Geometric and Thermal Characterization of a Machining Center Under Dry and Wet Conditions

Quantification of geometric and thermal characteristics of machinery is critical to the improvements in part dimensional accuracy and reduction of part to part dimensional variations in a high volume manufacturing operations. Assembly and alignment of different components in a machine result in geometric error over the machining volume of a machine. These errors, once quantified, can be corrected through offsets in positioning controls. The objectives of a good machine design should be to minimize the geometric errors during fabrication and assembly of the components, and replacement of the wear prone components during maintenance of the machine in operations. Thermal errors in machines are even more critical and have not been addressed sufficiently in improving part to part dimensional variations. Heat sources, such as motors, slides, and the machining operation have not been given due importance at the machine design stage even though it is understood that temperature gradients will be generated during the operation of a machine. These temperature gradients cause thermal growth in machine components, which in a typical manner are controlled through the use of coolants. It is not uncommon to see the use of chillers, which in turn control the temperature of coolants. These accessories contribute to the low reliability and low up time of the machining process. This paper describes a quick procedure for the geometric and thermal error characterization of machinery. The procedure is applied to quantify errors between the tool tip and the part, under dry and wet conditions. These errors are verified through actual machining and dimensional measurements of a test part. Contrary to expectations, the results revealed large temperature gradients when characterization was done under wet conditions as compared to the characterization under dry conditions. A thorough understanding of the thermal gradients in existing machinery will lead to improved designs in new machinery which will reduce the warm up time required in machinery, and provide for reduced dimensional variations in machining parts.