I talk dovetails with a lot of woodworkers. When I teach this subject, I notice that almost everyone makes the same common mistake. In the shop this week, I made the same mistake. As a result, I thought I’d write a “what to look for when your dovetails don’t fit as you planned” blog post. If you have this show up when you’re dovetailing, you’ll know what to check.
Far and away, I think the no. 1 problem in half-blind dovetails is the small gap where the tails fit into the sockets. In the pair of photos above, you see exactly what I mean. There’s a small gap at the top and that gap also shows at the front where the tails meet the socket. We know that this is not a layout problem because when the pins are transferred to the tail board, the drawer front (pin board) sits on or at the scribed line of the tail board. Also, if layout were wrong, the gap should be even along the entire front. Why, then, is there a gap?
In the left hand photo, you see where the problem is found – in the socket that the gap appears. The problem is the floor of the gap, as it’s seen laying on the bench, is not level from front to back. The floor actually slopes upward. What happens then is that you start the tails into the sockets and the fit is great, but as you drive the tail board farther in, the slope takes over and the board begins to push out away from the front of the socket. That also produces the small gap at the top.
If you’re sockets both had floors that were not level, then the entire tail board would be pushed back. Because the lower socket correctly fits, you know the problem is in the one socket.
The fix to bring this socket into shape is to pair away a bit of the floor so it slopes level to downward – I aim for downward, which is easier to hit than dead-level when working. Plus, it’s OK to have a downward-sloping socket because the glue surface at that point is end grain (tail board) to flat grain (pin board). While there is a small amount of strength at that spot, it’s not where the dovetail gains its best holding power.
Of course, it’s better to make sure you have the floors sloping prior to putting together the joint. The easiest way to do that is to hold the workpiece up to your eyes and level the piece looking down the board’s upper face. When that surface is level, check the floor for the proper slope. The reason you want to make sure of your floor prior to assembling the joint is that when the joint goes together with an ill-fitting socket, the tails board and socket are slightly misshapen as the two halves are driven tight. After that, the dovetail joint will never look as good (see below) as it would when the fit is perfect.
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