The lathe bed supports the headstock, tailstock, and tool-rest assembly and is in turn supported by a stand. The earliest lathe beds were no more than two wood planks, and wood is still used on some modern lathes. Wood has much to recommend itself as a bed material-it is relatively inexpensive, readily available, absorbs vibration, and can yield a lathe of any desired length between centers. The springiness of a timber bed has shock absorbing characteristics unmatched by metal.
Starting in the 18th century, cast-iron lathe beds began to displace wood. Cast iron is a good bed material because it is stable and has excellent vibration-damping characteristics. The casting process allows beds of intricate design to be made. In an iron bed, each of the wood planks is replaced by a strip, or rib, which is called a “way”. The bed ways are typically 1 inch to 11/2 inch apart. A number of lathes today offer modular cast iron beds.Under this scheme, the standard package gives the turner a decent distance between centers, but additional sections (typically 12 inch to 20 inch long) can be added to achieve a bed of any reasonable length.
Although structural steel does not have the damping ability of cast iron, it makes a good, solid bed if the weldments are designed properly. Steel also makes longer beds possible at reasonable cost. You can even make a “stretched” structural-steel bed by obtaining lengths of matching steel fr om a steel fabricator.
I’ve seen two lathes with beds made fr om aluminum extrusions. Extrusions don’t have much to offer as a bed material for anything but a miniature lathe, so I would avoid them on a full sized lathe.
In a bed, you should look for rigidity and workmanship. The ways should have a smooth surface, and the distance between them should be constant. Make sure the tailstock and tool base slide easily but lock solid where you put them. Te st the truth of the bed by checking the alignment of the headstock and tailstock.