The repeat accuracy of such a simple mechanism is absolutely amazing. Never in my life would I have guessed that you could make some many reversals without some accumulating error somewhere.
Check out the middle of the last picture on the page. It looks like it didn't go up as far as it needed two about twice in the same line, causing a noticeable gap. Not at all trying to knock it here, amazing work for sure, just wanted to point that out. It could be intentional as well.
Most of the motion is in the up and down, which I'd guess is why you get some gaps forming between the "layers", but it still seems to be lined up OK in the angular dimension.
This bites pretty hard on hektor, a graffiti robot:
http://hektor.ch
I met the kid who built this, year ago. He is a seriously rad ex-demo scene hacker, now art school prof. Lots of other good projects there worth checking out.
It's for hardware and electronics hackers (and programmers, of course). A good book on things like this is Programming Interactivity at http://www.sparkfun.com/products/9415 (that's the kind of thing you'll find on HT).
The use of polar coordinates is interesting, and it makes the images look a lot more interesting than a simple X/Y. (Probably less mechanical strain too.)
