most woodworkers fear about chairs: curves, and joinery. And joinery is definitely scarier; the image of your mother crashing down to the floor as your chair collapses due to poor joinery is the stuff of nightmares (at least of my nightmares). Cutting angled joinery certainly seems a whole lot harder than ordinary joinery, too.
So what do you need to know about chair joinery? Let’s start with the simple things. First, all chair joinery should be reinforced. That usually means corner blocks screwed into place You could glue them too, but unless you’re using some sophisticated techniques, you’re gluing endgrain and it won’t add much strength. Having corner blocks in place means that if (or when) the main joinery loosens up, the chair will still stay together. You should use corner blocks even if the chair has stretchers, which also provide additional reinforcement. Incidentally, corner blocks give you a convenient place to attach the seat.
Next, the joint of choice is the mortise and tenon. I know that ‘industry standard’ seems to be the double dowel joint. I’m reminded of the problems that dowel joints have every time I sit in a commercial chair (at a restaurant or hotel) that has loosened up. The fact that a dowel joint doesn’t have good long-grain to long-grain glue surface means it will almost inevitably loosen up. You want as much long-grain to long-grain surface as possible, and that’s what the mortise and tenon provides. The one caveat? The mortise and tenon joint has to fit well. The inner walls of the mortise and the cheeks of the tenon must be flat, smooth, and in full contact with one another. The joint should be snug: not so tight that you can’t push it together by hand with a little effort, but not so loose that it will fall apart if you pick one of the parts up.
Interestingly, with enough redundancy of a questionable joint you do get strength. But with this a dowel joint, that means lots of dowels, ideally spread out over a wide area.
Multiple biscuit joints are also a possibility, although I think the shape of a biscuit poses its own problem: it has minimal contact out at its ends where maximal force is concentrated in a chair joint in typical use.
The problem with loose joints is pretty obvious; most glues don’t fill gaps with any real strength. It’s best to patch a chair joint that’s too loose, rather than trying to use a glue that claims gap-filling properties. Just glue on a slice of veneer (or something a little thicker, if necessary) and re-fit the joint.
A joint that’s too tight is a different story. It’s not exactly the tightness that’s the issue (unless you can’t actually put the joint together without splitting the other part). A butt joint that’s clamped together is much ‘tighter’. The problem tends to be that in a very tight joint the glue will all get wiped off the joining surfaces as you put the joint together. If your fit is just a little too tight (you can put it together, but it’s more work than it should be), you can try something I’ve done on occasion. I’ve got a machinist’s vise that has knurled faces on the jaws.
I squeeze the tenon in the vise just enough to emboss the knurling on the cheeks. I’m not trying to crush the tenon to fit; just trying to leave some space for the glue as the joint is assembled (this does usually make it a little easier to put together as well). The glue in the embossed knurling lines stays in place on the cheek, and the moisture in the glue helps swell those lines back up: a tight fit with glue in the right places.