Measurement-based geometric reconstruction for milling turbine blade using free-form deformation

Highlights

• A measurement-based approach of geometrical reconstruction for blade was presented.

• Geometrical reconstruction was formulated as squared distance minimization problem.

• FFD volume was used to move the lattice points to modify the NURBS shape.

• The shape of blade was reconstructed by modifying each cross-section respectively.

Abstract

In aerospace engineering, the combination of hot forming and numerical control milling processes is an effective way to manufacture gas turbine blades nowadays. Due to the shape deviation, it is hard to mill the parts formed by hot forming process to the final nominal shape sometimes. To reduce the rejection rate and save the production cost, a measurement-based approach for geometric reconstruction of final nominal shape using free-form deformation (FFD) is presented in this paper. The original shape was firstly sliced into several cross-sections in its design manner, then each section was modified by FFD based on a set of organized measurement points, and at last the final nominal shape was reconstructed by lofting these modified cross-sections. An iteration process with knot insertion was developed to improve the FFD calculation accuracy. The results were found to be highly encouraging, which validates the feasibility of our proposed geometrical reconstruction method.

Introduction

Hot forming processes such as forging, casting and creep forming, are widely used in manufacturing complex structural parts in aerospace application, which can reduce material costs and improve machining efficiency [1], [2], [3], [4]. However, they cannot directly meet the high accuracy requirement of some key parts such as gas turbine blades and blisks. Thus, they are often followed by numerical control (NC) milling to ensure the final accuracy of these parts [5], [6], [7], [8]. That is, the output of the hot forming process is the input of the NC milling process.

A critical barrier has been encountered in the NC milling of these output parts that they were different from each other because of low forming accuracy. Moreover, some formed parts cannot be milled to the final nominal shape due to large shape deviation. For reducing the rejection rate and saving the production cost, the nominal geometrical shape should be reconstructed [9], [10]. In this paper, we investigate the problem of geometrical reconstruction of final nominal CAD model for milling gas turbine blade using free-form deformation.

The main contributions of our work are:

• We consider the geometrical reconstruction of gas turbine blade as a problem of shape modification with multiple measurement points. A least squares minimization approach regarding point-pair displacements was adopted.

• We describe the geometrical shape of gas turbine blade in non-uniform rational B-splines (NURBS) formats. Free-form deformation (FFD) was used to modify the lattice control points to update the NURBS shape. An iteration process of FFD calculation was developed to improve the calculation accuracy by knot insertion.

• The NURBS shape of the gas turbine blade was reconstructed in its original design manner. The NURBS shape was sliced into several cross-sections. Each cross-section was reconstructed first and then the final shape was lofted by these modified cross-sections.

The rest of the paper is organized as follows. In Section 2, some related works on geometrical reconstruction and FFD are described. Section 3 details the geometric reconstruction approach for milling gas turbine blade. Implementation of the proposed approach is given in Section 4. Results of a case study are depicted in Section 5. Finally, conclusions and outlook are presented in Section 6.