This thing is a part of Contraptor - a DIY open source construction set for building Cartesian robots: http://contraptor.orgLinear bearings ride on linear rail http://www.thingiverse.com/thing:1071, driven by belts or leadscrews. A pair of rails and at least a pair of linear bearings are needed to make a complete linear stage. There are three sizes of the linear bearing: 3.5", 2.5" and 1". The part list is for 3.5" size.Sketchup source: http://sketchup.google.com/3dwarehouse/search?q=contraptor+linear-bearingLinear bearing is basically half of aluminum rectangular tube, with R3 bearings mounted on 10-24 screws. The tube pieces are made using SVG templates. There is a 1/16" gap between bearings and the base wall. For 1" size, 1.5" square tube is used.Detailed howto: http://www.contraptor.org/make-linear-bearings