Workpiece accuracy criterion for the dynamic machine tool acceptance test

Abstract

A BASIC manufacturing system may be considered to be composed of a manufacturing process and a machine tool which includes a structure and cutting tool. The output of such a manufacturing system is a machined surface. The purpose of the machine tool is to provide the kinematic means, the mechanical positioning and the necessary energy to produce the geometric shape and dimensions required. However, instantaneous position errors between the cutting tool and surface due to elastic and thermal deformations, vibration, geometric errors, tool wear, etc., greatly influence the resultant accuracy of the manufactured workpiece. This investigation treats the surface accuracies of turned parts produced on a lathe. In this case the distortions from the desired cylindrical shape may be classified into macrogeometrical and microgeometrical surface deviations. The macrogeometrical deviations may be further divided into three subcategories. The first-order macrogeometrical deviations are depicted by the shapes illustrated in Fig. 1. The physical reason for this type of deviation from roundness is a result of geometrical errors of the machine and the elastic deformations from the applied cutting forces. The surface waviness represents the second-order macrogeometrical errors and results from relative motions which take place between the tool and the workpiece in the direction normal to the formed surface. The frequencies contained in waviness distortions are at least one order of magnitude higher than the first-order distortions and, as this investigation shows, lie in the region of the natural frequencies of the machine tool structure. Figure 2 illustrates the waviness distortion of a turned part. The tool track as shown in Fig. 3, shapes the cylindrical form of a turned part and is also a macrogeometrical deviation. This error type, which is strongly dependent on the geometrical form of the cutting tool, should be properly considered as a third-order macrogeometrical deviation. Surface roughness represents the microgeometrical variations of a workpiece from its desired shape. Surface roughness can be identified from its random frequency content which is superimposed on the various macrogeometrical errors. The geometrical deviations resulting from surface roughness contributions are due mainly to the physics of the cutting process and tool wear, not significantly to the machine and structure. The first-order and second-order macrogeometrical errors reflect respectively the static and dynamic behaviour of the machine tool. The present investigation treats these two error forms under the assumption that the system output--a machined cylindrical surface-may be related to the instability boundary of the machine under certain cutting conditions.