Drawboring is a technique that increases the strength of a mortise and tenon joint. Instead of relying on glue, it uses a dowel as a permanent interlock, held in the mortise piece, that keeps the keep the tenon in permanent tension. The second photo shows just such a joint cut to show its holding power. (This photo comes from Popular Wooding Magazine. They have done a lot to make this point popular again.)
This joint was popular in preindustrial times as glue was either of poor quality or non existent. It lost popularity in the 19th century as glues became better and the drive for cheaper furniture caused by a rising Victorian and American middle class. However did have a resurgent with the Art and Craft movement at the turn of the century. Because the tenon is pulled into place, it does not have to be cut as snug and is actually easier to make. The following steps will show how.
Step 1: Cut Mortises and Tenons
For this example I am putting in a rail at the bottom of two legs for my new workbench. A workbench takes a quite a beating and drawboring will help keep these joints strong.
Shown in the photo are two legs, each with a mortise, and a rail with a tenon on each side. They have already been cut and fitted and are ready to be drawbored.
Because these legs and rail are so massive I will be using 3/8" dowels. For more refined furniture I would most likely use 1/4" dowels.
Step 2: Drill Hole Through the Mortise
Drill a 3/8" hole through the mortise. This hole is drill through the leg so the dowel will go completely through it. The hole is centered in the mortise and 3/4" from the edge.
Step 3: Mark Tenons
Reassemble the pieces and use the drill bit to lightly mark the tenon.
Step 4: Mark and Drill Tenons
Disassemble to get the rail. Find the marks on the tenon and mark the holes to be drilled, 1/16" to 1/8" further in. Then drill the hole. Closely look at the third photo and you will see the hole is misaligned. If a dowel is pounded through these the holes, the tenon will be pulled into the mortise, keeping the joint in tension. I moved the holes 3/32" in. After doing a few you get a feel for what depth is right.
Step 5: Pound in Dowels
You will want to make sure that your dowel are an actual 3/8" diameter. The easiest way to guarantee this is to take a fatter dowel and pound it through a 3/8" hole in a piece of steel. I will post an Instructable on how to do this. Take your sized dowel and sharpen the tip. Apply glue to the joints and reassemble. Take the dowel and pound it in the hole. Because the leg is so thick I use spacers to help keep the long dowel from splitting as I hammer. Pound the dowel until it is completed through the leg.
Step 6: Trim Dowels and You're Done
Use a flush cutting saw to remove the excess dowel and then sand smooth. You have constructed your first drawbored joint.