Played with the bearing a bit and added tiewrap to put some outward tension on the belt and its way better now. Not elegant but ok for now.
Just watched the vid, there’s too much slack in the belt, you should have more tension on it so that it can’t flop around.
I also use tie-wraps instead of the tension blocks to apply tension. But I’ll pull it taught and then zip tie it.
@Tim_Sills Thanks. I’ll adjust that.
Have it running a 3x calibration angle (http://www.thingiverse.com/thing:53668) over the full width. will see how it goes.
your experiments are fine, mine are made from bits of IKEA left overs, cheap rails from china, lotsa manual approximated adjustments and badly cut wood bits lol. (http://3roomlabs.blogspot.sg/2013/04/049b-y-gantry-platform.html)