Review
Robotic grinding of complex components: A step towards efficient and intelligent machining – challenges, solutions, and applications

https://doi.org/10.1016/j.rcim.2019.101908Get rights and content

Abstract

Robotic grinding is considered as an alternative towards the efficient and intelligent machining of complex components by virtue of its flexibility, intelligence and cost efficiency, particularly in comparison with the current mainstream manufacturing modes. The advances in robotic grinding during the past one to two decades present two extremes: one aims to solve the problem of precision machining of small-scale complex surfaces, the other emphasizes on the efficient machining of large-scale complex structures. To achieve efficient and intelligent grinding of these two different types of complex components, researchers have attempted to conquer key technologies and develop relevant machining system. The aim of this paper is to present a systematic, critical, and comprehensively review of all aspects of robotic grinding of complex components, especially focusing on three research objectives.

For the first research objective, the problems and challenges arising out of robotic grinding of complex components are identified from three aspects of accuracy control, compliance control and cooperative control, and their impact on the machined workpiece geometrical accuracy, surface integrity and machining efficiency are also identified. For the second aim of this review, the relevant research work in the field of robotic grinding till the date are organized, and the various strategies and alternative solutions to overcome the challenges are provided. The research perspectives are concentrated primarily on the high-precision online measurement, grinding allowance control, constant contact force control, and surface integrity from robotic grinding, thereby potentially constructing the integration of “measurement – manipulation – machining” for the robotic grinding system. For the third objective, typical applications of this research work to implement successful robotic grinding of turbine blades and large-scale complex structures are discussed. Some research interests for future work to promote robotic grinding of complex components towards more intelligent and efficient in practical applications are also suggested.

Introduction

Complex components, such as turbine blade, wind blade, new energy bus body and high-speed rail body are widely used in aerospace, energy, automobile and rail transit industries, and their manufacturing level represents the core competitiveness of a country's manufacturing industry. Generally, the complex components can be divided into complex surfaces and complex structures. The former is characterized by freeform, thin-walled surface with difficult-to-machine materials, and requires high dimensional accuracy and surface quality. While the latter is featured with large size, high material removal rate, and multi-variety and small-batch production. After forging, casting, molding or mechanical machining, these components are universally required for grinding or finishing to further enhance the contour accuracy and surface finish. Therefore, mastering the high-efficiency and high-precision grinding technology for such complex components poses a serious challenge facing the manufacturing industry.

At present, the complex components are mainly ground by virtue of manual operation and multi-axis CNC machine tools. Two typical examples of grinding turbine blade and wind blade are shown in Fig. 1. Because of the time- and labor- consuming as well as the harsh operating environment experienced by manual grinding, the multi-axis CNC grinding has become the mainstream approach for manufacturing such parts [1]. Its in-depth application, however, is limited owing to the followings reasons: (1) high cost of precision machine tools, up to millions U.S. dollars, particularly for the large-scale multi-axis CNC machine tool; (2) fixed manufacturing mode, characterized by unavailable flexible and parallel machining capability; and (3) complex configuration without integrated "machining - measurement" function.

An approach based on the industrial robots offers new ideas for the manufacture of such complex components. Compared with the multi-axis CNC machine tools, robots are attractive due to their large extendable workspace and competitive price that makes them a cost-effective solution for machining of complex components, especially for large dimension parts [3]. Particularly equipped with the powerful sensing functions, such as machine vision, force-sensing [4], the robotic machining operations can optimize the running parameters in real-time based on the process knowledge model and multi-sensor feedback information. This breaks through the limitations of traditional manufacturing equipment which focuses only on the movement axis position and speed control, thereby leading to the active control of the equipment on the process. However, it is worth mentioning that the main obstacle for wide utilization of robots in precision machining is their relatively low accuracy and repeatability compared to the CNC machine tools, and a detailed comparison of CNC machine tools and industrial robots for machining is presented in Table 1 [5].

Nevertheless, a large number of effective solutions have been proposed to reduce the manipulator stiffness and positioning errors in industrial machining fields [6,7]. Particularly for the grinding operation of complex components, the robotic grinding is gradually replacing the multi-axis CNC machine tools, and becoming an alternative means for manufacturing such parts. During the past one to two decades, the research results of robotic grinding of complex components are gradually enriched, and the published literatures mainly focus on the feasibility study of robotic machining [8], [9], [10], as well as the modeling and analysis of machining dynamics [11], [12], [13]. The investigations are conducted mainly from the aspects of robot position/posture optimization [14,15], robot calibration and measurement [16], [17], [18], [19], [20], grinding path planning [21], [22], [23], material removal control [12,24,25], force control [26], [27], [28], [29] etc.. Furthermore, Table 2 compares the recently published review papers related to robotic machining. It is found that the existing ones are mainly elaborated from the perspectives of machining robots, machining properties based operation categories, mobile-robot machining, and robotic machining chatter.

Starting from the different perspective compared to the existing ones, the organization and writing of our review paper mainly focuses on the following three aspects:

  • (1)

    The specific machining operation category. We identified the machining operation category as robotic grinding, since this operation offers new ideas for the manufacture of complex components compared with the capacity of multi-axis CNC machine tools and conventional manual operation. Also this topic with respect to robotic grinding receives wide attention to enhance the current automated machining level.

  • (2)

    The geometry and size of the research objects. We focused on the complex components, including the freeform surfaces (less than one meter in length, i.e. turbine blades, blisk) and the large-scale complex structures (up to ten or dozens of meters in length, i.e. wind blade, high-speed rail body, and new energy bus body). This is mainly because we noticed that the development of robotic grinding in theory and application presents two extremes, one aims to reach micrometer level of grinding accuracy of freeform surface by the robot with sub-millimeter level of relatively low accuracy and repeatability, the other emphasizes on the enhancement of the grinding efficiency of large-scale complex structures to maintain the upstream and downstream production beats.

  • (3)

    The key technologies identified from this topic. For the robotic grinding of small-scale complex surfaces, the strategies from accuracy control and compliance control should be fully addressed, while for the robotic grinding of large-scale complex structures, multi-robot based cooperative control could be taken as an effective means to enhance the machining efficiency. To address the challenges, our research team identified four aspects of key technologies in robotic grinding of complex components, including the high-precision online measurement, grinding allowance control, constant contact force control, and surface integrity from robotic grinding.

The aim of this paper is to present a systematic, critical and comprehensive review of all aspects of robotic grinding of complex components. Thus, we aim to conduct the summarization of research works to (1) identify the challenges faced by robotic grinding of complex components, (2) report the research work carried out and various alternative solutions provided by the researchers in this area till date, and (3) discuss the typical applications of robotic grinding of turbine blades and large-scale complex structures, and finally propose some possible directions for future research interests.

Section snippets

Challenges faced by robotic grinding of complex components

The main issues which restrict the practical implementation of robotic grinding of complex components are discussed below.

Strategies and solutions for successful robotic grinding of complex components

To overcome the challenges faced by the robotic grinding of complex components, the following strategies and potential solutions can be conducted to construct the integrated “measurement – manipulation – machining” function for the robotic grinding system.

Background and existing problems

At present, the milled blades are mainly finished by virtue of manual grinding and multi-axis CNC belt grinding. Compared with the blade profile, the thickness of the leading and trailing edges of the blade is thin (to be R0.1 mm level). Both the curvature and machining path change greatly, and this poses a challenge to the precision grinding of blades [64]. Owning to the large randomness of positioning and the uncontrollable contact force at tool-workpiece interfaced during the manual

Conclusion and future research interests

Robotic machining is a kind of high-end manufacturing that conforms to the national situation. It is regarded as an effective means to solve the "pain points" facing the grinding industry. Despite the robots have obvious technical advantages in terms of dexterity, production flexibility, and functional integration, however, there are still gaps in machining accuracy and surface quality by robotic grinding, especially compared to the CNC machine tools. This is mainly owing to the high coupling

Declaration of Competing Interest

The authors declare no conflict of interest.

Acknowledgment

The authors received financial support from the National Nature Science Foundation of China (nos. 51975443, 51675394, 51535004), the National Key Research and Development Program of China (no. 2017YFB1303403), the Wuhan Applied Basic Research Project (no. 2017010201010139), the State Key Laboratory of Digital Manufacturing Equipment and Technology (no. DMETKF2018018), and the “111” Project (no. B17034).

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