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Mortise & Tenon Joint

Mortise & Tenon Joint

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.

The tools you need are:-

Tenon Saw, Mortise Gauge, Try Square,Mortise Chisel (you could use a normal bevel edge chisel), Bevel Edge Chisel, Mallet, Bench Hook (optional), Bench Vice or Workmate, Ruler, Pencil, Marking Knife or Stanley Knife, Masking or Electrical Tape


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For this guide, piece ‘A’ is going to be the tenon and piece ‘B’ is going to have the mortise. The wood I’m using is 45mm x 20mm pine.


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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.


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Using your ruler and pencil mark 6mm in from each pencil line on piece ‘B’ Then, using your try square against the first pencil line, draw a second line parallel with it, at the 6mm distance. Repeat this for both lines and you shuld now have 4 pencil lines.


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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.


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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.


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Step 2- Laying out the Tenon!

From the end of piece ‘A’, measure and mark the length of the tenon. I’m making my tenon 25mm.

The general rule for the length of a tenon is two-thirds the width of piece ‘A’.


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Using a try square and pencil, continue to mark all the way around piece ‘A’. (These new pencil lines are called ‘Shoulder Lines’)


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Holding piece ‘A’ firmly on the bench, with a try square and marking knife, cut along the waste-side of the shoulder lines. This helps to guide the saw and give a more accurate and cleaner cut.


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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".


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The last step in the layout is to mark the waste to stop you making a mistake when cutting.

This is what you should be left with (photo on your left)


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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.


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Once you’re near your depth (or before you start cutting the mortice) wrap some tape around your chisel to act as a depth gauge. I’ve marked my tenon to be 25mm so I’ve put the tape to 28mm – this small gap will give room for the glue.


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At this stage the mortice is nearly complete. Its down to its final depth and now just needs carefully paring back to the lines.


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One mortise all done. Once you do a few you get the hang of it and can cut them quite quickly.


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Step 4- Cutting the Tenon!

Holding piece ‘A’ in a bench hook or clamping it in a bench vice/workmate, cut down the shoulder line (waste-side) being careful not to go too far. Turn the piece over and cut the other shoulder line.


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Clamp piece ‘A’ into your bench vice or workmate at a 45° angle and cut along the gouge lines again being careful not to go too far. Flip the piece over to the other side and do the same.


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Now clamp the piece vertical in the vice and finish the cuts on one of the gouge lines. Unclamp the piece and flip over and cut the other gouge line.


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If necessary clean up the tenon with a bevel edge chisel.


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The next job is to do the cheek cuts. With a ruler measure and mark 6mm in from the sides of piece ‘A’ then with a try square or straight edge continue the marks around the tenon.


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Clamp piece ‘A’ in your bench vice or workmate and cut down both of the cheek lines you’ve just marked.


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Now clamp piece ‘A’ horizontal in your bench vice or workmate and carefully finish the cheek cuts.


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If all your careful laying out and cutting paid off, you should have a snugly fitting, accurate, flush mortise and tenon joint.


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