Robotic grinding and polishing for turbine-vane overhaul

doi:10.1016/S0924-0136(02)00114-0

Journal of Materials Processing Technology

Volume 127, Issue 2, 30 September 2002, Pages 140-145

https://doi.org/10.1016/S0924-0136(02)00114-0 Get rights and content

Abstract

A robotic grinding and polishing system was used to automate the manual operation of turbine-vane overhaul. In the system a passive compliance tool combined with an adaptive path planning approach was adopted to overcome intrinsic problems arising from part-to-part geometry variations. The system is believed to be the first of its kind in the world for turbine-vane repair. This paper focuses on the development of the passive compliance tool (PCT) with passive force control and adaptive path generation. Tool wear compensation and process optimization were also explored in order to meet stringent quality requirements.

Introduction

High-pressure turbine vanes are key components in jet engines. The vane consists of airfoils (concave and convex) and a buttress (inner and outer) (Fig. 1(a)). After service in high-temperature and high-pressure environments, the vanes are severely worn and distorted and the cracks are often formed on their airfoils due to heat fatigue. These vanes need to be repaired as replacement is far more expensive. To date, in the aerospace overhaul industry, manual operations are used to repair turbine vanes. In the overhaul process, defective areas on the vane airfoils are covered with a layer of braze material. Skilled workers then remove the excessive braze material using abrasive belt grinding and polishing to restore the original profile of the airfoil. The belt grinding and polishing process is labor intensive, time consuming and produces inconsistent quality. The automation of such a process can lead to significant cost reduction and improved quality.

Industrial robots have been proven to be an economic solution for automation in the last 2 decades. Examples of such applications include deburring [1], [2], [3], [4], chamfering [5] and finishing [6], [7] of workpieces that possess a constant geometry. The major difficulty in the automation of turbine-vane overhaul is that the airfoil geometry is presented in a severely distorted condition. In addition, the complex three-dimensional (3D) geometry of the airfoil (see Fig. 1(b)) makes automatic machining even more challenging. So far, an automated system for the grinding and polishing of turbine vanes has yet to be applied in the overhaul industry.

In this paper, the authors report a robotic grinding and polishing system that replaces the manual overhaul of turbine vanes. The project focuses on the development of passive compliance tools (PCTs), adaptive path planning and tool path optimization for robotic grinding and polishing. Process parameter optimization and tool-wear compensation are discussed in detail.

Section snippets

Overview of the robotic system

The robotic belt grinding and polishing system, shown in Fig. 2, consists of a 6-axis finishing robot, a robot controller, an interface and host personal computers, an in situ measurement station (ISMS), an index table and four PCTs. The flow chart of a typical working cycle is shown in Fig. 3. During one cycle, the robot picks up a vane from the index table and moves to the ISMS station for profile measurement. The measured profile of the airfoil is transported to the host computer for

PCT

Force control is particularly important for avoiding over- and under-cutting of the vanes as the material removal amount for force control systems is usually uniform along the whole tool path. To achieve the desired vane profile, it is crucial to maintain good control of the contact force and the compliance between the tool and the vane. The present system uses PCTs to grind and polish the distorted vanes, with a combination of adaptive path generation and path optimization. PCTs with passive

Robotic grinding and polishing process

The abrasive material of the belts used for the PCTs is ceramic aluminum oxide with a mesh size of 80. This abrasive is especially designed for grinding aerospace alloys and forged steels. The braze material to be ground is an alloy containing about 45% of cobalt, 24% of chromium, 25% of nickel and 3.5% of tungsten, which belongs to the category of hard-to-grind materials.

Concluding remarks

The robotic grinding and polishing system has enabled overhauled vanes to meet stringent quality requirements such as profile smoothness, surface roughness and minimum wall thickness. The system is believed to be the first of its kind in the world for jet engine turbine-vane overhaul. It has been already put into production. Benchmark tests have shown that the system achieves a better and more consistent quality and reduces cycle time by more than 42% in comparison with the manual operation.

The

Acknowledgements

The authors wish to thank Mr. S.L. Liau, Mr. E. Ang and Mr. J.A. Sylvestro for their suggestions, discussions and experimental assistance. They also wish to thank the National Science and Technology Board of Singapore for providing a grant under the Aerospace Technology Program.

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Robotic grinding and polishing for turbine-vane overhaul

Abstract

Introduction

Section snippets

Overview of the robotic system

PCT

Robotic grinding and polishing process

Concluding remarks

Acknowledgements

Ann. CIRP

Control of tool/workpiece contact force with application to robotic deburring

IEEE J. Robotic. Autom.

Keynote address: advanced deburring system technology

ASME

Force control for robotic deburring

ASME J. Dyn. Syst. Meas. Cont.