**1. Introduction**

Ultraprecision machining techniques have received much attention owing to the increasing demands for precision components with tight dimensional tolerance, high shape accuracies and excellent surface quality. Single point diamond turning (SPDT) or diamond turn machining is an ultraprecision machining technique used to generate the components with exceptional accuracy and surface finish [1]. The SPDT was first explored in 1960s with the need to advance in various fields such as defense, aerospace, computer, electronics and medical. In 1983, Taniguchi produced a plot which showed the evolution of machining accuracy with respect to every passing decade [2] and predicted the accuracy for the year 2000 as shown in **Figure 1**. After extrapolating, it indicated that the ultraprecision processes at micro/nanoscale would be able to achieve the machining accuracy of 0.01 μm. To achieve this extreme level of accuracy, the processing scale needs to be extremely small of the order of few nanometers, which in turn depends on the accuracies of the machine tool and

#### **Figure 1.**

*Taniguchi prediction of machining accuracies [3].*

cutting tool being used. Among the various mechanical micro/nano-scale machining processes, diamond turning technology has brought a revolution in the world of precision machining by achieving submicron level accuracies in the size and shape of the machined component. Other than the integrated technologies like granite bed, air bearing spindle, hydrostatic slide bearings, and optical linear scale feedback system in ultraprecision diamond turning technology, single crystal diamond tool is a key factor to obtain a high quality machined surface. Owing to its exceptional properties suitable for cutting tool, the single crystal diamond tool with edge sharpness of the order of nanometers is extensively used in ultraprecision machining.

Based on the material removal mechanisms, various manufacturing processes can be categorized into mechanical, physical and chemical processes (see **Figure 2**). The physical and chemical machining processes are limited to certain materials, mechanical machining processes can be applied to a variety of materials and applications. The mechanical based micro/nano machining processes are classified into mechanical tool based cutting and abrasive based machining. Mechanical tool based machining is deterministic in nature as the tool path can be controlled. Whereas abrasive based machining are force controlled and are random in nature. Size and shape control in abrasive machining are difficult to achieve through abrasive based machining processes. The cycle time is much higher in case of abrasive machining. Therefore, precision of the highest level in terms of size and shape control and surface finish is not achieved through abrasive based material removal process [4]. In addition, the cycle time is relatively higher [5].

Currently, the diamond turning is commonly used for various applications, as some examples are listed below:


#### **Figure 2.**

*Classification of various micro/nano scale manufacturing processes. {MRAFF: Magneto-rheological abrasive flow finishing; MFP: Magnetic float polishing; EEM: Elastic emission machining; EBMM: Electron beam micromachining; LBMM: Laser beam micromachining; MEDM: Micro electric discharge machining; IBMM: Ion beam micromachining; PBM: Plasma beam micromachining; PCMM: Photo chemical micro-machining; ECMM: Electro chemical micro-machining; RIE: Reactive ion etching}.*


Single crystal diamond tool is used in ultraprecision diamond turning of nonferrous materials because of its ability to maintain an extremely sharp cutting edge owing to its wear resistance property. Single crystal diamond tool has a round nose with rake angle ranging from 0o to highly negative and suitable clearance angle (see **Figure 3(a)**) enough to avoid the contact between clearance face and the machined surface. The tool consists of sharp edge with controlled waviness (< 1 μm) along whole edge. The cutting edge radius is in the order of tens of nanometers. The performance of single crystal diamond tools depends on the quality of the diamond, its crystal orientation, edge radius/sharpness (see **Figure 3(b)**), and edge irregularities (see **Figure 3(c)**). Among these, measurement of tool edge radius is a challenge due to the fact that the tool edge is extremely sharp.

**Figure 3.** *Cutting tool edge geometry.*

Asai et al. [6] put forward a measurement technique for diamond tool edge sharpness. They advanced the conventional SEM by employing two secondary electron detectors. The signals from these detectors are processed to produce the fine cutting edge. This method of measurement is efficient for diamond tool with edge radius of 45 nm or less. Yuan et al. [7] performed experimental investigations on different orientations of single crystal diamond tool to find out the optimum crystal plane for ultraprecision machining. They conducted the friction tests on rake and flank face of the tool, noticed the friction coefficients and observed the effect on shear deformation, tool wear and the machined surface quality. Based on the friction coefficients value on different directions of crystal orientation of (100), (110) and (111), it was found that the (100) plane shows highest anisotropy. The literature has contradictions regarding the optimum crystal planes for the rake and flank surface of the diamond tool.

Ultraprecision machining trials including the shear deformation, tool wear and surface quality has indicated that the (100) plane is most suitable for rake as well as flank surfaces. However, Uddin et al. [8] recommended (100) as rake plane and {110} as flank plane for the diamond tool in machining of Si. With theoretical calculations, Zong et al. [9] proposed that it is possible to achieve 1 nm sharp cutting edge radius for the (110){100} crystal orientation.
