A Tongue-in-Cheek Look at Mortise and Tenons:
Derek Jones describes a few options for taking the hard work out of large scale joinery
On paper or to the uninitiated perhaps, carpenters and joiners are often mistaken for furniture makers and vice versa. In fact, before the mid-18th century and the introduction of veneered casework (cabinets) carpenters and joiners made all our furniture. And while there are some obvious similarities there’s still the odd gap in skills and techniques when you consider the vast array of work that forms part of their remit. On the few occasions that I’ve spent time in a joiners ‘shop I’ve become aware of just how much more economical they are with their labour. Maybe it’s because the components are larger and not so manageable that they stay in one place for longer and don’t get moved around quite so much. Carpenters and joiners tend to be a little more relaxed about dimensions as well and not quite so obsessed with splitting the atom with every pass of a plane.
Then there’s the use of machines. Typically a single setting will be sufficient to complete a number of identical processes negating the need for a lot of marking out apart from a squiggle perhaps to identify feed direction and orientation. In this respect I envy the joiner’s skill at simplifying a task by knowing precisely where and when it’s appropriate to dial in the small stuff. In a typical hybrid workshop, that’s one with hand tools and machines, where there’s a need to flip between disciplines it’s possible to blur the lines and think more like a joiner.
More than a year ago I set about building a new bench for no other reason than it seemed like a good idea at the time and, to be honest, I fancied trying out something different. Hefting large heavy components around the workshop is never much fun on your own and a full-on solo dry fit assembly can be something of a challenge, if not a physical impossibility. For a cabinetmaker bypassing this part of the process means throwing caution to the wind and putting the text book to one side for a moment and adopting a more pragmatic approach to the construction process. Easier said than done.
Re-learn the basics
One of the basic principles of good cabinetry is to resist the temptation to work from measurements in order to achieve that piston fit drawer or gentle puff of air from a nicely hung door. If you find yourself wanting to calculate how much to shave off a leading edge to make it fit, you’re probably missing the point; this level of precision has more to do with what you can’t measure, but can feel than what you can quantify and the techniques required to achieve those ends. Large or small, the most accurate way to determine a good fit between two components made from wood is by touch or by using the components as a template or feeler gauge.
The undercarriage for this bench includes four stretchers that are mortised and tenoned into the legs and then draw bored. The front and back faces to the worktop complete the joints that connect the legs with the bench top and are the last pieces to be glued in place. In this unfinished state the legs can be dropped into their mortises and squared up to establish the shoulder lines for tenons on the stretchers. Clamping the stretchers to the outside of the legs holds them in place while you work around the frame. I used
the dimension at the top of the leg, where it joins the bench top, as a guide to setting out the frame in its intended position further down the leg. The shocking fact is I couldn’t achieve 90° at every junction in both directions, but rather than lose sleep over it, I made some adjustments to create a structure that was not unduly stressed at any one corner.
I’m always impressed when I hear about woodworkers that have their bandsaw set up as a re-saw capable of slicing veneers from a solid piece of timber apparently without any run-out or deflection. While this level of accuracy is not beyond the realms of anyone with a bandsaw it’s hardly typical. If the F&C workshop is anything to go by, the bandsaw gets a rough deal and is treated like the camp bike. Woodturners, for example, like to create bowl blanks on their bandsaws and therefore dull one side of the blade quicker than the other. General woodworkers like to resize offcuts on their machines to suit their wood burning stoves. And cabinetmakers? Well, we just want to be able to set the fence and cut a straight line. Considering the aforementioned, I find this works reasonably well on thin stock so I can usually rely on the bandsaw for trimming the tenons to width. Annoyingly, the same cannot be said when it comes to cutting the cheeks; the all too common problem of run-out being the result.
If this sounds like the bandsaw in your workshop do not despair, you can still use the machine to good effect. Like flying an aeroplane, the trickiest moment is take off and landing. In sawing terms this relates to establishing a good clean initial kerf (take-off) and not crossing the baseline at the other end (landing). Providing the table on your bandsaw is at 90° to the blade you can set a dead straight kerf to a depth of a couple of millimetres on the end of the board without the blade running out. This will be sufficient for an auto pilot take-off with a tenon saw. Admittedly the kerf will be wider than your tenon saw blade so just remember which side of the bandsaw kerf you need to place your saw. Cutting the cheeks before the shoulders means that crashing through a base line and into the tenon and substantially weakening it is less likely to happen.
Tenon short cuts
For the majority of time when I’m cutting mortise and tenons for furniture I’ll start with the mortise and gauge the thickness to suit the tooling, either hollow chisel or mortise chisel. I’ll typically gauge and knife mark the position of the mortise to help with registration on each component. It takes a little time, but gets me in the mindset required to create accurate joints. Larger scale joinery is more forgiving. A pin gauge line or pencil line is usually sufficient to set things out. The first mortise will get the offset mark from the face of the leg and the width of the mortise. The remaining mortises just get the width marks.
If you are using a bench top mortise machine you might need to get a little creative to work on large stock. The pillar on this machine can be unbolted and rotated 90° to operate off the base to one side.
Quality dowels from offcuts
There are a couple of things that can affect the strength of a draw bored tenon but no more so than the quality of your dowels. The dowels perform an important role and need to be made from wood with the straightest grain you can find. Don’t take chances here. I start by sorting through the offcuts first as this material has already gone through some kind of quality control. I’m in favour of using the same species for the dowels that the mortise and tenon are cut from. I don’t think there’s any empirical evidence to support my theory, but my hunch is that the degree of compression between the components is better off matched than introducing a rogue factor into the equation.
The next step to producing the best dowels is to split them into billets with a chisel and not cut them with a saw. Green woodworkers have been using the inherent benefits of riven timber for centuries. Your blanks must be free from any defects or grain that even slightly veers off course. If you’ve chosen wisely the dowel blanks will split from the mother blank in nice square sections. Any that don’t can be placed on the pile for kindling. To make your square pegs round, use either a dowel plate or a block plane to knock the corners off. Creeping up to the finished size in a couple of steps is advisable if you are using a dowel plate. A tip here is to mark the finished diameter of the dowel by tapping it through the dowel plate a short way first. The indent gives you something to aim for if you decide planing is a better option. The final step is to put a slight taper or point on the end of the dowel that enters the hole.
Draw bore 101
For maximum strength drill the holes for the dowels close to the edge of the mortised component, or in other words close to the shoulder line. This may look like a bad idea at first, but as the shoulder of the tenon will be drawn tight into the edge of the mating component any risk of splitting is minimal. This location also leaves the maximum amount of material behind the dowel hole on the tenon to resist splitting the tenon when the dowels are driven home. When you drill your dowel holes, place some scrap material into the mortise to prevent any breakout inside the joint. With the holes drilled you can now dry fit the joint and use a tipped drill bit to make a mark on the cheek of the tenon. To draw the joint together, the dowel needs to be forced off course slightly as it passes through the tenon and into the hole on the other side of the mortise. So the next mark you make is about as critical as it gets. Move the hole in the tenon about two millimetres back towards the shoulder, effectively off centre from those passing through the mortise. In the words of a well known TV drill sergeant ‘shoulders back, stomach in’.
The next part isn’t rocket science. Assemble the joint, use clamps if you have to and then drive the dowels into the holes. If everything has gone to plan the dowels will pull the joint together and render the clamps useless. In some circumstances there may also be a good argument for not using glue, but we’ll leave that discussion for another day.
Room for manoeuvre
The essence of good cabinetmaking is tight joints. Without them things start to move a little and then a lot until the whole ensemble falls apart. But, given that any rigid form, however well constructed, could encounter a force greater than that which it has been designed to withstand, we often need to introduce an element of flexibility to the structure. When building with wood the most destructive force will come from the material itself as it comes to terms with adjusting to various temperatures and levels of humidity. When making the top for this bench I tried hard to select boards that were quartersawn. These are easily spotted as the growth rings cross the section of the board on two faces, either a face and edge or two faces. Boards that display this feature are less prone to cupping, bowing and twisting so make for a more stable construction. However, expansion and contraction in a linear direction across the boards is still very much an issue. Armed with this knowledge we can introduce joints that allow movement in a known direction. Mortise and tenons that incorporate a loose tenon are a good example and those produced with a Domino are quick and easy to apply.
It’s not imperative that the bench top has an end cap, but it will certainly help to reduce the ingress of moisture through the end grain and therefore splits. The cap will also help to keep the top flat. This type of construction is typically referred to as a breadboard end and usually incorporates either a single sliding tenon or a series of draw bored tenons. For a bench top attaching the end cap with lag bolts is fast and effective. Creating slots for the bolts to pass through rather than holes allows for movement. No amount of glue will prevent the top from expanding if that’s what it decides it wants to do so going dry with this joint is perfectly fine. I’ll be applying a durable oil finish to the bench when it has been levelled making sure the joint gets a good soaking. The final step to complete the joint is to wind the bolts into the end grain with a socket set. Carry out a few tests on some scrap beforehand to establish the right size pilot hole, lubricate the bolts with wax.