On-machine noncontact scanning of high-gradient freeform surface using chromatic confocal probe on diamond turning machine

Highlights

• An on-machine noncontact scanning method for high-gradient surface is presented.

• Segmented motion is designed considering steepness angle and planning deviations.

• The chromatic confocal probe is integrated into the diamond turning machine.

Abstract

Inspecting a high-gradient freeform surface is a challenging task in manufacturing automation due to its complexity of the surface and high-aspect-ratio. On-machine measurement has shown remarkable achievements in ultra-precision manufacturing. In this paper, an on-machine noncontact scanning method using chromatic confocal probe for high-gradient freeform surface is proposed. First, a coordinate sampling model of scanning based on an ultra-precision diamond turning machine is established. Then, the steepness angle is defined to guide the scanning curves distribution, and global mesh scanning lines are generated by controlling planning deviations. Segmented motion trajectory of the probe is designed considering the vector direction of optical axis and sensor offset error. Finally, the scanning parameters are determined and the NC inspection program is generated. The proposed method can make a balance between accuracy and efficiency. Its effectiveness is verified by the on-machine inspection experiments.

Introduction

High gradient aspherical and freeform components are increasingly used in astronomy, aerospace, and automotive [1], [2], [3]. The slope is an important functional parameter of these parts. Optically, it changes the direction of light propagation through refraction or reflection. And in current aircraft, it can effectively reduce aerodynamic drag at high speeds. This kind of surface effectively enhanced the system performance and simplified the system structure. Nevertheless, the complexity of the surfaces gives more challenges to the fabrication and the metrology of these parts. Diamond machining, such as grinding, slow tool servo turning and fly cutting, has shaken off the limitations of machining some kinds of freeform optics or materials. Even so, the products may be not qualified due to various sources of machining error, such as tool wear, contour control error, vibration and environmental factors. In the whole process chain, suitable dimensional inspection is a key step to ensure that the products meet the quality requirements. Many techniques, both experimental and commercial, exist for measuring general freeform surfaces and precision optics, such as interferometry [4], [5], stylus profilers [6] and coordinate measuring machines [7], [8]. However, commercial instruments usually operate offline. The process is complex and time consuming since the workpiece must be removed from the machine after processing, transferred to a special measurement room and re-clamped on the machining tool.

On-machine measurement has shown remarkable achievements in ultra-precision manufacturing since it increased the inspection efficiency, improved the machining accuracy and increased the automation level [9], [10]. Many object-oriented on-machine measurement systems have also been reported [11], [12], [13]. The integration allows one machine and one 3D model to be used for both manufacturing and inspection. Therefore, it can avoid the systematic errors introduced by re-clamping the workpiece. And the on-machine measurement system can efficiently feedback deviations caused by machining errors achieving the maximum quality of machined parts and reducing both costs and overall cycle time.

When we select the probe to be integrated, it is necessary to consider the properties of the workpiece to be measured and the measurement accuracy, range, speed, and the integration of the probe. For example, Moore Nanotech provides an air bearing LVDT probe for on-machine measurement on a diamond turning machine. A high-precision C-axis encoder is mounted to the spindle and the machine controls have been adapted to allow for indexing of the spindle. Therefore, a cylindrical CMM is setup with an XZC configuration. With this contact probe, scanning measurement of aspheric and gentle slope free-form surfaces can be performed. But the measurement results are affected by the errors such as calibration accuracy and radius compensation of the measuring head. And contact measurement has its inherent defects, such as easy to scratch the surface and low measurement efficiency. The optical sensors have been applicated for achieving non-destructive, fast inspection of freeform surface. Boltryk [14] compared three different types of optical displacement sensors for measuring complex surface forms. The study observed that the chromatic confocal sensor has better measurement characteristics than triangulation laser sensor and confocal laser sensor. Chromatic confocal sensing is a well-known measurement technique and has been integrated to ultra-precision turning machine for non-contact on-machine measurement [15]. Convex spherical surfaces have been effectively inspected with the proposed system. However, for high gradient aspherical and freeform components, little work has been performed.

Currently, the research on continuous inspection planning of the high gradient aspheric and free-form components is scarce. Generating the effective inspection path for steep surface is still a challenging task, due to its high-aspect-ratio and complex freeform. Traditionally, according to a computer-aided design (CAD) model, path planning for the free-form surface measurement requires two main considerations: which points or lines on the surface should be measured and how to measure them. The crux of measurement point distribution is to balance measurement accuracy and time efficiency. The researchers have proposed some efficient measurement planning methods. Elkott and Veldhuis [16] studied three types of continuous scanning paths: automatic isoparametric line scan, curvature-based isoparametric line scan and isoplanar line scan. The inspection process is commonly constrained by the maximum number of scanning curves, and the minimum step-over distance between subsequent sample lines. The large surface gradients result in sparse measuring points at the top of the steep surface when iso-planar planning is applied as show in Fig. 1(a). Spiral strategy is commonly used for generating the tool path for CNC machining of these complex steep surfaces. For inspection, the size of the pitch affects the accuracy. The spiral inspection trajectory with a constant pitch causes the measuring points to be sparse at the bottom of the surface as show in Fig. 1(b). Based on the curvature information, scanline with equal curvature of the surface can be obtained. The top view is shown in Fig. 1(c). However, the curvature-based scanlines add significant complexity to the movement of measuring device.

In recent years, optimization methods have been employed to improve the inspection planning strategies. Zahmati [17] proposed a new hybrid sampling strategy based on Particle Swarm Optimization to determine the best location of sampling points. Compared with uniform sampling strategy, the method improved the measurement accuracy and has a good effect in point-by-point coordinate measurement. Li [18] developed an interference-free inspection path for five-axis on-machine measurement of blades using contact probe. The accessibility cone was defined to determine the probe attitude. The generated trajectory is smooth, and it can avoid the sudden change of the approach direction vector of the probe. Jiang [19] proposed a method to inspect complex blades along their sectional curves for coordinate measuring machine. Their method can obtain better measurement accuracy with less sectional curves. The maximum chordal deviation was used to distribute the sample points.

The presented literature review shows that a lot of work has been done for improving the measurement accuracy and optimizing the inspection planning strategy for freeform surface. However, relatively few investigations have been conducted for on-machine noncontact scanning of high-gradient freeform surface using chromatic confocal probe on a four-axis ultra-precision machining tool. And almost all the developed sampling strategies are focus on the geometric characteristics of the measured surface. Actually, the inspection path planning cannot be independent of the inspection machine and the performance of the measurement probe. The inspection motion must be decomposed into each single-axis movement by inverse kinematic transformation. Continuous scanning measurement with an unreasonable trajectory may damage the stability of the servo system or lead the optical probe out of range, thereby introducing excessive measurement errors.

Motivated by the limitation of the capability and efficiency to measure the steep freeform surface, we aim to propose an effective inspection planning method for the steep surface using an on-machine chromatic confocal probe. The basic idea is to define the steepness angle of surface and create mesh scanning lines based on the change in the steepness angle. The mesh scanning lines consist of two sets of curves in two different directions on the measured surface and it can be used to construct a substitute surface to calculate the deviation from original design. After the continuous inspection paths are determined, an effective measurement process is designed, mainly involving the kinematic characters analysis, optical axis vector optimization and scanning measurement parameters selection.

In this paper, we conduct a systematic study of on-machine sweep scan and propose a practical method of automatically generating scanning paths for measurement of the steep free-form surface form error. The rest of the paper is organized as follows. First, the principle of on-machine scanning using a chromatic confocal probe is introduced in Sections 2. In Section 3, a CAD-based scanning path planning method for the high-gradient surface is developed. In Section 4, inspection experiments are carried out to demonstrate the proposed method. Conclusion is given in Section 5.