Engage the workpiece with an indicating device and observe the movement of the needle as the part is rotated. Note that the difference between maximum and minimum size, whether ovality or TIR, is not roundness. Measuring roundness requires a roundness or formtesting machine. Measuring minimum and maximum sizes at various locations can approximate roundness, but actually provides a measure of ovality. Early machines used a simple chart recorder to graph roundness.
Current machines use dedicated processors or computers to monitor and analyze the gage head signal.
Modern electronics have made the inspection process simple and comprehensive, but the heart of a roundness machine remains in the precision spindle. Roundness errors in the spindle have the greatest influence on the measurement results. Most equipment manufacturers offer roundness machines with spindle deviations less than 0.000003 in. Spindles can run on either air or mechanical bearings. Air-bearing spindles are typically less expensive than mechanical bearings in the same accuracy range but require connection to an air supply and regular maintenance.