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Technique Woodworking

Dovetails

For Christmas, Fi got me a place on a woodworking course, which I took over the weekend…

Not far from where we live is an organisation called the Sylva Foundation. They are an environmental charity who work to connect people and trees. They look at forest, land and wood resource management, and at the uses of home-grown wood once cut. Sylva host a Wood Centre which acts as a hub for small businesses and craftspeople who work with wood. Within this centre lies a Business Incubation service, a Community Orchard and Future Forest, an Anglo-Saxon House of Wessex reconstruction, and a Wood School, dedicated to lifelong learning.

It’s to the Wood School I went to learn how to make dovetail joints.

On a day when the wind was blowing pallets across the adjacent field, eight of us descended on the coffee machine, and then took our places at work benches. Our aim was to get to grips with the fine art of scribing, sawing and chiselling in order to get small pieces of wood to fit snuggly together. This is new territory for me, moving a step closer to ‘proper’ woodworking and into the sorts of work that a biscuit cutter can’t reach. As I’ve mentioned in other posts, I’m familiar enough with the use of a machine that cuts nicely measured grooves into two pieces of word so that they can be glued together with a biscuit between. I can also drill holes and insert dowels and cam locks to create a secure join. These joints are fine when the wood is in planks and the piece is allowed to look a little rustic. If I’m going to work with finer wood on smaller scales, I need more skills. Understanding the mechanics of a dovetail joint, and the process of cutting and assembling dovetails, will be a good step in that direction.

A Dovetail is a piece of wood that is shaped to look like a dove’s tail. It slots into a hole in another piece of wood, that has been cut to exactly the same shape – the bits remaining in the other piece to hold the Tail are known as the Pins. The cuts of the tail and the pins all need to be precise so that the one fits into the other without movement. The dovetail fans out in such a way that any force on the joint acts to pull the two bits of wood together rather than letting them part. This adds a lot of strength and rigidity to the joint, making it very unlikely to come apart. Plus, it looks nice, especially when used repeatedly over a long section.

So, the kit: There are lots of new bits to buy and new things to master. Dovetails are absolutely 100% dependent on the care that goes into measuring. Pencils must give way – their lines are too thick and imprecise as a result. Marking Knives and Scalpels will make finer and more definite lines, grooves which other tools like Chisels and Dovetail Saws can slot into for guidance. This is far more precise than using tired old eyes like mine to line things up. Though they are more precise than pencils, marking knives have a limitation; they rely on Rulers, Set Squares and Bevel Squares to provide a guide and keep them straight, which can be tricky to manage on small pieces. So, we should also master Marking Gauges, which can mark a groove parallel to an edge. Marking Gauges have the added advantage that they can be used to measure up to a mark, and so transfer that measurement from one place to another. Dovetails rely on having identical things marked on two different pieces of wood, so good rulers, good straight edges and good gauges are a necessity.

Saws and chisels come into play next. A Dovetail Saw is a fine-toothed, thin and stiff saw that can cut cleanly and with lots of control. Chisels can then remove slivers of wood to achieve the exact match between the pieces. Both need to be really, really sharp. Blunt or dull edges will bump over obstacles and get pulled by the grain of the wood, resulting in cuts where you do not want them. Before we began to work, we took a quick lesson in how to sharpen, though that’s a subject for another blog probably, so for now we’ll take the need for sharpness as read.

The first technique we learned was to prepare to make a saw cut. Saws have a thickness, and will remove some wood as they are cutting, so you think of one side of the cut wood being the work you want and the other side being scrap. You want to line up the saw so that one side of the blade lines up exactly to the cut mark, and the saw teeth will scrape away at the scrap part, leaving just the wood that you want. It’s time for some PowerPoint, and here’s the trick:

Start by making the mark across the top of the wood with the knife or gauge against a straight edge. Take a wide chisel, with sloped side over the scrap and gently drag along the wood so that its tip falls into the mark cut by the knife. Hold the chisel perpendicular to the wood and tap it with a mallet. This will make a deeper cut, with one edge flush with the mark. Next, turn the chisel around, put its tip into the scrap about a millimetre or so from the mark, lean the chisel, then ever so gently, allow it to cut a thin line of scrap out up to the mark. This will create a narrow channel alongside the mark, just about wide enough for the saw blade. You can then run the saw down this channel to start cutting, and the saw will cut through the scrap with its edge up to the mark, removing exactly what you want and nothing more.

Marking up a dovetail requires an angle to be measured. It’s easier to calculate suitable angles as ratios rather than as degrees. That is, if you think of an angle as being a certain amount you move across for every amount you move up, then you can quickly create the angle you need by drawing a triangle. For dovetails in hard wood we wanted a ratio of 1:6. That is, we would go across the wood 1 unit for every 6 units we go into the wood. We got this angle by drawing a right-angled triangle with one side 12cm, the other side 2cm. Lining up a Bevel Square to this triangle gave a guide that can transpose this angle onto the wood.

(pencil lines added as the camera wouldn’t pick up the knife mark.)

The first dovetail we constructed was a Half Lap Joint. This joins two pieces of wood of the same thickness in the same plane. It works by cutting away half the thickness of each piece so that the two cuts together add up to the thickness of the original. We cut the tail first, then transferred its shape onto the other piece to mark out the pins. Here’s an important point. We traced the tail to mark out the pins rather than measuring them. This was so that any slight error we might’ve made in the angles of the tail would be matched in the pins, giving us a snug fit.

Chiselling looks like a nice way to get things precise. Ironically, it can be less accurate than sawing. A saw will generally remain straight once its cut has begun, whereas a chisel might readily slip and alter the angle of the surface. So, the trick is to get the saw cut as precise as possible and minimise the amount of chiselling necessary. Chiselling is needed around the pins however, since the saw cannot create a hollow in the wood. Chiselling also comes into its own in the internal corners in the wood, and here is where the most detail is required. The smallest of burrs in a corner will prevent the joint from coming together. A chisel can get these out and allow the pieces to fit flush.

One other strange thing about chisels: you don’t need to hit them with a mallet to make a cut. You can simply push the chisel and its super sharp end will do the cut for you. However… surprisingly, you can be much more accurate and precise by tapping with a mallet than pushing by hand. The pressure exerted by a mallet tap can be controlled carefully and is applied accurately down the length of the chisel, whereas pushing by hand can apply a variable pressure and the hand can wobble and twist as it pushes.

So, there is my first dovetail joint. Time to glue, and then onto the next.

This time we practiced a more traditional joint, suitable for connecting two pieces at right angles, and such as you would get in a box or a drawer. In this piece, the tail was marked out in the same way as for the half lap, except that it spanned the whole thickness of the wood. The pins were also cut the same way as for the half lap, except that it took a bit of mental processing to imagine the wood rotated 90 degrees, and it took a lot more nerve as the pins looked remarkably thin! Here is my second dovetail, just about cut and ready to be tidied up.

And with that, we ran out of time, and the day came to a close. I need a heck of a lot more practice before I’m ready to include anything like this in a piece of work, but I think the day has given me a good selection of new skills, a better appreciation of precision, and a lot of courage and enthusiasm to try more complex work.

I would very strongly recommend the courses at Sylva. No expertise is needed, there’s no competition or worry if things don’t work first time (or even second, third, fourth time. It’s all good practice.) The Wood Centre is worth a visit too, just to see some brilliant work and to support some local craftspeople. Oxfordshire Artweeks is approaching, and Sylva usually host open days throughout, so that might be a good time to pop in and see what goes on.

By nickcnickcnickc

I spend my working life staring at computer screens, so in my spare time I look for things to do with my hands, preferably involving wood. It's a little ironic then that I've now starting writing a blog about my woodworking, and thus introducing computer screens to my main hobby..!

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