Introduction: How to Draw Bore a Mortise and Tenon Joint
When you really want to lock in a tight joint line around a mortise and tenon, you can add a draw bore pin. In fact, if you don't want to use glue, then a draw bore pin will hold your joint perfectly well on it's own.
The theory goes:
to hold a mortise and tenon joint together, you can drill a hole right through the assembled joint, and insert a snug fitting pin - make sense?
well, what if the hole in the tenon was made a little closer to the shoulder? The holes wouldn't be concentric, but they would overlap. Now, wood is a little springy, so if the overlap were sufficient, it would be possible to drive the pin through, with it flexing a little to accommodate the mis-alignment of the holes. The flex, or spring, would be constantly trying to align the holes, with the result that the tenon would be pulled tightly home. Tight joint lines, stiff joint, good for years.
So, let me show you how I draw bored the mortise and tenon in my short planning board (photo).
Step 1: Drilling the Holes
Assemble the mortise and tenon joint, and drill through to the mortise from the side, as shown. Use a depth stop, or indicator on the drill bit, to prevent drilling the tenon. You should be aiming for about half way along the tenon's length. Preferably use a brad point drill bit, as this will leave a crisp indication of the centre of the hole on the face of the tenon.
Now remove the tenon. See the centre mark left by the drill bit?
Complete the hole through the far side of the mortise. You don't need to break through the far face, if you would prefer not to see the pin.
On the tenon, mark a new centre point, closer to the tenon shoulder. The exact offset isn't too critical, but will be governed by the materials being used and how easily they compress. I was using a hardwood (beech I think) dowel rod as a pin, and the joint was also in hardwood. I offset the centre by approximately 1/4 the dowel's diameter.
Using the new centre, drill through the tenon.
Finally, flare the hole a little by rocking a steel rod in it. This just temporarily compresses the wood, making the pin's entry a little easier.
Step 2: The Pin
Clearly the pin should be the same diameter as the hole, and to help it through you should bevel the end.
Assemble the joint, with or without glue (it's your choice), and insert the pin.
The pin will bind as it jogs through the mis-aligned holes, and you will need to drive it with a mallet or hammer - if you can just push it through by had, then either you're very strong, the pin isn't big enough for the hole, or the holes aren't offset sufficiently.
You should immediately notice that the joint is pulled up tight...
Notice how I have accentuated the pin here, by creating a divot around it - this is not obligatory!
Step 3: The Result
With well fitted tenon shoulders, the joint will pull up extremely tight, and rigid. In fact, even if the tenon to mortise fit is a little off, the joint should be okay - something that wouldn't be the case if you were just gluing it.
Obviously you need to cut off the excess, and plane or sand the peg flush for a neat finish.
Thanks for reading my Instructable :-)
If you would like to see the process in a video, you can do so here: