Getting the Y axis belt tight enough can be a real pain. That's why I've designed this printable upgrade to allow increasing the tension on the Y axis belt / pulley by turning a screw.
You can find my X axis belt tensioner here http://www.thingiverse.com/thing:1673
V1 - Based upon a single captive nut.
V2 - Fixing an alignment issue.
V3 - First printable version. The smaller piece that holds the captive nut was too long.
V6 - Same concept, but a different design using two captive nuts on each side of the Y axis pulley. My test print is suffering from some X stage problem, so the prints aren\'t coming out right yet. However, I wanted to post this in case I could get feedback from others.
V7 - Same concept with a few improvements.
I created the screenshot from Unrepentantgeek\\'s Sketchup version of the Cupcake CNC. http://www.thingiverse.com/thing:1449
There are two pieces in this one STL / Sketchup file.
The smaller piece should fit around the hex nut under the Y axis pulley. The second piece should fit around the edge of the Y axis.
Drop two M3 nut into the slots in the smaller piece that fits around the Y axis pulley.
Insert two M3x16 bolts.