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A pair of this part together with two of the diameter 3 half circles (https://www.shapeways.com/model/1958865/rokenbok-half-circle-diameter-3.html) creates a bearing that can spin freely inside a 3x3 opening.
These designs are owned by Rokenbok Education (http://www.rokenbok.com) and published under license by kid*spark (http://www.kid-spark.org), a nonprofit corporation, for nonprofit and educational uses. Drawings of these designs are licensed under Creative Commons Attribution-NonCommercial-ShareAlike (BY-NC-SA) 3.0: http://creativecommons.org/licenses/by-nc-sa/3.0/ The drawings may be freely downloaded, modified, and used, providing that attribution is given to Rokenbok Education and the drawings are for non-commercial purposes. Any modified drawings are licensed under the same terms. The drawings are intended for educational and hobby uses only; 3D prints of these designs should not be considered toys or for use by children without adult supervision. The drawings are not guaranteed to print perfectly.