Step 6: Drawbore the Mortise and Tenon Joint
In this last step, you’ll learn an amazing historical method for tightening a mortise and tenon joint using drawboring. Drawboring is the process of driving a wooden dowel through slightly offset holes in the mortise and the tenon.
First, find the center of the joint, where the drawbore hole will be bored with an auger bit:
Then start boring a hole through the wall of the mortise, using an auger bit that is the exact same diameter of the wooden dowl you’ll be driving in.
It’s a good idea to test the auger on a scrap piece of wood beforehand to make sure your dowel fits perfectly in the hole. Make sure the auger bit is square and perpendicular to the face of the board:
Bore until the auger bit breaks through into the mortise. Then you should slow down so you can stop just as the tiny lead screw point starts to exit the other side. If you don’t, then the auger bit will blow out the back side. Every few rotations, use your hand to feel if the point is exiting the back side.
Here you can see the tiny holes where the auger bit lead screws safely exited the back side:
Next, place the auger bit lead screw into the tiny holes, and start boring carefully back into the hole. It will quickly produce a clean hole:
Now place the tenon inside the mortise, and make sure it’s snugged down tightly:
Take the same auger bit that you used to bore the hole, put it back in the hole, and push straight in through the hole until the lead screw makes a mark on the tenon. This will mark the center of the hole on the tenon.
Pull the tenon out, and use a pencil to darken your little mark if you need help to see it better:
Next you’ll bore a hole in the tenon. But NOT on the little mark you made with the auger’s lead screw. Move the auger bit toward the tenon shoulder just slightly. 1/16th of an inch or less:
This will allow the hole going through the mortise to be slightly offset from the hole in the tenon, which will tighten the tenon against the mortise after driving the wooden peg through both holes. Go ahead and bore down through the tenon on this new mark. There’s no need to bore from both sides of the tenon, as blowing out the other side won’t matter since the tenon will never be visible after it’s assembled.
Here you can see the offset of the two drawbore holes once the mortise and tenon joint is assembled:
If you want, you can use a pencil to trace the offset, so you can see it on the tenon.
If the offset looks too extreme, you can use a rat tail rasp or file to move the hole toward the pencil mark a little more. Just don’t get rid of the offset.
Once you feel comfortable with the dry fit of your mortise and tenon joint you can add some glue inside the mortise if you want to.
I (Joshua) don’t always add glue to my drawbored mortise and tenon joints, because the peg holds the joint so tightly. And I figure one day someone may want to take the joint apart to make a repair to the furniture. You can also put a bit of glue on the tenons if you want, like Will does.
I recommend that you use a hardwood dowel, like these oak dowels, so the peg doesn’t break when driving it into the holes:
Also notice the tapered ends. You can use a chisel to shape the tip, or you can use a pencil sharpener, like I do! The tapered ends helps the peg move through the holes as it snakes around the offset.
If you want, you can also apply glue to your drawbore pegs. I use wax on mine, since I don’t usually glue up my mortise and tenon joint, as I mentioned earlier. Wax helps the peg move through the holes a bit easier.
Place the drawbore peg inside the hole, and then go ahead and use a wooden mallet or hammer to drive the drawbore peg down through the mortise and tenon joint:
I prefer to stop hitting the mallet before the peg is driven flush with the board, so I don’t accidently go too far and put a big dent in the wood. I can come back later and use a flush-cut saw or a backsaw to trim the peg flush:
And you can also flush cut the tapered part of the drawbore peg if you like. On some furniture, I actually leave the tapered drawbore peg protruding out the back side so people in the future can know that the joint is drawbored.
You can see that the joint has drawn itself “as tight as a tick” as Will likes to say!