Efficient offline thermal error modelling strategy for accurate thermal behaviour assessment of the machine tool.

Abstract

Positional accuracy of the machine tool is affected by the instabilities caused by the thermal gradients produced from the internal and external heat sources. Thermal gradients cause linear and non linear thermal expansions of the complex machine parts which result in the generation of the positional error between the tool and the work piece affecting the machining accuracy. Thermal gradients due to internally generated heat and varying environmental conditions pass through structural linkages and mechanical joints where the roughness and form of the contacting surfaces act as resistance to thermal flow and affect the heat transfer coefficients. Measurement of long term thermal behaviour and associated thermal deformations in the machine structure is a time consuming procedure and most often requires machine downtime and is therefore considered a dominant issue for this type of activity, whether for characterisation or correction. This paper presents the continuation of the efficient offline technique using Finite Element Analysis (FEA) to simulate the combined effects of the internal and external heat sources on a small vertical milling machine (VMC). The complete simplified CAD models of the machine were created and used to simulate the thermal behaviour of the machine structure by using the evaluated experimental data. The FEA simulated results obtained are in close correlation with the obtained experimental results which enables the offline thermal assessments for short and long term thermal behaviour and the extraction of the nodal thermal information for the development and enhancements of robust thermal compensation models