A New Method to Measure Runout of Motorized End-Milling Spindles at Very High Speed Rotations

摘要

The error motion of a machine tool spindle directly affects the dimensional accuracy of its machined parts. Spindle runout is a simplified indication for the spindle error motion. Traditional runout measurement techniques usually are applicable only when the spindle is at rest or at low rotational speeds. At very high speeds, such as 25,000 RPM or above, there are several factors, such as airflow, vibration, and difficulties in sensor fixturing, that could prevent proper measurements of the spindle runout. This paper introduces a new, practical method that can measure spindle runout at very high speed rotation. The spindle runout is determined by cutting marks produced by an end mill on a workpiece. The cutting marks are circular and their diameters are related to the spindle runout. The increase in diameters of these cutting marks at high speeds, with respect to the one by hand rotation, is related to the runouts at those speeds. Quantitative precision analysis was carried out to confirm the accuracy, repeatability, and resolution of this new runout measurement technique. This technique was applied to determine the quantitative relationships between the spindle runout and the spindle speed, the level of unbalanced, and the front bearing preload. The results confirm that the spindle runout increases as the spindle speed increases mainly due to the centrifugal forces generated by the unbalanced mass, Which is consistent with the prediction by a comprehensive spindle thermal-mechanical-dynamic model previously developed by the authors. One way to compensate for the effect of unbalanced mass is by increasing the spindle stiffness. Experimental results confirm that a higher front bearing preload can render the spindle stiffer, thus reducing the spindle runout. This new and practical technique for spindle runout measurement is an effective tool for verifying actual running accuracy of spindles at their actual operating speeds, which is highly valuable in quality assurance.