Abstract
Achieving high workpiece accuracy is the long-term goal of machine tool designers. There are many causes for workpiece inaccuracy, with thermal errors being the most common. Indirect compensation (using prediction models for thermal errors) is a promising strategy to reduce thermal errors without increasing machine tool costs. The modelling approach uses transfer functions to deal with this issue; it is an established dynamic method with a physical basis, and its modelling and calculation speed are suitable for real-time applications. This research presents compensation for the main internal and external heat sources affecting the 5-axis machine tool structure including spindle rotation, three linear axes movements, rotary C axis and time-varying environmental temperature influence, save for the cutting process. A mathematical model using transfer functions is implemented directly into the control system of a milling centre to compensate for thermal errors in real time using Python programming language. The inputs of the compensation algorithm are indigenous temperature sensors used primarily for diagnostic purposes in the machine. Therefore, no additional temperature sensors are necessary. This achieved a significant reduction in thermal errors in three machine directions X, Y and Z during verification testing lasting over 60 h. Moreover, a thermal test piece was machined to verify the industrial applicability of the introduced approach. The results of the transfer function model compared with the machine tool's multiple linear regression compensation model are discussed.
Highlights
The heat generated by moving axes and machining processes causes thermal gradients inside the machine tool (MT) structure, resulting in the thermal elongation and bending of MT elements; this substantially deteriorates the MT accuracy [1]
The principle of indirect compensation is the readjustment of the positioning of the axes by the MT’s control system, which here is based on corrections computed by a mathematical model
The current paper presents compensation for the main internal and external heat sources affecting the 5-axis MT structure
Summary
The heat generated by moving axes and machining processes causes thermal gradients inside the machine tool (MT) structure, resulting in the thermal elongation and bending of MT elements; this substantially deteriorates the MT accuracy [1]. The developed compensation grey-box model is restricted to a description of rotary and swivelling axes and how they impact thermally induced errors; here, other effects are neglected (e.g., the influence of the environmental temperature change, thermal displacements as a result of heat induced by rotation of the main spindle, linear axes movement, etc.). Despite the fact that different heat sources are permanently com bined in real machining conditions, which cause complex thermal errors at the TCP, much of the research has focused on an approximation of only one active heat source in the overall MT thermal errors This is a typical drawback of many existing compensation models of thermally induced errors; for example, only spindle thermal error compensation is considered in Li [9], time-varying environmental temperature influence in Zhang [19], the rotary axes movement in Blaser [20] and Bitar-Nehme [21] and so forth.
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