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 mother-child rivets are needed，and you need to prepare these metal parts yourself.Parts are designed for FDM 3d printing(PLA material). You may need to sand some parts due to the 3d printing quality.And you can add some glue as needed too.The mother-child rivet specfication is included in the images. You can buy them by yourself.Ignore the FBX,AMF，3mf and OBJ format files, they are only for the platform's requirement.