Following the war, Hammond moved to Detroit, where he was chief engineer for the Gray Motor Company, a manufacturer of marine engines. A partner in the company, Col. John H. Poole, with whom he had served in France, knew of his engineering skills, and paid him an extra $300 a week under the table to stay with Gray Motor. In 1919, he invented a silent spring-driven clock. This invention brought him enough money to leave Gray Motor Company and rent his own space in New York City.