Taiji mechanism(iris mechanism) designed for 3d printing. It is best to use resin material when 3d printing. To assemble the parts togther,some M2 screws are needed. When you rotate the “Drive Part”or the middle plate, the center hole can open or close gradually. The center hole is diameter 20mm, the outer size is diameter 54mm and thickness is 8.2mm. What you get is STEP file. Video on Youtube.