In the history of machine tools, spindles have been very good relative to other bearings and structures on the machine. So quality professionals have developed a cache of tools—-ball bars, grid encoders displacement lasers, etc.—-to help them characterize and understand the geometry of the structural loop. But as machine tools have improved in their capability and precision, and the demands of part-geometry and surface finish have become more critical, errors in spindles have become a larger percentage of the total error.