Mortise and tenon joints are indispensable if you’re making furniture that’s both strong and good-looking, and are particularly useful for making the most popular pieces of furniture – tables and chairs.
Take a piece of wood, shape the end to form a ‘tongue’, then fit the tongue into a matching slot in the side of another piece, and you’ve made a mortise-and-tenon joint.
The tenon is the tongue and the mortise is the slot, and the joint has proved its usefulness over centuries in all kinds of wooden frameworks because of its strength and resistance to movement. It’s the best joint for fixing horizontal pieces of wood – ‘rails’ – into uprights such as table and chair legs.
Once you’ve got the knack of cutting it cleanly, you’ve mastered a joint which will stand you in very good stead. Whenever you’re joining two lengths of wood in a T or L shape, and you want something stronger and more elegant than a halving joint, go for a mortise and tenon joint. The only time it won’t work is on thin, flat pieces – boards, planks and panels: use housing joints instead.
There are numerous types of mortise-and-tenon joint at your disposal. Think carefully about the job the joint has to do before deciding which to use.
Choosing the right joint
A passes right through the mortise piece (which makes it easier to cut the mortise). Because you can see its endgrain, it’s used in rougher work or as a decorative feature. It can also be wedged from the outside for strength and/or visual effect.
A is one which doesn’t pass right through, but fits into a ‘blind’ mortise – a hole with a bottom. The most familiar kind, especially in furniture, has shoulders all round which conceal the joint.
A has the tenon cut with only one side shoulder instead of two – useful if the tenon piece is already very thin; or the tenon may be reduced in width by having edge shoulders cut in it – see
A is a compromise often used at the corner of a frame to keep it from twisting. The haunch – an extra step between the tenon and the piece it projects from – can be square or sloping. A sloping haunch is hidden and easier to cut – see again.
A is just a pair of tenons cut on one piece of wood – used if the piece is very wide and you don’t want to cut a single enormous mortise to take one wide tenon.
An is simply one which isn’t in the centre of the tenon piece.
Making the joint
Let’s assume you’re making a basic stub tenon joint. It doesn’t really matter whether you start by making your mortise or your tenon; the important thing is to get them to fit together. However, cutting the tenon first means you can mark off the mortise from it, possibly getting a better fit. This is easier than the other way round. Either way, play safe by making the tenon a little too large (or the mortise a little too small), rather than the reverse. You can always cut off a bit more.
Marking and cutting the tenon
Begin by scoring round the tenon piece with a knife and try-square to mark the length of the tenon, using the width of the mortise piece as a guide. A through tenon should be a little bit over-long to allow for planing flush to give a neat finish; a stub tenon should go about halfway through, and be about 3mm (Vain) shorter than the mortise to leave room for any excess adhesive.
A mortise gauge is very useful for the next stage. Choose a mortise chisel which has a blade about one third the thickness of the tenon piece (under rather than over, though you can use a wider one if the tenon piece is much thinner than the mortise piece), and set the gauge’s twin spurs that distance apart. Then set the stock so as to place the resulting ‘tramlines’ in the centre of the timber thickness – unless you’re deliberately off-setting the tenon – and try it from both sides, adjusting the position of the stock till the two sets of tramlines coincide.
Now you can score the edges and end of the tenon piece to mark where the tenon will be cut. If you don’t have a mortise gauge, use an ordinary single-spur marking gauge and mark the tramlines separately.
For a straight tenon, that’s all the marking-up you need. If you’re cutting shoulders in the width as well, set a marking gauge to one sixth the width of the tenon piece and mark down both faces and across the tramlines on the end.
If you’re including a haunch, use the gauge to mark its width across the end and down the faces; then mark its depth with a knife and try-square. For maximum strength, the haunch should be not more than one third the tenon’s width, and its depth not more than one quarter the length (or 12mm/ 1/2in long, whichever is smaller). We will be dealing with these joints in more detail in another section.
To cut a tenon you need, not surprisingly, a tenon saw. All you have to do is grip the piece upright in a vice and saw down each side of the tenon; then lay the wood flat and saw off the shoulders. The vital thing is always to keep your saw-cuts on the waste side of the lines.
Marking and cutting the mortise
At this stage, you can lay the tenon on the mortise piece and mark the mortise length on it. Then score its width with the gauge.
To cut the mortise, cramp the timber in position. If working near the end of a piece, leave extra length – a ‘horn’ which you saw off later – to prevent the wood from splitting as you chisel into it. If you have a carpenter’s brace or a power drill, you can start by drilling holes close together along the length of the mortise. Make quite sure you keep the drill vertical – a drill stand will help.
Then chop and lever out the waste with the mortise chisel, and cut the recess for any haunch. Lastly, clean off the sides and ends with a bevel-edged chisel.
If you have no drill, use a mortise chisel by itself, keeping the bevel away from you and working from the centre of the mortise towards the ends – stopping just short of them so as not to bruise them when you lever out the waste before going deeper. On a through mortise, chisel halfway and then work from the other side. Clean up with a bevel-edged chisel.
Assembling the joint
Now you can fit the pieces together. Don’t be tempted to force them, or you may split the wood; if the joint is impossibly tight, carefully shave the tenon with a chisel and glass-paper checking all the time. When it’s a neat, close fit, glue it, cramp it and leave it to set.
Ideally, you need sash cramps – long steel bars with one fixed head and another which you tighten – plus scrap wood to protect the work.
STRENGTHENING THE JOINT
For extra strength and decorative possibilities, consider wedging or pegging the joint once it’s fitted. Hardwood wedges go either into previously made saw-cuts in the end of a through tenon (A), or into the mortise above and below it (B). The mortise needs to be slightly tapered. Pegging is done with one or more dowels inserted into holes drilled sideways through the joint.
TIPS FOR BETTER JOINTS.
– a through tenon should be cut too long, and made flush once the joint is assembled
– some people find it easier to start cutting the tenon while holding the piece upright, then to re-position the wood and saw at 45°, and to finish off with it upright again
– set your mortise gauge from the exact width of your mortise chisel
– if mortising near the end of your timber, leave it over-long to prevent splitting, and cut off the extra bit later
– to keep drill or chisel vertical, stand a try-square on end beside the tool as you’re working.
– leave it till last to pare down the mortise ends, so as not to risk bruising them while levering out the bulk of the waste.
– to stop yourself drilling too deep when starting a mortise, fit a depth stop (an item you can buy) or wrap masking tape round the bit as a depth indicator.