Determination of the Dynamic Characteristics of Machine Tool Structures

Abstract

The ‘operative harmonic response locus’ of a machine tool structure, which is required for the prediction of machining stability, must be determined by vibration tests carried out under conditions which simulate closely those arising during cutting. This requires that such tests be performed while the tool or the workpiece spindle is rotating, the vibrations being excited by a small harmonic force superimposed on a large constant pre-load. It is not possible to satisfy these conditions with vibrators working on electro-dynamic or electro-hydraulic principles and so an electro-magnetic vibrator was designed and developed to meet the above requirements (2). This excitor can be used with all types of machine tools, but in the present investigation its application is restricted to lathe and horizontal milling machines. The effects of the rotational speed of the lathe spindle and milling machine spindle, of the exciting force amplitude and of the pre-load magnitude and direction on the response data, are investigated. It is found that the lathe structure behaves in a linear fashion whereas the behaviour of the milling machine is non-linear.