Elsevier

Precision Engineering

Volume 30, Issue 4, October 2006, Pages 396-405
Precision Engineering

Nanometer edge profile measurement of diamond cutting tools by atomic force microscope with optical alignment sensor

https://doi.org/10.1016/j.precisioneng.2005.11.008Get rights and content

Abstract

This paper describes an atomic force microscope (AFM) based instrument for nanometer edge profile measurements of diamond cutting tools. The instrument is combined with an AFM unit and an optical sensor for alignment of the AFM probe tip with the top of the diamond cutting tool edge in the submicrometer range. In the optical sensor, a laser beam from a laser diode is focused to generate a small beam spot with a diameter of approximately 10 μm at the beam waist, and then received by a photodiode. The tool edge top and the AFM probe tip are brought to the center of the beam waist, respectively, through monitoring the variation of the photodiode output. To reduce the influence of the electronic noise on the photodiode output so that the positioning resolution can be improved, a modulation technique is employed that modulates the photodiode output to an AC signal by driving the laser diode with a sinusoidal current. Alignment experiments and edge profile measurements are carried out.

Introduction

Diamond cutting involves the use of a single-crystal diamond tool with a very sharp edge having a radius on the order of 10–100 nm for fabricating precision parts [1], [2], [3], [4], [5]. In ultra-precision cutting, the micro/nano-wear of the tool edge becomes a big problem as it influences the quality of the machined surface [6]. Diamond cutting has been used to generate three-dimensional (3D) micro-structured surfaces. In such cases, the fabrication accuracy is influenced not only by the tool edge sharpness, but also by the local 3D profile on the tool edge [7]. It is thus important to conduct nanoscale measurements of the edge wear as well as the 3D edge profile. From the viewpoints of measurement and fabrication efficiency, it is also desirable to conduct the edge measurement in on-machine conditions without removing the tool from the machine tool.

Conventionally, the tool edge is monitored by an optical microscope or a scanning electron microscope (SEM) [8], [9], [10], [11], [12], [13], [14], [15]. The optical microscope is easy to use and can even be used in on-machine conditions. Its resolution, however, is limited to the micrometer range by optical diffraction phenomena. The SEM is excellent for nanometer resolution, but the measurement has to be carried out in a vacuum chamber. In addition, an SEM image is basically a 2D projection of a 3D object, and thus the SEM cannot be used to measure the 3D tool edge profile.

Unlike optical microscopes and SEMs, scanning probe microscopes (SPM), such as atomic force microscopes (AFMs), are well-suited for measurement of 3D profiles of micro-objects with nanometer resolution in the X-, Y-, Z-directions [16], [17], [18], [19], [20]. The AFM has been used for characterization of edges of diamond tools [21]. On-machine measurement of a tool edge profile is also possible through mounting a portable AFM unit on the machine tool. In that case, the most difficult issue is the alignment of the AFM probe tip with the tool edge. Taking into consideration the AFM tip/tool edge geometries and the scanning range of the AFM, it is necessary to ensure alignment in the submicrometer range. The AFM probe tip in a conventional AFM is aligned by using an optical microscope (Fig. 1). As can be seen in Fig. 1, however, the AFM design constrains the optical microscope to focus on the back of the AFM cantilever, instead of directly on the probe tip. The extremely small working distance and the low resolution of the optical microscope further complicate its use for AFM tip and tool edge alignment.

Another critical issue for applying the AFM to on-machine profile measurement of a diamond tool edge is how to evaluate the sharpness of the AFM probe tip on the machine tool. The sharpness of an AFM probe tip, typically on the order of 10–20 nm, is comparable to the edge sharpness of a single-crystal diamond tool. Although it is possible to image the sharpness of an AFM tip by a calibrated SEM, it is desirable to evaluate the tip sharpness on the machine tool because the wear of the AFM probe tip progresses during the measurement. One idea is to periodically measure a reference artifact with a sharp edge by the AFM on the machine tool. The artifact can be a single-crystal diamond tool, as long as its edge sharpness has been measured by a calibrated SEM or AFM. For doing this, however, it is still necessary to align the AFM probe tip with the artifact edge so that the measurement can be conducted on the machine tool.

As the first step for profile measurement of a diamond tool edge on a machine tool, an instrument that combines an AFM unit and an optical sensor for alignment of the AFM probe tip with the tool edge top in the submicrometer range is developed. After demonstrating the alignment capability of the optical sensor, the instrument is used for 3D edge profile measurement of single-crystal diamond cutting tools.

Section snippets

Alignment concept

Fig. 2 schematizes the method used for aligning the AFM tip with the tool edge top. A collimated laser beam from a laser diode is focused by a condenser lens (Lens 1) to generate a small beam spot at the beam waist. The laser beam is then received by a photodiode after passing through Lens 2, which is used for collecting the beam. The Y-axis is set along the propagation direction of the laser beam, and the X- and Z-axes are chosen such that the three axes are mutually orthogonal. The origin of

Alignment experiment

Fig. 14, Fig. 15 show the results of positioning the tool edge along the X- and Y-directions, respectively. The error bar in the figure shows the full band of deviation of the output data at each position. The data set in each figure is fitted with a least mean square curve. In the experiment of Fig. 14, the FTS unit was moved toward the center of laser beam in steps of 0.5 μm by the stepping motor-driven stage. It can be seen that the curve of the lock-in amplifier output, which corresponds to

Conclusion

An instrument equipped with an AFM unit and an optical alignment sensor for the measurement of 3D edge profiles of diamond cutting tools has been constructed. The laser beam with its optical axis along the Y-direction from the laser source of the optical sensor is focused to form a beam waist near the AFM probe tip and the top of the cutting tool edge. The laser beam is then collected through a lens and directed to a photodiode. The output of the photodiode, which is verified to be a function

Acknowledgement

This work was financially supported by a grant from the New Energy and Industrial Technology Development Organization (NEDO).

References (30)

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