The Mortice and Tenon
Wednesday 26 November 2008
Robert Ingham looks at this classic joint
Mortice and tenon joints are probably the most popular way to combine pieces of wood to form a construction and there have been extensive variations on the theme. From the earliest times, craftsmen used sections of wood that were not planks and boards converted from large trees. They often worked with pieces of wood in the round, taken from branch stock and worked directly. There is evidence to support the fact that even in those early days, load-bearing constructions consisted of components set at right angles to each other. The simplest and strongest way to achieve this was to bore a hole in one piece and insert the end of the other piece into it. We still use this method of jointing today, in the form of turned rails inserted into holes for the construction of seating, such as the Windsor chair. But for some reason this type of joint is not referred to as a mortice and tenon.
Control issuesIn its simplest form the tenon or male component is a rectangular section inserted into a rectangular hole, as is quite commonly used for fencing. The problem with this method is the disproportionate strength of the rail to the stile. Also it is very difficult to control the depth of the mortice slot accurately if you are cutting it by hand with a chisel. A construction often needs several components to be structurally successful and it needs to have stiles that are parallel and dimensionally accurate. However, it is impossible to rely on tenons fitting to the bottom of mortices.
This would not be a problem today, as cutting mortices by machine would obviate this shortcoming. Not so with a hand-cut mortice, so the way this was overcome in the past was to cut the tenon so that it had shoulders. The distance between shoulders could be easily controlled, which was absolutely essential for the construction of early pieces of furniture such as chests and the wall panelling with which sophisticated dwellings were lined. The introduction of shoulders must have marked a major step forward in terms of technique and this would only have been possible with the parallel development of metallurgy to produce tools with which to control the sawing process.
Simple mortice & tenonMy guess is that most woodworkers today would not make this joint by hand. This is probably true of all the constructional joints we use to make wooden objects and it is a sign of the times that even the home handyman will go straight to his local hardware supermarket and buy a machine or jig to do the job. Having made this statement, it is still possible to buy all the original hand tools associated with this joint.
Technology is of its time and we cannot ignore it but the down side to it is that if you cut out the experience of doing it by hand, you risk losing the understanding of the underlying principles of many craft skills. I always say that wood is a complex material and in my opinion working with hand tools gives you a greater understanding of its properties. So, for the benefit of a more complete understanding, I will outline how to make a mortice and tenon by hand.
Marking outThis method is universal to all mortice and tenons that you would need to produce a flat frame. This was probably the construction used most historically, and is still very popular today.
The two parts of the joint have a lot in common dimensionally so it is a matter of choice whether you start with the mortice or the tenon. But, because the mortice is the first in the title it is probably first to be made. I was taught to mark out the location of the mortice using a pencil. This makes sense if you plan to cut the mortice through the component, as you can remove the material from both edges effectively and project the dimensions easily from one edge to the other with a try square. It is easy to remove the pencil line later, with little loss of the thickness of the component. Be sure you consider the pencil as a precise marking tool. The mark it makes is not just a line but an important definition of position. I use a click type of propelling pencil with an 'HB' lead for consistency of line width.
If you sharpen a wooden pencil, do it with a chisel and create the effect of a blade, so that the line is thin and the drawing action is consistent. For a stopped mortice I prefer to mark out the position with a knife line. Make sure your marking knife is sharpened with only one bevel and the bevel is orientated to be on the waste side of the line. A tool such as a Stanley knife is inappropriate, as it has two bevels that produce the cutting edge, so one will always compress the defined corner, producing a small chamfer. If several mortices are involved it is better to mark out all the positions on one stile and then clamp all the other stiles together with the first and square all the lines across from the master component.
Next, mark out the shoulders of the tenons. Here, a single bevelled marking knife is absolutely essential to ensure crisp, square corners to the shoulders. It also makes it easier to pare cleanly to the shoulder line after removing the cheeks of the tenon.
Uniform thicknessThe next stage is to mark out the thickness of both the mortice and the tenon. You can carry out this task most effectively with a mortice gauge. Select a mortice chisel that is proportionally suitable to the thickness of the components. If the construction is to be a frame with two rails and two stiles, the tenon should not be less than a third of the thickness.
The choice of chisel has a direct bearing on the mortice gauge setting - it is easier to cut a slot for the mortice and fit the tenon into it than the other way round. This is a common engineering principle - it is more difficult to adjust the inside of a hole than it is to remove material from the inserted piece.
Having selected the appropriate chisel size, set the mortice gauge to its width. Bear in mind that the spurs of the gauge are tapered so it is necessary to adjust the setting to the points. Then centre the two spurs in the thickness of the components. Centring at first by eye and pressing the spurs into the edge surface can do this quite effectively.
Repeat this from the corresponding face. If the two sets of marks coincide, the spurs are centred. If not, gently lock the gauge stock in position and tap one end or the other of the stem to move the spurs until they are centred. Once you are satisfied, lock the stock firmly in place.
Using a gaugeMortice gauges and marking gauges should, in principle, be easy tools to use. In practice, however, it takes some time to master control over their action. There are essentially three points of contact to carry out in sequence. Firstly, place the inside face of the stock against the face side of the component. Then do the same with the corner of the stem and finally the spurs so that they trail in the direction that the gauge is pushed. I have tuned my gauges for smoothness of action and this includes the removal of the sharp corner that makes contact with the stem. In fact I have rounded this corner so that the stem rolls smoothly to make contact with the spurs easy to control, both visually and mechanically.
To mark the lines, always work from the face side so that any variations in the thickness of components will only be evident in the assembled joint on one side. Face surfaces are bound to align, provided you have cut the two parts of the joint competently.
Here is a little tip to prevent over-shooting the limits of the mortice position. Press the spurs into the far line of the mortice to make a pair of indentations. The spurs will click into these when they reach this position while marking the lines. You can also use this for marking out a tenon on the end grain and both corresponding edges.
Working from the face side and face edge is regarded as general good practice and is essential when preparing timber by hand. If you are using machines for timber preparation there should be no variations, unless you have changed thickness settings and then reset them. The final act of marking out is to identify the waste with a visual mark to minimise the risk of cutting on the wrong side of the line, particularly when sawing the tenon.
Chop the morticeMortice chisels are sturdy tools that have to be strong enough to withstand a power blow from a mallet and the levering forces used to remove the waste. Nowadays most manufacturers produce the registered pattern with ferrules at both ends of the handle. The blade is quite thick and the surface area of the sides resists twisting when removing the waste, keeping the sides of the mortice clean.
Support the component on the surface of the bench and hold it down with a clamp. Position it, so that you can stand square to it and try to avoid standing to one side. This will enable you to hold the chisel upright - the only check that can be applied to vertical accuracy is visual.
The fact that mortices have always been chopped with a chisel, starting the first cut in the centre with the cutting edge across the grain, shows a significant realisation of the natural structure of wood. This first cut produces a V-shaped indentation that creates a void into which all subsequent cuts displace the material being removed. I am sure that this was clearly understood by craftsmen from very early times as the mortice and tenon has been around for many centuries.
I use two handsI like to hold the chisel with both hands to carefully guide the cutting edge into position between the gauge lines. Then I hold the handle before picking up the mallet to strike the end of the handle. By doing this I can use the entire length of the blade as a visual marker to ensure that the cut is upright and parallel to the eventual sides of the mortice. Some craftsmen hold the blade only and position the cutting edge between the gauge lines with one hand. It saves time, as the chisel is held in one hand and the mallet in the other. However the length of the blade is obscured, so you lose a valuable sighting tool.
After the first cut continue with a series of cuts and stop a few millimetres away from the end lines. This will leave enough material to provide a fulcrum for levering out the waste. When removing the chisel after each cut do not move it from side to side or the mortice will be too wide. Hold the flat face of the chisel towards the end lines until the series of downward cuts are made. Then turn the chisel round and lever out the displaced fibres and repeat in the opposite direction. The depth of cuts will depend on the density of wood and the ease with which the fibres shear between cuts. Practice and experience will enable you to judge the effective depth but if it is too deep the levering action to remove the chips will require a lot of force. Be prepared to chop a mortice in stages. If it goes through, turn the component over and repeat from the other edge. Finally, cut back to the end lines to square up and accurately dimension the mortice. I use a marking knife for these end lines as a chisel's cutting edge can be firmly trapped while being struck with a mallet. For small mortices I pare back to the lines using hand force and a vertical paring action. There should be no need to clean up the sides of the mortice unless tearing occurs. Rough sides improve the glue bond.