Hi Daid, I think you got the "rounded square" thing, it's not much like a square but I think that may be the issue.
Is there any way than printing circles to see if it's an issue related to belt tensions??
I have printed 3 sets of tensioners but I need an advice on what to do with my current situation, please see below my experiments and results:
http://www.thingiverse.com/thing:19892
Good tensioner but doesn't work on the Y belts, it blocks between the top wood and the belt block...shame :(
http://www.thingiverse.com/thing:19818
Interesting design but difficult to regulate properly. Easy slides back during machine operation, creating random uneven tensions across the belts.
http://www.thingiverse.com/thing:10082
Basic design but clever, I'm currently using this one on all belts but I see that they have default uneven tensions. Any ideas? Can't put more than one per belt or it will prevent the axes from reaching the end-stops.