Drawboring Demystified

This ancient mortise-and-tenon joinery technique needs no glue, no clamps.

by Jennie Alexander & Peter Follansbee
Pages 53-57

Simply put, drawboring is an intentional misalignment of the holes bored in the mortises and tenons.

These holes are bored through each component separately, and they are offset so that a tapered pin driven into them will pull the tenoned rail up tight against the mortised stile.

Follansbee’s marking technique. The thinnest awl in the shop is best for marking your drawbore location.

We know this is a period practice because we have seen it in surviving works, sometimes in disassembled pieces, or in those worn down by misuse. The photo at left shows clearly the kink in the pin resulting from snaking its way through the offset holes. In addition to this sort of evidence, we have a documentary record for drawboring as well. Joseph Moxon describes it in his section on joinery in “Mechanick Exercises:”

Then with the Piercer pierce two holes through the Sides, or Cheeks of the Mortess, about half an Inch off either end one. Then knock the Tennant stiff into the Mortess, and set it upright, by applying the Angle of the outer Square, to the Angle the two Quarters make, and with your Pricker, prick round about the insides of the Pierced holes upon the Tennant. Then take the Tennant out again, and Pierce two holes with the same Bit, about the thickness of a shilling above the pricked holes on the Tennant, that is, nearer the shoulder of the Tennant, that the Pins you are to drive in, may draw the Shoulder of the Tennant the closer to the flat side of the Quarter the Mortess is made in. Then with the Paring-chissel make two Pins somewhat Tapering, full big enough, and setting the two Quarters again square, as before, drive the Pins stiff into the Pierced holes.

Alexander’s marking technique. The centerpunch needs to fill the hole completely, so make one with material chosen to fit your boring tool.

We’re using the joint stool shown in the book from which this is excerpted as the example – but the following applies to any drawbored mortise-and-tenon joint. At this point, the holes through the mortises have already been bored, two per mortise, approximately a mortise chisel’s width back
from the edge of the workpiece.

With the test-fitted frames on the bench, mark the tenons for the pins that will secure these joints for the next few centuries. The faces of rails and stiles must be in the same plane. The rails’ outer shoulders should be a tight fit against the stiles’ arrises. The inside corners should not touch the stiles. Check that the apron’s upper edge lines up properly with the marks on the stiles that define the top of the stool. At the stretcher level, be sure there’s no gap in the mortise above the stretcher, where it will be visible in the finished stool. If there is a gap, bump the stretcher upward, shifting the space beneath the stretcher. You might need to check that the ends of the mortise are cut square to the edge, so they don’t interfere with the tenon’s fit.

When all that is checked, scribe inside the hole onto the tenon face with a thin, sharp awl. Alexander prefers lightly tapping a centerpunch into the pin hole. Make a punch by pointing the shank of an old drill bit or steel rod that fits snugly into the pin hole.

Disassemble the frame and bore the holes in the tenons. It is critical to remember to move in the proper direction: toward the tenon shoulder. You can eyeball this placement, or you might find it helpful to mark the center of the tenon’s pin hole. With the awl, prick the new centerpoint about 1?16〃 closer to the tenon shoulder.

Offset. This offset looks like it will work fine. Experimentation will help you get the hang of it. There’s leeway involved; it’s not an exact science. Too much offset can be fixed more easily than not enough.

Reassemble the joint and sight through the offset holes. The picture at near right shows the general idea of what we’re after; the offset should take up about one-quarter to one-third of the hole. When you have bored all four tenons for one frame, you can test-assemble this frame yet again, and lightly pull the joints tight by driving in tapered metal “drawbore pins.” These pins can be easily made by adapting a machinist’s alignment pin. Any rod or awl that tapers from 5?32〃 to 5?16〃 along 4〃 can be used. Installed in octagonal cross-sectioned wooden handles, they can be tapped in with a hammer and easily removed by hand.

If there is a question about a particular joint, remove the pin and sight through the hole against the light. If the interference is too great, use a handheld square-tapered reamer to enlarge the tenon hole. This tool is nothing but a drawbore pin that has been filed to a sharp-edged square cross-section.

You are approaching final assembly of the joint stool. Don’t hurry. Drawboring is the heart of the matter. (Again, our example applies to the build in
“Make a Joint Stool from a Tree,” but the technique can be applied to any drawbored mortise-and-tenon project.)

After the two straight frames are fitted it’s time to take up the angled side frames, test-fit the aprons and determine the length? apart, and set the straight rails aside. Take two mating stiles that define one side of the stool and ensure you have the correct stiles.

Now pare and fit the tenons from one apron to these stiles. Take this apron all the way to the drawboring stage, then test-fit the two stiles and the apron. The stretcher’s shoulder-to-shoulder length is found by scribing, not measuring. Place a side stretcher on its stiles with the stretcher’s upper edge lying across the stile’s faces. Line the stretcher up so its front shoulder meets the stile’s arris right at the point that marks the top of the mortise.

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