Gap or extension-type lathes are similar to toolroom lathes except that gap lathes can be adjusted to machine larger diameter and longer workplaces The operator can increase the swing by moving the bed a distance from the headstock, which is usually one or two feet.

In a gap bed lathe, the gap is provided just below the nose of the headstock spindle.

Cast iron is commonly used for machinery housings or base due to its damping characteristics. It is also known for holding its shape when it is subjected to contraction and expansion due to temperature fluctuations. Hence lathe bed is usually made up of cast iron.