A Christmas ago I was slowly hand making 6 piece burr puzzles as gifts, but as the weekends past I needed a faster method. The router jig design I came up with enables accurate and fast reproduction of burr puzzles. Using accurate spacer blocks (made in shop), accurately milled square timber, a router and router table and a bit of planning you can produce a burr puzzle in minutes. The best website for plans and patterns of puzzles comes out of a IBM Research Group.
To understand how it works you need to know a little about the puzzles. 6-Piece Burr Puzzles are a popular wooden riddle made of 6 interlocking pieces which are moved in a certain order to allow assembly and disassembly.
Puzzle difficulty varies, some require one or moves to release a piece, whereas Bruce's Love Dozen requires 12 moves before the first piece is removed.
A standard piece can be thought of as 24 small cubes half the width of the piece. The interlocking piece is created by removing cubes in a certain pattern. (More advanced patterns remove single cubes which requires handwork, like a half blind dovetail.)
Each piece in the puzzle must be the same size, but sizes may vary. In my case pieces 20x20x60mm worked well. This made each of the little cubes 10x10x10mm. I recommend choosing a size twice the width of a good router bit. Take time with the milling, it needs to be accurate, or slightly under, to allow the pieces to move easily.
The jig's T shape design allows the piece to meet the router at 90 degrees by running along the router fence. Mine is constructed of scrap pine and plywood. Few of the exact measurements are important. The slot must be accurate to the width of your piece so there is no wobble. The router piece must protrude from the table far enough to remove the cube section in one pass. In my case 10mm above the ply. The slot is produced on your first pass.
You can see how the cube lines up in the slot and is carefully removed by the router. By using various spacers (seen here as the grey block) you can repeatedly remove the various sections. (I used a dozuki saw, sander and caliper to make my spacers. A well tuned shooting board would also work.)
Once you have your jig and pattern the process is rather easy.
- Lift the router to a height where a single pass will remove half the width of your piece.
- Select a side to start with.
- Select the correct spacer to remove required row (2, 3, 4, or 5.)
- Firm against your fence push the jig through the router (careful not have hands behind the jig, for improved safety and accuracy I used another piece (seen in blue) to hold the piece down and towards the fence.)
- Replace the spacer if another row is needed to be removed, repeat as need.
- Rotate to the next side, and repeat steps 3-6 as needed.
- A little sanding, a little oil, and you're done.
As an upgrade you could make multiple slots allowing multiple pieces to be cut in one pass.