Dynamic Acceptance Tests for Horizontal Milling Machines Based on a Statistical Theory of Machine Tool Chatter

Abstract

Machine tool chatter is essentially a statistical phenomenon. The dynamic characteristics of machine tools as well as the dynamic coefficients of the cutting process are statistical quantities which show scatter when determined in repeated tests. They can therefore be specified only in terms of mean values with confidence limits. The statistical theory of chatter developed in this paper deals with the scatter of dynamic data of the machine structure but assumes that the dynamic cutting coefficients have discreet values. This is used for analyzing two series of tests carried out on horizontal milling machines. In the first series a particular machine was tested ten times and from the results thus obtained the mean threshold of stability with its confidence limits was established. In the second series a batch of the same type of machines taken at random were analyzed in a similar manner, with the aim of establishing “type behavior”. In the second series of tests large scatter in the dynamic characteristics was found, causing a large scatter in predicted cutting performance. Thus, machines which have the same specification and which to all appearances are identical may have widely varying resistance to chatter. It is recommended that machines should be “graded” on the basis of the dynamic acceptance tests evolved so as to ensure that the user will obtain “value for money”.