Master Carpenter Video: How to Drill and Assemble a Timber-Frame Joint
After you've cut a mortise on one timber and a tenon on the other, it's time to drill a peg hole.
We finished the mortise and tenon on the other timber, and now it’s time to drill the peg hole. Drill the peg hole in the mortised piece first, test-fit the joint, and mark where the peg will go on the tenon; then offset the hole in the tenon for the drawbore. This peg will be a ¾-inch peg, and we’ll center it on the tenon and mortise. Use a framing square to locate how far the peg hole is off the shoulder of the housing. If I’m a little off, it doesn’t matter; I’m going to drill the hole and use it to mark the location on the tenon.
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Now I’m going to use one of my favorite tools. This is a neat trick I learned from a Japanese carpenter. I place this mirror over the peg hole and use it to see if I’m perpendicular to the surface that the mirror is resting on. If I see any dogleg in the drill bit, then I’m not drilling perpendicular. Peg holes are drilled all the way through to the other side because the peg hole has to be straight. Always drill a peg hole from the side that’s closest to the mortise—the reference face. That way, it will go through the mortise closer to your layout line than if you were to drill from the side farther away.
Once I go through the mortise, I have a chance to realign the drill if I’m a little bit off. I don’t want to blow out the other side with the drill because that might be a visible surface inside the building. One trick with the auger bits that have a feed screw on the end is that I can lift up as I’m nearing the end of the bore. When the feed screw comes through the lower surface, as I lift up, it’ll release, and I’ll be able to pull the drill out before the main part of the bit goes through the other side. Once I’ve rolled the timber over and drilled through the other way, there’s a nice clean hole on the opposite face.
Now we’re going to fit the timbers together, mark the peg hole on the tenon, and offset it slightly for the drawbore. Use a commander to pound the timbers together.
This is close enough for us to do the drawbore. I put a framing square in this corner to see how square the timbers are—and they’re pretty much right on the money. There’s just a slight little gap here. We’re going to offset the peg hole on the tenon about 1/8 inch that way. When I drive the peg in, it will pull the joint tight and make a slight curve, hooking the tenon and pulling it in. As the timbers shrink, that peg will act as a spring and keep the joint tight.
Use the tip of the auger bit to mark the tenon. Pull the joint apart, and there’s the pricked point from the drill bit. There’s a rule of thumb we use to decide which way we want to drawbore, because it’s very easy to offset the hole in the wrong direction. Offset the hole in the direction you want the mortised piece to move. You want the mortised piece to move in toward the shoulder, and so drawbore in that direction. Another way of drawboring is to tilt the bit in the direction of the shoulder. Make sure you’re not drilling away from the joint, but drilling toward the joint or the shoulder. Pound the timbers together to see how you did on the drawbore.
Here’s an old peg from a 100- or 200-year-old barn that had the joint drawbored. You can see the curve in the peg as it went in, hooked onto the tenon, and came out the other side. That peg acts as a spring. These are riven pegs, which means they’re split from a round log, red oak, and tapered so that the point can go in and grab that drawbore. The rest of the peg will fill up the hole as it pulls the joint tight. Here’s a modern riven peg that might take 10 minutes to make. They’re very strong because the grain is continuous. This is a turned peg, which most timber framers use because they’re much less expensive and most are produced by one company. It is also tapered to go into the drawbore. It doesn’t have continuous grain, though, and is more likely to split than a riven peg. Theoretically, once the frame is up, the peg shouldn’t do much work.
We’ll use the riven peg in our joint. It will tighten up as I drive in the peg.