I’m a big fan of loose tenons. Because I have a slot mortiser, they are usually quicker to create than traditional tenons. I still use traditional tenons when the situation calls for it (haunched tenons, for example), but my first choice is loose tenons if everything else is equal.
In traditional tenon joinery, the mortise is typically made a little longer than the tenon is wide. The terminology gets a little confusing here, since mortise length is the same as tenon width (and mortise width = tenon thickness, and mortise depth = tenon length). The “slop” in traditional mortise-and-tenon joinery allows for wiggle room to get the aprons aligned with the tops of the legs.
Of course, the width of the mortise and the thickness of the tenon have to match, but I’ll get to that in a minute.
Here’s a photo of the mortise in the apron. I have marked out the ends of the mortise using a combination square, referenced off the top edge of the apron, as described here. Note that I have carefully cut the mortise right to the layout lines.
On the legs, things are a bit different. The layout lines are in the same relative locations, but this time I cut about 1/32″ past the lines. Why? Because my tenons, which will be glued into the aprons in the next step, are cut to fit the apron mortises with no gaps, ensuring their precise location. The 1/32″ overcut on the legs ensures that I will have the wiggle room I need later to align the aprons with the leg tops.
If both apron and leg mortises were overcut, the tenon may end up in the wrong location on the apron and not give me the adjustment necessary for alignment. Here’s the leg mortise below.
And here’s a photo of the tenon in the apron:
The thickness of the tenon is, of course, very important to the strength of the joint. A few thousandths thick or thin can be an issue. What makes this tricky is that the mortiser bit cuts differently in end grain than it does in face grain. The face grain mortise is almost always slightly larger than the end grain mortise.
To accommodate the difference, I mill the tenon material thickness to fit the face grain mortise (in the leg). I aim for a snug fit, but not so snug that it won’t go together with glue. I’ll surface the stock in my planer and use a handplane to fine tune the fit if necessary. I then cut the tenon stock to final width, round the edges on my router table with a quarter-round bit, and cut to length on the table saw. They are too thick, however, to fit correctly in the aprons. I fix that by sanding half the face of the tenon on a MDF board that has a half sheet of 120-grit sandpaper glued to it (a handy thing to have around). I also chamfer both ends of the tenon on the sandpaper. The tenon now fits nicely in the apron and can be glued in place. I know this sounds like a lot of steps, but it goes very quickly once you figure it out.
Here’s a photo of the setup for cutting the tenons to length.
This is a handy setup for cutting small parts to length. It’s not safe to use a stop in this situation, as the cut parts would be trapped between the stop and the blade. Instead, the length is indicated by the end of the 1/4″ MDF. I just position the end of the MDF relative to the blade, clamp it in place, and locate the tenon material by eye. If I want more accuracy, I can hold a temporary stop in place against the MDF end, locate the stock, remove the stop, and make the cut. Easy-peasy.
Here’s a photo of all the apron and leg parts. The #7 parts are the end aprons, which at the time had not been cut to length. Those lengths could not be determined until the side aprons were located on the center apron assembly.