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 Interlocking Puzzle in aluminium 
This is I think a great little project for beginners. It was my first try to cut aluminium, and it sorted out well. 
I found the plans for this puzzle and many other interesting links related to interlocking puzzles) on  Tom Almy's Web site. I just adapted a little the size to metric values (Yes, I'm metric impaired, as all Frenchmen are)