Tighten your Ultimaker belts Parametric Design Slim, fits in small space of Y belts Adjustable tightening (via cable tie) I have printed and fitted 4 on my machine todayTry this one http://www.thingiverse.com/thing:24336 , It's better.Print, they are small so use thin layers The Scad file and STL file is small size for Y belts You can increase height of Scad file for X belts Place over belt underneath at opposite end to the block Place flat side up for the Y belts Rap 2.5mm cable tie around and tighten, make sure block part of cable tie is not on the flat side or it will scrape Cut end off cable tie Move belts by hand and make sure nothing is scraping Start printing round circles again