Doors and auxiliary axes on machine tools need different linear motion components than spindles.
Doors, in particular, are an axis that are often designed with less-than-ideal motion components. From an accuracy standpoint, door motion may seem like an open and shut case, but they do present some challenging design issues of their own. They tend to be large sheet-metal structures that are subject to deflection and alignment problems. They also have to work reliably in a contamination-prone environment filled with metal chips and cutting fluids.