Mystery dovetails

Words and photos: Robert Howard

Many of us have seen these intriguing double or quadruple dovetails before, but to the uninitiated they can be a visual mystery.
The major difficulty is that, unlike a normal dovetail where the second half is cut from a tracing of the first half, here we can’t trace anything because the mating surfaces are angled and not flat on to each other (study the photos and imagine how you might try to trace one onto the other).

Here's how I did it

The photos below should give some clues as to the process, but here it is step by step:

1. Check that both blocks are the same size, and square.
2. Mark both shoulders with a cutting gauge, to whatever measurement looks good to you.
3. Mark the tails on two adjacent faces of one block (in my case the Huon pine).
4. Use a 45° mitre square to mark lines from the outer tips of each tail across the end of the block.
5. From where these lines cross the edges of the remaining two faces, mark two more sets of tails, using the sliding bevel on the same setting used for the first two sets of tails.
6. Saw close to the lines (but not touching them) in the waste down to the shoulders. Remove the waste, either with a coping saw or a chisel. Clean up the shoulders. Then very carefully pare back to the marked lines. If you make a mistake and pare beyond a line, you will have to pare the matching line on one of the other sides, and the internal surface connecting them, to the same new angle.
7. Now comes the tricky bit: marking the second half of the joint. If you have been able to pare exactly to all your lines on the tails, you can use the same measurements to mark out the pins.
If you think there are likely to be variations, then you need to use the cut tails to mark out the pins. Unfortunately, you can’t lay one face on the other and trace the pins from the tails, because they will be back to front. I did it this way: lay the blocks down end on end, in the positions they will take when the joint is assembled. All four faces will now be aligned in their final positions relative to each other. Imagine the tail block hinged to the pin block on the edge of the top face, and tip the tail block over on this hinge line until it lies on top of the pin block (take note of the position of the reference face and edge marks as the photos progress). Slide the tail block down until its top edge lines up with the shoulder line of the pin block. Carefully mark where the top corners of the tails meet the shoulder line (these points will be together when the joint is assembled).
8. Use the sliding bevel, set to the same slope as the matching surface of the pin you are copying, to mark from these points back to the end of the block. This outlines the shape of the pins on this face.
9. Repeat this procedure for the remaining three faces.
10. Join the points up by scribing the lines across the end of the block. Make sure you connect the correct pairs by checking their relationship with the face and edge reference marks.
11. Cut the pins using the same method as that used for the tails: saw, coping saw and/or chisel.
12. After the cutting is finished, sight across all the surfaces to see how flat they are. All the shoulder surfaces should be slightly undercut, and no surfaces should have any humps in them.
13. Line up your reference face and edge marks, and try sliding the joint together. Lay a piece of scrap wood on an upper face and try gently tapping them together with a fairly heavy hammer. Don’t overdo this. If they don’t go fully together, tap them apart, and examine the internal surfaces for pressure marks.
Check that you have previously cut to all your lines. Make some small paring adjustments and try the fit again. It is helpful to remember that none of the internal faces will be seen, but the outer edges will. So make corrections to internal surfaces, cutting where necessary up close to the edges but without actually reducing them. Only remove wood from where it will be seen when you are certain it is necessary, and even then take away less than you think you need to.
14. Eventually, you will get it together, and if you aren’t very happy with the result, my advice is to do it again.

This story first appeared in AWR#76. Robert Howard is a woodworker who also teaches weekly classes. Learn more at roberthoward.com