A review on spindle thermal error compensation in machine tools

Highlights

• Thermal error compensation is an effective and economical way to reduce the error.

• Analyzing, testing, modeling and compensating are main aspects of the research.

• Various devices and systems for temperature and thermal error testing are introduced.

• Different kinds of methods used for thermal error modeling are studied.

• Two commonly used mechanisms of thermal error compensation are explained.

Abstract

Thermal error caused by the thermal deformation is one of the most significant factors influencing the accuracy of the machine tool. Among all the heat sources which lead to the thermal distortions, the spindle is the main one. This paper presents an overview of the research about the compensation of the spindle thermal error. Thermal error compensation is considered as a more convenient, effective and cost-efficient way to reduce the thermal error compared with other thermal error control and reduction methods. Based on the analytical calculation, numerical analysis and experimental tests of the spindle thermal error, the thermal error models are established and then applied for implementing the thermal error compensation. Different kinds of methods adopted in testing, modeling and compensating are listed and discussed. In addition, because the thermal key points are vital to the temperature testing, thermal error modeling, and even influence the effectiveness of compensation, various approaches of selecting thermal key points are introduced as well. This paper aims to give a basic introduction of the whole process of the spindle thermal error compensation and presents a summary of methods applied on different topics of it.

Introduction

As the demand of high accuracy of the machine tool becomes higher and higher, thousands of scholars and engineers dedicate to the research of errors. These errors can be broadly divided into several groups as follows [1]:

• geometric and kinematic errors,

• thermal errors,

• cutting-force induced errors, and

• other errors such as the tool wear and the errors induced by assembling and chattering [2], [3].

Among these errors, the thermal error, generated at that time on account of deformation or distortion of the machine elements caused by the heating and temperature rising, is one of the principal causes of the inaccuracy [1], [4]. According to Bryan's research published in 1990, the thermal error accounts for 40–70% of the total error [5]. In general, there are two kinds of heat sources in machine tools, namely internal and external heat sources, bringing about the temperature rising and thermal errors [4], [6].

• Internal heat sources:

• heat generated from cutting process;

• heat generated from frictions in ball screws, spindle, gear box, guides, etc.;

• heat generated in motor;

• heating or cooling influences provided by the various cooling systems.

• External heat sources:

• environmental temperature variation;

• solar and personal radiations.

As the core component in machine tool, the spindle would generate large amounts of heat when it is running at a high speed. Among the heat sources listed above, the spindle is considered as an important one [7]. Therefore, this review focused on the thermal error of the spindle. In order to minimize the influence of spindle thermal error on the accuracy of the machine tool, kinds of methods have been proposed by scholars all over the world. All the methods fall into the following three categories [8].

(1) Thermal error avoidance

This strategy is supposed to reduce the generation of the heat or the thermal deformation in the spindle system. For example, by replacing the metal bearings with the hybrid bearings which have ceramic balls, less heat are generated due to less friction. Besides, advanced material, such as carbon fiber reinforced plastics (CFRP), invar and the polymer concrete are used to build the spindle parts instead of metal. Because these advanced materials have low coefficient of thermal expansion, they are insensitive to the temperature change which means when the temperature rises, less thermal deformation would be generated [5], [9], [10].

(2) Thermal error control

Another method of minimizing the thermal error is to control the amount of heat transferred into the spindle system or to avoid the generation of the non-uniform temperature distribution. For example, by incorporating the cooling jackets around the spindle bearings, the heat dissipation is enhanced so that there would be less heat left in the spindle system leading to the thermal expansion [6]. Temperature controlled boxes and rooms could be used to reduce the heat transferred from the environment into the spindle system [5]. Also some researchers tried to control the heat flows by placing a layer of thermal insulation between the spindle shaft and the inner race of the spindle bearings [6], [11]. In addition, optimizing the machine tool structure and applying heat pipes and thermal actuators are other ways to equalize the temperature distribution and control the thermal error [5], [12], [13].

(3) Thermal error compensation

Thermal error compensation is a process where the error is corrected by adjusting the position of the tool and work piece, usually using the existing machine axes [6]. Compared with other two types of methods, thermal error compensation is more convenient and cost-efficient [14]. This is because that on one hand, it needs no extra expensive hardware such as the advanced material and heat pipes, and on the other hand it can be implemented at any stage of the machine tool designing or building, while some tactics above (e.g. optimization of the structure) cannot be implemented after the machine tool has been built or the structure has been determined [8].

Generally, there are several main aspects in the real time thermal error compensation, namely analyzing, testing, modeling and implement of compensation. Analyzing, testing and modeling are the basics and preparatory work of the compensation. The general process of the thermal error compensation is

• analyze and study the temperature distribution and the thermal errors of the spindle theoretically, numerically and experimentally;

• build the thermal error models which are mostly describing the relationship between temperatures and thermal errors based on the analyzing or testing results;

• predict the thermal error according to the model and based on the predicted data the thermal error compensation is completed by incorporating the compensation values in the respective axes or adjusting the origins of the coordinates.

In this paper, all these aspects of the thermal error compensation mentioned above are discussed. Firstly, the spindle thermal error is analyzed in Section 2. The theoretical computations of the heat source, heat dissipation, temperature distribution and thermal error are stated. Various numerical analyses of the spindle thermal characteristics are discussed as well. In Section 3, measurement set-ups for testing the temperature and thermal errors are introduced. As the numbers and locations of the temperature sensors are vital to the accuracy of the further thermal error modeling and compensation, the optimization of thermal key points is also discussed in this section. Then in Section 4, the relationships between the temperatures and thermal errors are mapped and the modeling methods are listed. In Section 5, two basic techniques for implementing the thermal error compensation, namely the feedback interception method and the origin-shift method are studied. Finally, the conclusion of the spindle thermal error compensation research in the past are stated and the problems and directions which scholars should be worked on in the future are pointed out.