Have you now stood in your kitchen an considered how many joints you are surrounded by? Probably not.
Continuing on from the first part of this series, Douglas Coates looks in detail at the mechanics of the blind tenon and dovetail joint, including optimal tail angles. He continues by discussing methods for managing moisture in joinery, and offers tips on how to glue joints that last.
Blind tenon
This joint is one of my personal favourites.
Although it may look complicated, it’s construction isn’t too laborious, and what it gives in return is well worth it. As the ultimate test of its durability, I recommend making one and then trying to break it – if you can.It also gives you the thrill of a once-only fit. No dry run. Just precision and faith. Tip: on assembly, use a lot of sharp small taps, as opposed to one big bang. And use a glue that’s not ‘grabby’, and has a decent cure time.
Dovetail cleat
A clever use of the dovetail form as a cleat.
In this example, two boards are joined together to form a tabletop. The two-part bog oak cleat has a box wedge that forces the dovetails into the long grain of the oak top – therefore drawing the top boards together. From an Arts & Crafts table by Gimson and Barnsley. This one was made by Richard Arnold.
About dovetail angles
From a mechanical viewpoint, the dovetail needs to be a tight fit, with the pin under very slight compression.This compression applies load into the tail and helps prevent shear along the tail. The angle is there to achieve an interlock. At less than say, 1:10, we may as well have a finger joint. At more than 1:4 is excessive as it can cause shear in the tail.
So, around 1:6 to 1:8 is generally regarded as the correct angle. Furthermore, these angles have been popularised because companies can sell pre-set marking gauges.
A few years ago, I visited a mothballed joinery shop full of old Wadkin machines and a few well-used workbenches. On one of them, a strong gouged line was scribed at an angle on the front of the skirt beside the vice. This was certainly a guide to ‘eyeball’ when setting dovetail angles. Time was money, and I doubt these makers could afford to take the time marking out dovetails the way we might.
Under normal loads, the dovetail is at risk of shear along the grain of the tail, and excess compression (hence distortion) in the pin. Compression in the pin is applying pressure into the tail, and this pressure helps reduce the risk of shear. A such, there is an optimal range for dovetail angle. The main variable is how soft (compressive) the wood is. It is therefore vital to consider how soft or hard a wood is, as opposed to simply is definition as a soft or hardwood.
Dovetails are a lot stronger than they look. In this example (poplar), the joint opens a small amount as the pins compress, but then ultimate failure is caused by a deep fracture down the grain. In this case, the whole side came away as well.
Avoiding joint failure
The sure way to prevent a future failure is to over-engineer everything, but that is neither efficient or attractive. From race car engineers to bridge designers, the efficient and elegant approach is to design for purpose, and add a safety (or error) margin.
In joinery, we are dealing with a great number of variables, such as the inconsistency of wood, so it is impractical to design furniture that is within the margins of possible failure. By taking a little care in selecting the correct woods, and making choice joints with accuracy and care, we can maximise the durability of the furniture. None of this really takes longer or costs more.
Furthermore, by selecting workpieces with a straight and compact grain structure, and identifying bearing and bonding surfaces in joints, we can greatly increase the strength of our work piece. Ensure your degree of accuracy is compatible with your joint. I want bearing faces to be a really good fit, whilst bonding faces need to be slightly freer. Accurate cuts can be made with a chisel (it looks and feels like a fine shaving off a well-tuned smoother).
With this kind of fit and careful use of the adhesive, we are creating a joint that will last.
Managing movement
Drying timber can often be referred to as movement. I’m my years of experience I can’t recall problems arising in a piece of furniture because the wood was too dry.
Furthermore, I have never been in a workshop here in the UK that was too dry, though I’ve certainly seen the opposite. These days I take more care to manage moisture content before and during build. Our homes have central heating and, increasingly, air conditioning. As such, most applications we are being challenged by shrinkage.
My own approach is:
Minimising movement
Process all the movement possible out of the wood before the build. This means achieving the moisture level to where it will be in the home, over the long term.
Minimise stock size
Rough size every component to release any stresses from excess wood. (often stored in a board from its time in the kiln), then acclimatise down to the target dryness. There will still some movement over time, but following these steps will help to minimise the extent.
Moisture levels
The first and obvious course of action is to reduce the moisture level to its long-term stable state. If you have a moisture meter (a good investment) you have probably been around your house testing various woods. I generally get about 6%, maybe less. The hardwoods I get from a reputable supplier are generally around 10–12% moisture content, which is typical I think, but too high.
So, going back to our joints, we need to get as much movement out during the build as we possibly can. I think there is an argument for slightly over-drying, partly because the amounts of movement at these low levels of moisture are very small, and a tiny swelling within a joint is better than shrinkage.
The adhesive interface
This is an extensive topic. Specifically, we are considering mechanical properties of joints, and the glue is a membrane or interface between the two components. That interface needs to transmit loads (stresses) through the joint as efficiently as possible – ideally, as if it wasn’t there.
The need for glue in many joints is essential. The joint may be well-fitted, but if the glue membrane can flex (even very slightly) then mating faces will distort. Distortion increases stress in critical areas and failure will ultimately follow.
As stress will focus on the weak point – wherever that is and no matter how small it is – it is important to ensure the entire mating area is coated with glue. If stress is a gremlin looking for a weakness, then a sudden discontinuity in the glue membrane is exactly the kind of place those gremlins will get to work.
Good wood gluing practice
Viscosity
The glue should be low viscosity to flow and penetrate. Increasing its temperature will increase its viscosity.
Application
Cover all areas within the joint, on both components. Applying glue thoroughly to both components may take slightly longer, but it’s a fraction of the time it took to make the joint. This will avoid discontinuities in the bond membrane.
Seal
Apply to end grain (the tenon end for example) – it won’t bond usefully, but it’s an end-grain sealer, and end grain is mostly where we get movement from.
Give space
Give bonding faces enough clearance for the glue to do its job – 5 thou per face is sufficient, but a nice slip fit is about right.
Listen to the makers
It may be ‘stronger than the wood itself’, but there are terms and conditions in the small print. Working temperature is important, and cold winter workshops are less than ideal setting conditions.
Use a brush
I have a cheap brush stuck into a hole in a pot lid. Keep water in the pot and the brush will last many months.
Breadboard ends on tops must accommodate movement. This 100mm ash workbench top will live in a dry workshop, but it will be subject to small cyclical movements – a reminder that we have to consider the environment the piece will live in. The front corner is built using a double dovetail. A sliding tenon (tongue and groove) then locates all the way back, fixed with coach bolts into slotted holes. The big dovetails sit tightly, requiring a once-only assembly with no dry test-fit.
The bonding faces on dovetails should be rather tight-fitting. Using a short bristle brush, I can scrub the adhesive into the surface of both components, aiding penetration where there is hardly any clearance for glue. A warm day and a low viscosity adhesive help a lot.
Summary
In joinery, there are a great deal of variables. I set out to look at joint making from a different point of view – specifically what goes on inside a joint, and how this bares relation to the choice of joint specific to the application.
By following the steps discussed above, you will achieve strong joints that will look good and last. A nice idea would be to summarise the points and hang them on your workshop wall for easy reference.I hope that this two-part series provides you with a broader perspective on joints, and helps inform your choices when designing and building your woodwork. Take it from me – it is well worth spending the time to understand these time-tested principles.