Monday 9 July 2018
This dovetail alignment board by David Barron is an amalgamation of designs, combining versatility with accuracy
Cutting accurate dovetails by hand is not easy, but I have developed a number of aids and techniques which help greatly with this difficult task. My alignment board is designed to make the marking out of the pins from the tails accurate and stress free.
The conventional way of doing this is by balancing the tail board on a bench plane, something that is far from easy as the board needs to be positioned in three directions at the same time – flush with the sides, flush with the front and square. If the board moves during the marking process it is virtually impossible to reposition to make the remaining marks.
A better option is the method suggested by Christian Becksvoort where the groove for the drawer bottom is cut in both pieces and a piece of tight-fitting scrap is inserted. This aligns the sides of the boards but still requires the adjustment for flushing the front as well as keeping everything square.
A better method is suggested by Robert Ingham in his book Cutting-edge Cabinetmaking and involves a jig which holds both pieces firm and keeps them square.
I have used his jig and it works very well, although it is designed for a specific width of stock which he uses in his small dovetailed boxes.
Marc Fish showed us his version of Robert's jig in F&C 164. This copes with varying-sized stock and looks as if it works very well. He also incorporates an offset for marking the pins which allows you to place the saw in the knife line rather than next to it, and I would be interested to see this in action.
My own solution combines aspects of both jigs in a simple but very effective aid. By using a fence to work from the bottom edge I can combine the versatility of the Becksvoort method with the accuracy of the Ingham jig.
The pin board is pressed against the fence and then held in a vice to allow the tail board to move backwards and forwards against the fence of the other leg.
This keeps the bottom edge of both pieces in perfect alignment as well as the tail board perfectly square with the pin board, so all you need to worry about is positioning the tail board flush to mark out the pins.
The tail board can be removed halfway through marking and repositioned with no loss of accuracy on the remaining knife marks. The ability to do this serves no practical purpose but does demonstrate the control you now have over the whole marking operation.
With all these references to accuracy it comes as no surprise that the board itself needs to be made spot on. The wood must be bone dry and ideally quarter sawn to minimise movement. It may be tempting to use two contrasting species to highlight the dovetails but it is much better to use the continuous grain of one board for maximum stability.
The dimensioning needs to be as square and perfect as possible, indicating use of a large good-quality square.
The best way to join the two pieces is with through dovetails. The cutting gauge will reference off two perfectly square ends and the knife lines will maintain the 90 degree angle.
As long as the joint pulls tight things should remain square and the dovetails should stand up to workshop bumps and knocks.
In a perfect world it would be nice to have a dovetail board to make your dovetail board, but the width of the boards makes manual alignment easier if you are careful.
Cut the dovetails by hand or use a magnetic guide which cuts squarely and at a consistent angle, then line the pin board up in the vice flush with the work surface and at 90°. Use a square to line up the edges of both boards and keep things at 90 degrees to the work surface.
After marking and cutting out the pins, check for a good fit and assemble the joint. Make sure the joint has gone fully together by holding it up to the light and looking for gaps, then check it for squareness on the inside of the joint â€“ the slight protrusion of the tails and pins would not give a true reading on the outside. It is then on to the board assembly.
After the glue has dried the dovetails can be cleaned up. It is best to use a sharp plane and good technique to keep the surfaces level and straight. It may be tempting to use a random orbital sander but this will inevitably result in a rounded edge and a domed surface, with resulting loss of accuracy in use.
The next thing to check is the angle of the sides on which the fence is to be attached. This is the most crucial of all and must be exactly 90° if you are to avoid any twist in the marked joint.
If you have been accurate up to this point you should find that the angle is spot on. If not some careful trimming with a block plane should correct matters.
The fence is sized to protrude 5mm which is enough for a firm hold as well as coping with thin stock.
The corners are mitred and trimmed to a perfect fit on the shooting board. Both pieces are left over length and cut and trimmed to match the ends of the board. This is much easier than working the mitres to perfect length. The fences are screwed rather than glued so that they can be removed if the angles need adjusting in the future. To finish I apply four coats of Osmo Hard Wax Oil, remembering to do the same on both sides to equalise moisture movement.
When I demonstrate this jig, I can see light bulbs switching on among the audience. It is such a simple device, but if accurately made will work really well.