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How to make a tapered mortise and tenon joint with a tapered spoon bit

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A tapered mortise and tenon used in chair making 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.
An example of chair legs that attach to the seat via tapered mortise and tenon joint, created with a tapered spoon bit 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!
A caution sign to advise that you need to make sure your spoon bit and tapered tenon cutter match if your project is going to be successful 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).
Image showing matching tapered spoon bit and 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

A tapered tenon cutter, which works like a pencil sharpener to shave the tips of chair legs and wooden rods to a point

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.

A pencil sharpener, which works in a similar way to a tapered tenon cutter This works in much the same way as a pencil sharpener.
A chair leg that has been fully tapered. The tenon cutter cannot cut any further down the leg. Shave the ends of each leg until the top reaches the end of the tenon cutter.
Wonkee Donkee advises DIYers to make sure they keep the tenon cutter straight so that their tenons do not end up wonky
Drilling a pilot hole with a Forstner bit so that a tapered spoon bit can be inserted to widen the hole

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?

Check if the spoon bit is too deep for your seat

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!

DIYer reaming a hole with a tapered spoon bit to create the perfect tapered mortise in which to fit a tapered tenon

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.

A green light for a chair maker to start widening their leg hole with a tapered spoon bit 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.
Image of a tapered tenon being inserted into a tapered mortise 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

A tapered reamer which is the alternative to a tapered spoon bit when cutting tapered mortise and tenon joints 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.

For more information on reamers, see: Reamers.