Hello, everyone! It’s July 2024 and things are weird, generally. The Mets, despite being a pretty shaky team, are in the playoff hunt. The world is on literal and figurative fire. And I have a new workshop.
More on that soon.
But today I’d like to revisit something I’ve talked about previously: rectangular tenons on low benches. It’s no secret that I’m a fan of the traditional Chinese workbench. And in the past I’ve showed off several versions of a sawbench or sitting bench that use angled, rectangular tenons. But today, I’d like to do a deeper dive on how I lay out the angled mortises. Because if you lay them out correctly, it’s tough (but not impossible) to screw things up.
By definition, for any angled mortise, the mortise hole on the top of the slab is in a different position than the mortise hole on the bottom of the slab. Unless you want a super tippy bench, the top of the mortise hole should closer to the center line than the bottom of the mortise hole. And if it’s a compound angle, the mortise hole on the top of the slab will be closer to the midpoint of the slab than the bottom of the mortise hold (which will be closer to the end of the slab). This means that since the mortise (and the corresponding tenon) is a three dimensional object, there will be a portion of the entire mortise is completely vertical. Making angled tenons is, essentially, just strategically relieving the wood around that vertical mortise.
Take a look at the below image, which shows the layout for the mortise on the top of the slab. The greyed out area is the entirely vertical part of the mortise. The unshaded boxes between the other knife lines are the extants of the angled mortise. You’ll see that on the top of the slab, the mortise angles inward. In this case, because of the compound angle, it’s both inward toward the center line and inward toward the mid point of the slab.
Compare that to the underside of the slab (shown below). The vertical part of the mortise is greyed out again. Because the legs are angled both forward and out, the mortise angles outward (both outward from the center line of the slab [and toward the edges of the slab] and outward from the mid point of the slab [toward the end]).
One more note: when laying out, I find it helps to carry your lines down the sides of the slab. This also can show the vertical parts of the mortise more clearly and make sure that is accurately carried around the edge.
I also use a trigonometry calculator to figure where to start the mortise on the top of the slab so the legs don’t stick out too much beyond the plan of the slab itself. For most slabs in the 10-12″ range, the mortise on the top side of the slab should be centered around 2.5″ away from the center line.
So what is my preferred method for cutting these angled mortises?
First, I would bore out the vertical part of the mortise with a brace and bit and then pare roughly to the lines. Second, set up a bevel gauge and chop down the angles until I get pretty close. Finally, use a block of wood as a paring guide to refine the mortise until the angle is perfect. Then cut the tenon to fit each mortise, as there will likely be some slight variance in mortise width (and the wedge will take care of any variance in mortise length).
What are some other viable techniques? If you go the chisel only route (i.e., no boring it out first, which is the traditional method), I would still recommend chopping the vertical mortise first. Then you can pare it down to the lines as described above, with or without the guide block. I do not recommend freehanding the angled mortise right off the chisel. At least not for furniture grade pieces. For a workbench, it’s probably fine.
Once you have the vertical mortise, you could use a floats, rasps, or files to shape in the angles. These tools are also helpful to refine the angles and make everything as uniform as possible.
The nice thing about rectangular mortises is that you really don’t need any of the above special tools. You could do the entire thing with a chisel and mallet no problem (and I’ve heard that the lack of boring tools in China is how the form came to be). Although, although a bevel gauge and a rasp or coarse file will definitely help.
Next time, we’ll talk about cutting the tenons. Including how to cope the shoulder so it mates perfectly with the underside of the bench.
JPG
The Apartment Woodworker 