A novel five-axis on-machine measurement optimization method for complex curved surfaces

Abstract

On-machine measurement (OMM) stands out as a pivotal technology in complex curved surface adaptive machining. However, the complex structure inherent in workpieces poses a significant challenge as the stylus orientation frequently shifts during the measurement process. Consequently, a substantial amount of time is allocated to calibrating pre-travel error and probe movement. Furthermore, the frequent movement of machine tools also increases the influence of machine errors. To enhance both accuracy and efficiency, an optimization strategy for the OMM process is proposed. Based on the kinematic chain of the machine tools, the relationship between the angle combination of rotary axes, the stylus orientation, and the calibration position of pre-travel error is disclosed. Additionally, an OMM efficiency optimization model for complex curved surfaces is developed. This model is solved to produce the optimal efficiency angle combinations for each to-be-measured point. Within each angle combination, the effects of positioning errors on measurement results are addressed by coordinate system offset and measurement result compensation method. Finally, the experiments on an impeller are used to demonstrate the practical utility of the proposed method.

1. Introduction

Due to their weak stiffness and complicated structures, complex curved surface parts, like turbine disks or impellers, are difficult to process [1−3]. On-machine measurement (OMM) is a critical technique in determining part machining deviation during adaptive machining [4−9]. Touch-trigger probes, owing to their affordability, ease of installation, and high precision, have become widely adopted. However, it is challenging to meet the efficiency and accuracy requirements for measurements in adaptive machining because of the poor openness of curved surface parts, which necessitates frequent tool movement during the measurement process. This increases the time required for probe movement on the one hand and introduces more errors between the probe and the machine tool on the other.

One typical method to raise the efficiency and accuracy of OMM is to optimize the stylus orientation and measurement path. The efficiency of measurement is directly correlated with the measurement path length, and measurement errors will be introduced by changing the measurement position and stylus orientation [10−13]. Based on this, some academics examined various sources of error and proposed innovative approaches to measurement planning.

Some scholars have concentrated on improving the accuracy and efficiency of measurements by optimizing the measurement path. LEE et al [14] and CHO et al [15, 16] optimized the measurement sequence of features by analyzing the information of to-be-measured features on complex mechanical parts. They also planned the appropriate number of measurement points and measurement paths for different features. YEO et al [17] proposed a measurement path that eliminated errors in the measurement path due to the conversion of features to non-uniform rational B-spline (NURBS) surfaces. The methods mentioned above concentrate on how the features that will be checked will affect the measurement’s accuracy and efficiency. However, the errors introduced by the machine tools and probes as components of OMM equipment cannot be ignored. In particular, the machine tool, serving as the carrier of OMM, exerts a significant impact on measurement errors. Some scholars have focused on reducing the errors introduced by machine tools. YAN et al [18] analyzed the influence law of positioning errors on shaping effect and identified the critical errors that should be minimized in operation. ZHAO et al [19] established a mapping relationship between machine tool volumetric error and measurement error, and selected the measurement scheme that minimizes the measurement error. The adaptive point placement approach used by LI et al [20] is based on the curvature of the curve and chooses the direction of force measurement to determine the compensating direction for probe inaccuracy.