Drawboring a
Mortise & Tenon Joint

Compiled and illustrated by Loren Hutchinson

The following is in response to a question about 'drawboring' a mortise & tenon joint. Author's names preceed their answers.

Wyomingbob -

A drawbore is a tenon pinned from the side; but instead of assembling and driiling straight thru the joint, you offset the holes a teeny bit, like 3/64 inch. Then hammer in a chamfered peg at glue up (or a tapered one -- 3/8 inch walnut tapered nails are my trademark) and the pieces are drawn together, putting the M&T joint under tension & tranferring pull-out strains from the glue faces to the peggy. Not that much extra work, if you use round nails. S-loads more if you go [with] square [pegs].

[Figure 1]
[Figure 2]
[Figure 3]

I find a taper of about 5 degrees is plenty on the peg; more that that, you'll have trouble driving it home or will as Dave says, blow the end off your tenon.

Some tips on drawboring:

  • Wooden handscrews are simply the best way to seat the pegs -- squeezing them in gently instead of whacking them with a mallet. Plus you can angle the jaws to counteract the peg's obvious tendency to go off sideways.
  • Locate your hole as far away from the end of the tenon as possible. To futher prevent blowout, you can glue veneer slips at 45 or 90 dregrees to the rail tenons' faces just before cutting them to final size. It's like plywood tenons. Square pegs have greater area to distribute the force but are, as mentioned, a pain to make many of. Still my choice.
  • You can drill the hole before cutting the mortise, or after. If after, have on hand a bunch of over-long fake tenons milled out of scrap to stick in the mortise before drilling/chopping the peg hole. Nothing turns your stomach like realizing the drill just splintered the wall of your pretty mortise into a useless mess. Then, after drilling/chopping to depth, remove the dummy & stick in the real thing (keep parts mated!), using the center spur of the drill to mark a dot on the tenon. Withdraw, move the dot over about a long 1/32 inch toward the tenon shoulder, and drill -- again using a backer board to prevent tearout.
  • Slow glue is a nice idea with these things.
  • I've made a bunch of practise joints with differing woods, peg sizes & tapers, seating depths, glues, etc., & hammered 'em to test strength. Band or handsawing apart the joints will reveal much, like is your tenon end breaking, or did the peg seat thru the joint. F'rinstance, pine or poplar are too soft for real drawboing; the tenon's hole just deforms. Walnut makes boffo nails (all I use) but you need to watch splitting with it as frame rails. Oak works very well, as will ash or cherry.

[The] trick is to pound as deep into the far wall as possible without breaking thru the backside, giving the peg the broadest possible shelf to sit on. One helpful jig is a 1/4 inch thick piece of wood with a fence on one edge and a good square sample hole cut [at the] right distance. You can drill your holes with a forstner (best for flat-bottom, won't overfeed like a bradpoint), center the square hole over the round, & use it to guide your paring chisel. Practise lets you do it by eyeball pretty good.

Dave Mount -

One other thing to be aware of -- it is possible to break your tenon doing this if

  1. Your peg is large
  2. The tenon is small (thin)
  3. The offset is too large
  4. The wood cracks easily

When you pound the peg, you are pulling the tenon in by pulling on the side of the hole toward the end of the tenon. Obviously if the strain here is too great, you can break out the wood along the grain toward the end of the tenon. Don't mean to scare you off the technique, because it doesn't happen under ordinary circumstances; just keep your offset small (1/32 to 3/64 inch as Bob suggested). Moreover, with modern glues, even if the thing does break, you can clamp it and the loss of strength will probably not be an issue.