Master Carpenter Video: How to Mark a Timber for Cutting a Mortise-and-Tenon Joint
Veteran timber-framer Will Beemer demonstrates the preliminary steps for joining timbers: laying out and marking tenons
Senior editor Chuck Bickford: Before you start to lay out and cut, there is some preparation. First, it’s important to create a level working surface for the stock. Second, you’ll have to establish the timber’s reference faces, as there’s usually some variation in size between timbers.
Timber-framer Will Beemer: Large timbers, such as the ones we are going to be working on, have some special issues associated with them: they’re usually rough-sawn; they may not be of consistent dimensions; they may shrink.
So I’ve set this timber up so it’s reasonably level. I’m on flat ground. I don’t have a lot of obstructions around me. The saw horses are a little bit shorter than is usual for carpentry; these are about 24 inches high. This is because the timber has considerable depth to it and I want the surface to be at a comfortable height.
The next thing I’m going to do is find the two reference surfaces of our timber. This gets into layout issues with timbers that come right off of the sawmill. For instance, this 7-by-7 timber is actually 7-1/8-in. in one direction and 7-1/16-in. in another. It also may be a little bit out of square. What I’d like to do is have this post that I’m working on have two square faces that I can reference off of. This might be the corner of a building. I’m going to check both ends of the post and look for the squarest faces. Once I find my two reference faces, I mark them with two arrows pointing towards the arris, or common edge between the faces.
I have here a 7×7 timber that I’m going to put a tenon on the end of. And I’m going to mate that with a mortise in a similar-sized timber. The tenon on this timber could be the top of a post, for example.
So, using my reference edge, I’m going to mark my tenon, keeping my square on my reference surfaces. And I’m going to bring my line down the side to mark my cut-off at the end of the timber. I try to have a nice flat surface for 1/2-in. of the pencil lead so when I go in next to my square, I can get in nice and tight and flush. I usually like to place my pencil where it’s going to make its mark and slide the square over to meet it, using the flat side of my pencil right against the square.
The length of the tenon is a nominal four inches long. In reality, I usually cut the mortise a little deeper or cut the tenon a little shorter so the tenon doesn’t bottom out. So I’m going to come back to 3-7/8 inches and make a swallow tail there. And then a line drawn across the timber at this point will represent the shoulder of the tenon.
I’m going to switch tools at this point. I have what is called a Borneman layout template. The two sides of this template have different-thickness fences so that one side registers every half-inch and the other registers every inch. I’m going to come down first 2 inches and then another 1-1/2 inches to mark the thickness of the tenon. The tenon thickness is determined by a rule-of-thumb of 1/4 the thickness of the timber. I only mark lines that are going to be cut, so I continue the shoulder lines around both sides of the tenon I just marked.
Now I’ve got my 14-inch circular saw and have it set to a depth of a little more than halfway through the timber. After making the first pass to cut the end of the timber past the tenon, I roll the timber over and complete the cut the remainder of the way through the timber. I have to be sure to support the saw as I cut through so it doesn’t drop off.
Next, Check the cut to see how square it is, and then I’ll carry the tenon-layout lines across the end of the timber, and I’ll be ready to finish cutting the tenon.