The bed is the main part of the lathe provides the foundation for the whole machine and holds the headstock, tailstock and carriage in alignment. The surfaces of the bed that are finely machined – and upon which the carriage and tailstock slide – are known as “ways”.
Some beds have a gap near the headstock to allow extra-large diameters to be turned. Sometimes the gap is formed by the machined ways stopping short of the headstock, sometimes by a piece of bed that can be unbolted, removed–and lost.
Some very large lathes have a sliding bed where the upper part, on which the carriage and tailstock sit, can be slid along a separate lower part – and so make the gap correspondingly larger or smaller.
Cast iron is used as the “material of choice” for many machinery housings or bases because it is extremely stable in its structure. Certainly it is subject to thermal expansion and contraction, but the crystal structure of cast iron makes it “hold its shape” in applications ranging from machines and machine parts to cookware,ie, keep stability. Cast iron also suppresses high frequencies generated during machining. Note that the “ways” (the parts of the bed that the carriage slides on), are hardened and machined to provide lasting and accurate surfaces.