Constructed with heavy duty metal and plastic Dial to show rotation Unit is open system to study and sandwiched by two clear plastic plates. Nice bench top or desk top item Includes: (1) unit This demonstration shows the fundamentals of how a worm gear works . The system has a hand crank. Perfect for the classroom. Constructed with heavy duty metal and plastic. Unit is open system to study and sandwiched by two clear plastic plates. Nice bench top or desk top item