Hi Craig - My search for small harmonic drives continues. The planatary I'm looking at has 30arc min backlash (from stepperonline, a couple of years ago they used to be 10 arc min). I'm going to use this on Lanky's Z as a test as I have them in stock. I have had 5:1 and 10:1 on a small router X&Y belt driven and was quite happy. As soon as I find suitable harmonics then that's the future. Working on Lanky's Z axis at the moment.. Trying to use up some old parts!! Peter