Common in chair making, tapered mortise and tenon joints are used to create joints that strengthen whenever pressure is applied. The tapered sides of the tenon push further into the conical mortise under this pressure, forcing the joint to fit together even more securely.
This type of joint is often used to connect the legs to a chair’s seat. This means that the whole time you’re relaxing in this type of chair, you’re strengthening the joints!
For this process to be successful, and for your mortise and tenon to fit together snugly, you need to make sure that your tapered tenon cutter and tapered spoon bit match. You can do this by making sure they are both the same size (e.g. a 16mm (5/8″) spoon bit will match a 16mm (5/8″) tapered tenon cutter).
Matching tenon cutters and tapered spoon bits will create the same degree of slope on both the mortise and the tenon, and will also help you to match the depth of the mortise to the length of the sloped end of the tenon.
Creating a tapered mortise and tenon joint
Step 1 – Create the tenon
It’s best to create the tenon first so that you can use it to test your mortise later. For this, you will need to use a tapered tenon cutter.
This works in much the same way as a pencil sharpener.
Shave the ends of each leg until the top reaches the end of the tenon cutter.
Step 2 – Drill pilot hole
Drill a 13mm (½”) pilot hole for your mortise using a Forstner or brad point bit.
For more information on these bits, see: What is a Forstner bit? and What is a brad point bit?
Step 3 – Check depth
Check your spoon bit against the side of your chair seat and mark the bit with tape so that you know how far down you can drill before you have to stop. You don’t want the legs to poke through the top of the seat!
Step 4 – Widen the pilot hole
Attach your tapered spoon bit to your hand drill and insert the tip into the pilot hole. You don’t need to worry about seating the bit this time as the pilot hole should keep it on course.
Start drilling. This will widen the mortise and create a tapered hole. Stop to clear out wood shavings regularly so that your bit does not get clogged.
You’ll also need to check the depth of your hole regularly by inserting the tenon into the mortise and checking how well it fits.
Alternative tools
As an alternative to a tapered spoon bit, you could use a tapered reamer, which will widen the pilot hole you have drilled to create a tapered mortise.