I mentioned a few days back the need to find the center of a rod. I ended up buying the Groz tool for this, and it works--but I confess, my experiences so far suggest that this approach to finding the center of a round rod is at best accurate to hundredths of inch. It is definitely inferior to using a lathe--but then again, I did not have that option.
When I bought this tool at Woodcrafters here in Boise, they salesman mentioned that it works on a lot of shapes besides round. And sure enough, it does. I am rebuilding some wind chimes for my wife. Wind chimes up here have to be very strong to survive our weather conditions, so I am rebuilding these sets with aluminum replacing the wood, and galvanized steel picture cord replacing the twine. The center finder works pretty well for finding the exact center of a square or rectangle, as you might expect, and in the same way: put it in the corner, scratch a line along the diagonal, turn it 90 degrees, and repeat. Where the lines cross is the center point--at least, accurately enough for wind chime work.