It is a necking mechanics or shrink mouth mechanism. Parts are designed for 3d printing. The center hole's diameter is 25mm. The outer gear's module is 2, and pressure angle is 20 degree. You can get another gear to drive the mechanism with a motor(a 3d model of small gear is included in the attachment too).You need to get some M2 screws to assemble the parts together. If you need a similar mechanism that can be used in industrial machines, we can design that too.STEP files are included in the attachment. Ignore the FBX,STL and OBJ format files, they are only used to meet the platform's requirement.If you need a similar design plan for CNC machining, visit: https://www.cgtrader.com/3d-print-models/science/engineering/d50nma01-pipe-necking-mechanism-for-cnc-machining-metal-part