Words and photos: Hamish Hill
The first time I encountered this simple joint was in a table made of 600 year old Spanish oak. The table consisted of one solid slab of oak, probably over two metres long and over a metre wide. Fitted to it, using large sliding dovetails, were three ‘lumps’ of oak probably about 170 x 70mm. Attached to the two pieces of timber closest to the ends were some simple wrought iron legs with wrought iron braces angled up to the central lump. The reason for describing the timber used for this table as slabs and lumps is because that’s what it was, and despite travelling all the way from the dry Spanish countryside to a humid Queensland climate the table remained a reasonably flat, very useable kitchen table. The top was able to expand and contract as much as it wanted across the grain, and was well restricted from warping, cupping or buckling by the joins which held it together.
I now utilise the sliding dovetail whenever possible and practical. Often they are better than a conventional open mortice and tenon or for any joint which needs to allow the timber to expand and contract, but remain flat. With some simple routing techniques and some basic guides, producing classy sliding dovetails is almost foolproof and pretty quick.
I always cut the slot or mortice section first. For small delicate joints up to about 10mm deep, one pass with the dovetail bit will suffice. For deeper joints it’s best to cut the dovetail slot in two passes. First cut with a straight bit not quite as wide as the narrowest part of your dovetail bit and to a depth just a millimetre or so less than the eventual dovetail. The second pass with the dovetail bit should then produce a very clean dovetail section. Two passes are particularly important if you want to cut to a substantial depth. If your dovetail will be wider than the width of your router bit it should be fairly simple to then widen the slot, remembering to run your router bit against the direction of cut.
For a simple joint attaching a stretcher of timber across the face of a wide board (see photo left) you need a device to guide your router across the board without it deviating. I generally use two straight-sided pieces of plywood or chipboard joined parallel, with screwed and glued pieces attached across both ends. These hold the ply solidly and exactly the width of my router base apart. A guide of this type is quick and simple to make and its only drawback is a tendency for dust to clog between the router base and plywood guides. A vacuum or dust extraction system can help alleviate this problem. There are also some extremely useful and well-designed, guide systems available for straight cutting with a router, which certainly make the operation easier. Alternatively I know several people who would be happy fitting the circular guide plate to the bottom of their router and just running it along a straight edge.
Above: Router fitting between two guide rails of chipboard jig after cutting groove. sash clamps hold the workpiece, waste strips and jig firmly together while all is clamped solidly to the bench.
Having cut the slot, the hard part is creating a tail that will slide in neatly without sticking fast half way or wobbling around loosely. The easiest way to cut most tails is with a bench-mounted router and an adjustable fence. The adjustments on the fence should be extremely fine, as the final fitting of a dovetail requires some very fine adjustments.
When machining a joint like this I always have several scrap pieces of timber machined to the same thickness as those I plan to use—this allows me to run test pieces as I go, ensuring I don’t remove too much material at any time. For the depth of cut for the tail, I recommend you set up your router to cut a tail that is between a ‘pumpteenth’ and a 0.25mm shorter than the depth of the slot. The shoulders should fit tightly for the sake of stability and appearance. Remove a small amount of timber at a time with the bit protruding the full depth required from the table and less than half its total width from the fence. Follow with a block of timber to reduce breakout. Generally the tail is in the centre of the timber you are machining so it’s simple enough to turn your timber around and machine one side straight after the other until you are very close to the required width.
Above: Router bench set up with featherboard to cut one side of tail in jarrah workpiece.
Always machine your test piece first to eliminate problems. When reducing the final tenths of a millimetre, keep checking your test piece in the slot to see when it fits. It’s easy to take off too much timber without realising—if your test piece almost fits just run it past the router again without adjusting the fence but holding it firmly, sometimes that’s all it needs. If you find the tail is half way in but not keen on going further, I find using a clamp to help ease it in generally works well. Don’t hit the wood with a mallet or hammer as you may find it impossible to adjust the joint further, and you might damage the timber as well.
A problem frequently encountered when trying to pass long or large pieces of timber across the router bench, particularly when they are being held vertically, is maintaining stability and moving them smoothly. There are many aids you can produce, particularly useful are finger boards clamped firmly to the bench top, extended fences, even push blocks composed of right angles to which you can clamp the timber you are machining.
The production of sliding dovetails with a router can be very quick and easy once you have cut a few. If you haven’t tried them before get some scrap timber and experiment. Always ensure at least some part of the piece you are working on is firmly held in place. The timber and guides should be solidly clamped and the router held with two hands. Alternatively the router should be well fixed in the table or bench and the timber firmly held in the hands, using push sticks, finger boards and guides where ever possible.
• Often when using a router you will experience break-out or chipping at both the entry point and the exit point—clamping waste strips on either side of the slot or section you are routing can help here.
• Whatever the job, you can never use too many clamps!
• When fitting bits in the router, put them all the way into the collet, then withdraw them a fraction before tightening. If the bit widens towards the cutting end there is a danger of just gripping the shoulder of the widening part causing the bit to soon come free. (The collet is the equivalent of a drill chuck, but as a parallel-sided sleeve it needs to grip the entire length of the router bit shank.)
Hamish Hill's story is reprinted from Australian Wood Review magazine issue 34. Hamish is a furniture designer/maker, learn more about him here
An oak garden bench made by Hamish Hill. sliding dovetails join the front legs to the arms, the seat slabs to the rails and the back to the legs. Photo: Terence Bogue.