In the year 1806, Terry signed the Porter contract to produce 4,000 wooden clock movements (other shops would make the cases). According to historian Diana Muir writing in "Reflections in Bullough's Pond", at that time a skilled craftsman could produce six to ten clocks per year. In the third year he produced 3,000 wooden clocks. He sold his manufactory to two of his assistants Seth Thomas and Silas Hoadley and retreated to his workshop to create the first machine in the world to be mass-produced using interchangeable parts.