Probably the most essential machine in the general machine-shop is the lathe. It is used for turning all cylindrical work, both straight and tapered; for boring and thread-cutting, both internal and external; and for turning spherical work or parts of spheres by means of special devices. Most of the other machines used in the machine-shop have been evolved from the lathe. Machine-shop lathes have been perfected and specialized during recent years, until there are now many forms of this machine. They may be classified into: (1) turning, (2) engine, (3) speed, and (4) turret lathes.