This thing is a part of Contraptor - a DIY open source construction set for building Cartesian robots: http://contraptor.orgLinear rail comes in several lengths: 6", 12", 18", 24" etc. Attached DXF file and part list are for 12" long linear rail. The rail can be attached to perforated angle, channel or T-slot profile, such as 80/20 Fractional 10 series. A pair of rails and at least a pair of linear bearings http://www.thingiverse.com/thing:1075 are needed to make a complete linear stage.Sketchup source: http://sketchup.google.com/3dwarehouse/search?q=contraptor+linear-railLinear rail is assembled from perforated aluminum channel and a pair of 1/4" drill rods, placed on square nuts and held by flat head screws. Perforated aluminum channel can be made in a way similar to that of perforated angle.Detailed howto: http://www.contraptor.org/make-linear-rail