Drawbored Mortise and Tenon: The Joint That Tightens Itself

Glue fails. Wood shrinks. Joints loosen over time. Every woodworker knows these truths, yet we keep gluing mortise and tenon joints and hoping for the best. Drawboring offers a centuries-old solution that transforms hope into mechanical certainty. Once pinned, a drawbored joint tightens itself and stays tight forever.

The Principle

Drawboring offsets the pin holes between mortise and tenon. The hole through the mortise cheeks sits slightly closer to the shoulder than the hole through the tenon. When you drive a tapered pin through these offset holes, it pulls the tenon deeper into the mortise, jamming the shoulder tight against the mortised piece.

This mechanical pull replaces clamp pressure during glue-up. More importantly, it remains active permanently. As wood shrinks and glue crystalizes over decades, the pin continues applying its drawing force. The joint can’t loosen because it’s physically locked.

Offset Calculation

The offset amount depends on wood species and pin size. Hardwoods tolerate less offset than softwoods because they resist the bending force on the pin. Most furniture applications use 1/16 to 1/8 inch offset with 1/4 inch pins.

Mark the mortise hole location first, drilling through both cheeks at the same position. Then insert the tenon and transfer the hole location by running a drill bit through the mortise holes to mark the tenon. Remove the tenon and mark your offset toward the shoulder from these transfer marks.

Drilling the Holes

Drill the mortise holes before assembly, while you have clear access. Use a drill press or careful hand drilling to ensure perpendicular holes. Elongated or angled holes reduce drawing action and can split the pin.

The tenon holes can be slightly undersized since the pin will find its way. Some craftsmen drill these holes slightly larger to reduce the bending stress on the pin. Either approach works if your offset is appropriate.

Making the Pins

Traditional drawbore pins are octagonal in cross-section, split from straight-grained billets. The corners of the octagon bite into the hole walls better than round pins, preventing any rotation or back-out.

For ease, you can use dowels. Choose straight-grained stock and taper one end slightly with sandpaper or a pencil sharpener. The taper helps start the pin in the offset holes without immediately binding.

Pin length should exceed the total mortise depth plus tenon thickness by about an inch. You’ll trim the excess after driving.

Assembly Process

Apply glue to the joint as normal. Insert the tenon and use a drawbore pin, which is a metal rod with a tapered end, to test that the offset holes align close enough for the wooden pin to navigate. If the test pin won’t pass, reduce your offset and re-drill the tenon holes.

Drive the wooden pin using a mallet, stopping when the joint pulls fully tight. You’ll feel the resistance increase as the offset forces the tenon home. If the pin breaks, your offset was too aggressive or your wood too dry.

Multiple Pins

Wide tenons need multiple pins to prevent racking. Space them evenly across the tenon width, with each pin offset the same amount toward the shoulder. The combined drawing force multiplies with each pin.

For visual effect, you can stagger the pins in a pattern or use contrasting wood species. Drawbored joints were often proudly displayed in traditional furniture as evidence of quality construction.

When to Drawbore

Use drawboring on structural joints that must never loosen: chair legs, table aprons, bed rails, and workbench legs. Skip it for joints that might someday need disassembly for repair.

Drawboring also eliminates the need for complicated clamping setups. When you’re building without a shopful of clamps, the self-clamping action becomes invaluable. Timber framers raised entire buildings using only drawbored joints and wooden pegs.

Master this technique and you’re connecting to an unbroken tradition of woodworking that trusted mechanical advantage over chemistry. The joints you make today will still be tight when your great-grandchildren inherit the furniture.