Drawbore Mortise and Tenon: Offset Peg Joinery

A drawbore joint is a pegged mortise and tenon in which the hole through the tenon is deliberately offset toward the shoulder, so that driving a peg through the mortise cheek and the tenon pulls the shoulder tight against the mortise face. The joint locks mechanically and holds without glue, which is why it is traditional for timber frames, chairs and doors where the joint may need to be tightened or taken apart later.

What the drawbore does

In an ordinary pegged mortise and tenon, the holes in the mortise cheek and the tenon are aligned. The peg locks the joint but does not pull it together; any gap at the shoulder stays a gap. In a drawbore, the hole through the tenon is shifted a small fraction of the peg diameter toward the shoulder. When the peg is driven, it bears against the far side of the tenon hole and the near side of the mortise hole, drawing the tenon forward until the shoulder seats. The peg is then in shear and the joint is under tension.

The offset is small. Too much and the peg either breaks or splits the tenon cheek; too little and the draw is ineffective. The right amount is a small fraction of the peg diameter and varies with species: it depends on the species, the peg stock and the size of the tenon. There is no universal figure that suits every wood; the offset is judged by the peg diameter and the species in hand.

Why it is used

Drawboring is traditional where a joint must stay tight under movement and where glue alone is unreliable: green timber frames that shrink as they dry, chair joinery that is racked constantly, and door frames that swell and shrink with the seasons. The peg can be re-driven or replaced, and the joint can be taken apart by drilling out the peg. A glued mortise and tenon cannot be serviced the same way. A drawbore keeps the shoulder closed even if the glue line fails, and it can be assembled without clamps.

Cutting the joint

Cut the mortise and tenon as for any mortise and tenon: mortise walls parallel, tenon cheeks flat, shoulder square to the face. The drawbore does not excuse a sloppy fit. A loose tenon will not be pulled straight by the peg; it will rack in the mortise. Aim for a fit that can be assembled by hand with light pressure, with the shoulder making contact all round before the peg goes in.

By hand, chop the mortise with a mortise chisel, keeping the walls parallel along the length. Cut the tenon with a tenon saw and clean the cheeks with a shoulder plane. Mark the shoulder from the mortise face with a marking gauge rather than measuring, so the shoulder sits where the mortise actually is.

By machine, cut the mortise with a hollow chisel mortiser or a router and template, and the tenon with a tenon jig on the table saw or a dado stack. The same rule applies: the shoulder must register on the mortise face, so cut the tenon slightly long and trim the shoulder to fit rather than cutting to a number.

Boring the peg holes

The holes are bored in two separate operations, never in one pass through the assembled joint. Bore the peg hole through the mortised piece first, with the tenon out, backing the far cheek with scrap so it does not break out. The tenon is then marked, taken out again, and bored on its own with the hole shifted toward the shoulder.

A common approach is to bore the mortise cheek first, then insert the tenon and use the same hole to mark the tenon with a brad point or a dowel centre, shifting the mark toward the shoulder before boring the tenon. This keeps the offset consistent with the actual fit of the joint rather than with a drawing.

Riving the pegs

Use riven pegs, not dowel rod. Dowel rod is turned or drawn and has run-out: the grain does not run the full length, so the peg can break at a short grain line under the driving force. Rive pegs from straight-grained stock by splitting along the grain with a froe or a chisel, then shave them round with a knife or drive them through a dowel plate, and taper the leading end to a long point. The tapered end is what lets the peg find its way through the offset holes and bend slightly as it is driven; a blunt peg catches on the edge of the tenon hole and breaks or tears out the far side.

Drive the peg with a mallet, not a metal hammer, and support the mortise cheek on the far side with a block or a hand to prevent the cheek from splitting out.

Dry assembly first

Always assemble the joint dry before glue and before driving pegs. Check that the shoulder seats, that the peg holes align enough to start the peg, and that the draw pulls the shoulder home. If the joint will be glued, glue only after the dry fit is confirmed; the peg then holds the joint while the glue cures, and the drawbore keeps the shoulder tight even if the glue starves.

Telling a good fit from a bad one

A good drawbore shows a closed shoulder line all round after the peg is driven, with no gap opening as the peg tightens. The peg should drive with increasing resistance and stop solid; a peg that drives too easily is too small or the offset is too little, and a peg that stops short or mushrooms is too large or the offset is too much. Check the far cheek for a split before driving the peg fully home.

A bad fit shows a shoulder that stays open on one side, meaning the tenon is racked in the mortise or the offset is uneven. A tenon that rocks in the mortise will not be corrected by the peg; recut the tenon or shim the mortise.

Where it fails

• Offset too large: the peg breaks in shear, or the tenon cheek splits along the grain from the hole to the end. This happens most in dense hardwoods and in thin tenons.
• Peg with run-out: the peg snaps at a short grain line, usually partway in, leaving a stub that is difficult to remove.
• Blunt peg: catches on the edge of the offset hole and breaks or splits out the far cheek before the shoulder seats.
• Loose tenon: the peg pulls the tenon but the tenon racks, opening the shoulder on one side.
• No support on the far cheek: the cheek splits out as the peg exits.
• Green timber assembled too tight: as the timber dries and shrinks, the peg can crush the mortise or the tenon can shrink away from the peg. Drawboring is designed for this movement, but the offset must suit the species and the moisture content at assembly.

Comparison

The mistake most often made

The most common mistake is making the offset too large in an attempt to get a stronger draw. It happens because the draw is invisible: the peg goes in and the shoulder closes, so more offset seems better. But the offset puts the tenon cheek in tension across the hole, and past a small fraction of the peg diameter the cheek splits or the peg shears. The failure often appears after assembly, when the joint is loaded, which makes it hard to trace back to the offset. Keep the offset small, test on a scrap joint of the same species, and increase only if the draw is clearly ineffective.

Related

For the parent joint, see mortise and tenon.