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Fixture for mitred through dovetails

A mitred through dovetail looks like this …

For years I have simply sawn the mitre freehand, which tended to require a little paring to achieve the accuracy/tolerances for a tight fit. The mitre complicates this dovetailed joint by adding a third dimension – it is not enough that the pin and tails must join without a gap, but the mitre must as well. A mis-fit with one will create a mis-fit with the other.

I recently began building a toolbox which will require a total of 24 mitred corners and a grand total of 120 through dovetails. After the first two mitred corners, I decided that this was crazy, and that it should be possible to speed up the build with a fixture. Up until this point I was sawing them freehand - reasonably accurately, but still needing some paring to finish.

To understand what is needed, this is the layout for sawing the through dovetails.

Saw all the tails except the ends, as below. This is where the mitre will go ...



Now reverse the board and mark off the mitre at the ends ...



Saw the outside of the tail at a diagonal to the top of the board (not through the board as with the other tails).

This is the fixture, a guide to saw the mitre ...

With a Japanese saw, which has minimal set, I have not needed to adjust for a saw kerf …

The desired result ...



If necessary, the guide helps fine tune with paring ...



At first I simply held the guide against the top of the board, but it would move, and this affected accuracy. I attempted to clamp the fixture but this was impractical. In the end I built in a clamp, taking the screw from my box of saved "precious parts that never get thrown out" (don't you do this as well?) ...





This is the fit off the saw. Very pleased with the result ...



Regards from Perth

Derek

March 2024