How to make a drawbored mortise and tenon joint
|Mortise and tenon joints are commonly used in woodworking where strength is important. They are regarded as one of the most resilient ways to join two pieces of timber together, and used throughout the furniture-making industry to produce chairs, tables, beds and other objects that bear weight.
|The principle for mortise and tenon joints is simple. One piece of wood is thinned down at the end to create a tenon.
|The second piece has a hole of matching size cut into it, called a mortise.
|The tenon slots into the mortise and is held in place either with glue, or by hammering in small metal wedges that put pressure on the outsides of the joint to hold it in place by friction.
|Sometimes this type of joint can be reinforced by a dowel that is inserted into a hole drilled through both pieces of timber. This anchors the tenon inside the mortise and prevents it from moving.
Most often, the holes in the tenon are slightly offset from the holes in the piece of wood into which the mortise has been cut. This is the first step in a technique known as drawboring.
What is drawboring?
|Drawboring is a technique that lends even more strength to a reinforced mortise and tenon joint. Not only does the dowel act as an anchor, it actually pulls the tenon into the mortise tightly, mechanically holding it in place.
This effect is produced by the off-centre hole in the tenon, which causes the dowel peg to pull down on it as it is hammered into the hole.
|This is possible because wood is a fibrous material that can be flexed or compressed. The dowel peg in the joint is flexed around the off-centre hole in the tenon, and wants to straighten itself out. The resulting force compresses the end of the tenon into the bottom of the mortise, making an extremely secure joint.
Making the joint
Step 1 – Drill through the mortise
Take the piece of wood into which the mortise has been cut and mark the location of two holes so that they will penetrate through the mortise itself.
|Using a brad point bit that is the same diameter as your dowel peg, drill through your piece of timber so that you reach the wood on the other side of the mortise and bore a short way into it. If your dowel peg has no flutes, you don’t need to make the hole any bigger – any excess glue will be able to escape through the top of the hole as you hammer in the peg.
Step 2 – Drill through the tenon
Assemble your joint by pushing the tenon into the mortise.
|Remove the brad point bit from the drill chuck and Insert it through each of the holes you have just drilled in turn. Push down just hard enough for the sharp point to make an indentation in the tenon.
|Disassemble the joint and mark a new point with a pencil that is 2mm (1/16“) further away from the tip of the tenon. This will be enough for your dowel peg to exert mechanical force on the tenon once it is assembled and in place.
|Using your brad point bit, drill a hole centred on each of the pencil marks you made on your tenon.
Step 3 – Assemble joint
Your dowel peg will need to be tapered at one end to make it easier for it to enter the reduced gap between the hole in the mortise and the hole in the tenon. Once it has entered the hole in the tenon, it will start to pull it into place as it is hammered through.
|Apply wood glue to the tenon and the inside of the mortise.
|Reassemble the joint once again.
|Apply wood glue to the dowel holes, then insert the tapered end of the dowel into the hole and hammer it in with a rubber-tipped hammer. This may require a little elbow grease.
You can alternate between dowels during this stage. If you do, you will avoid putting all of the strain from the joint on one dowel while you’re hammering the other one in.
|As the dowel enters the tenon, it may tilt. This is normal, and it will level off as more of the dowel pushes into the tenon hole.
Step 4 – Finishing off
Once your dowel peg has straightened up and will not enter the wood any further, it can be left to dry. You will not need to clamp your joint, as the dowel pegs are already doing that job for you!
|Drying should take 24 hours.
|Once all of the glue is dry, level off any protruding wood with a chisel.