The mortise and tenon is a very traditional joint and is popular due to its strength. A mortise is simply a hole in the piece wood and the tenon piece slots inside. There are many different forms of mortice and tenons and also uses – but they are mostly used in tables and doors.
The key to a strong mortise and tenon is a snugly fitting joint. If the joint is too tight there will be no room for glue, and there is a chance of the mortise piece splitting. If the joint is too loose and you need to ‘pack’ the tenon, the joint will be weak. For a strong joint you should be able to push the tenon into the mortice just with hand pressure.
Step 1 – Laying out the Mortise!
Lay the mortise piece (‘B’) on your bench so it’s face edge is facing the ceiling & then rest piece ‘A’ across it at the location where you want the mortise to be. Using a try square and a pencil, mark the width of piece ‘A’ onto piece ‘B’ as shown in the photo to your left.
We want to set the mortice gauge now, so with your ruler measure the thickness of piece ‘A’. The mortise should be one-third of the thickness of the tenon piece (‘A’) but is normally rounded up to your nearest chisel size. For this joint I’m going to use a 6mm chisel.
Set your mortise gauge to the exact width of the chisel and to the center of piece ‘A’. I like to do this by taking an offcut the same thickness as ‘A’ and holding the gauge over it, set it by eye and then working from both sides, adjust the gauge until the marks it makes are the same from either side.
With your mortise gauge, from the face side gouge between the two inner lines on piece ‘B’
A good tip so you don’t go past the line is to put a small indentation just before the line, using your mortise gauge – so when you gouge the lines, the pins on the mortise gauge should fall into them, therefore stopping you going too far! When using the mortise gauge it’s best to hold it at a slight angle and to take several light passes. By doing this you should be left with nice straight gouge lines.
With your mortise gauge, gouge lines on both sides from the shoulder line to the end of the wood. Also gouge a line on the top of the piece (the end grain)
As I’ve said above "A good tip so you don’t go past the line is to put a small indentation just before the line, using your mortise gauge – so when you gouge the lines, the pins on the mortise gauge should fall into them, therefore stopping you going too far and when using the mortise gauge it’s best to hold it at a slight angle and to take several light passes. By doing this you should be left with nice straight gouge lines".
Step 3- Chopping out the Mortise!
Clamp piece ‘B’ into your bench vice or workmate. You want to start cutting the mortise a few mm in from each end while you chop out the bulk of the mortise. You can then pare back to the lines once you’ve reached the depth you want.
With your chisel and mallet make a cut 90° to the timber, a few mm in from each end of the mortise & then do a couple more cuts further in towards the middle. Now working from one of the cuts that you made near the end, hold the chisel at a slight angle and start cutting towards the middle of the mortice. Don’t try and cut too deep at once, just remove the waste bit-by-bit.