A general purpose thermal error compensation system for CNC machine tools

Thermal effects cause the majority of machining errors on many types of machine tool, with linear expansion and distortion of the structural elements causing unwanted movement between the tool and workpiece. Heat inputs that cause temperature elevation and gradients come from many sources internal and external to the machine tool and make thermal errors difficult to control without some form of compensation. Many thermal error modelling and compensation systems have been proposed which use neural networks, multi-regression analysis, heat modelling or probing techniques. However, each method suffers from one or more major drawbacks that limits its effectiveness when used in a practical machining environment. One feature of all the thermal error compensation techniques is their lack of flexibility that makes them difficult to apply to more than one machine type in a timely and cost-effective way. This paper describes a combined thermal and geometric error compensation system with a flexible structure that is general purpose in its application to any machine tool. The system can accept input from any number of temperature sensors. Information from the temperature sensors is acted upon by a novel programming language based model that estimates thermal movement and directs error components to a number of outputs for compensation by axis position modification. The entire compensation system can be applied either in a stand-alone computer that accepts a wide range of feedback signal types, or integrated into an open architecture machine controller. The system allows the management of temporary or permanent input failures and displays every thermal error component as an aid to fault diagnosis. Both position independent and position dependent thermal errors can be reduced through compensation. The system has been applied to several machine tools.