When you drive the top cover, the center hole can open or close gradually. The center hole is diameter 50mm. To assemble the parts togther,some M2.5X6 screws are needed.Parts are designed for FDM 3d printing.And you may need to sand some parts due to the 3d printing quality. STEP format files are included.Ignore the FBX,STL and OBJ format files, they are only for the platform's requirement.