Master Carpenter Video: How To Mark and Cut a Mortise for a Timber Frame
Will Beemer demonstrates techniques for creating the second half of the quintessential timber-frame joint
Timber-framer Will Beemer: We’ve finished the tenon. Now we’re going to cut the mortise that the tenon is going to go into. I’m going to keep my square on the reference face on one side of my housing. The width of my post is 2 inches, so I’ll measure and draw a line for the other side of the housing.
The mortise is going to 2 inches over and 1-1/2 in. thick. I’ll use the 2-in. wide blade of my framing square to measure to the edge of the mortise, and I’ll use the 1-1/2-in. wide blade to lay out the mortise.
I’ll grab my Borneman layout template and mark down 6-1/2 inches. That will be where the shoulder of my tenon rests.
Score it, soar (saw) it, and bore it. I’m going to saw this housing down so, when we bore it with the drill, it doesn’t tear this out. I’m going to set the depth of my circular saw. This is a visible line, so I want to be pretty careful.
We’re going to bore out the mortise now. When I set up my timber, I made sure that it was level in both directions. I’ve installed a bubble level on the end of my drill so I know I’m boring plumb and perpendicular to the surface of the timber.
So the boring part is done [smirk]. So now we’ve got to go with our chisel. We’ve severed the end grain with the drill. Now we’re just going to pare away all of the material. And it should come out fairly easily with a sharp chisel. I’ll go in a little ways and then pare down. The bevel is towards the waste side; if anything, my chisel is going to want to dive in a little bit as I go in there. It’s not a serious problem if the mortise flares out a little as you go in there, but you don’t want to do it excessively.
Now that we’ve cleaned out the mortise roughly, we’re going to check to see if the sides of our mortise are square to this surface. The sides of our mortise need to be parallel to our reference face. So I’ll hold the framing square against the reference face and check the mortise with a combination square.
Now we can remove the housing material. I’m going to do that by kerfing down almost to the depth of the housing. I hold the bevel down on the chisel for removing a lot of material. I’m going to use the mallet. And I’m going to go about half-way down, and then I’m going to go about half again. And then, theoretically, we never get there. Now one reason we kerf this is because if we didn’t it would have a tendency to dive in to the mortise because there’s nothing backing it up. Now I’m trying to slice with my chisel using a slight slicing motion.
So you can see that we’ve dished out this housing a little bit so that the middle is lower. As the timber shrinks, it’s going to want to belly up in the middle a little bit, so this will offset that.
Next, we’re going to bore for the peg hole.