**6. Diamond tool and workpiece material interactions during machining**

With the tool wear, the cutting edge radius at every section of the tool edge interacting with the work material increases and it changes the a/r ratio which in turn shifts the range of these regions. Therefore, it is not only the chip thickness which varies along the chip profile but the tool edge radius also continuously changes with increasing tool wear. **Figure 14**(**a-d**) shows a schematic which shows the variation in cutting edge sharpness (r) at any uncut chip thickness (a) as the tool wear takes place. **Figure 14**(**b-d**) shows the condition of tool edge at three different stages of tool wear. In initial cutting stages when the tool is fresh, it possesses a sharp cutting edge with a very small radius (*r*1) for an uncut chip thickness '*a*' as shown in **Figure 14(b)**. During the intermediate stages of cutting, as the tool wears and the edge becomes blunt, the cutting edge radius increases from '*r*1' to '*r*2' as shown in **Figure 14(c)**. The increased edge radius causes a decrease in the a/r ratio. This is a stage where uncut chip thickness and edge radius become approximately comparable i.e. a/r ~ 1 at that section. With a further increase in cutting edge radius from '*r*2' to '*r*3', the a/r ratio decreases to

**Figure 14.** *Schematic of tool edge condition and cutting mechanism with tool wear [3].* a value less than 1. The material removal becomes difficult and thereby no chips are removed from the workpiece material at the final stages of cutting (**Figure 14** (**d**)).

### **6.1 Wear characteristics of the diamond tool**

Tool wear influences the cutting forces, chip formation and the surface roughness. Tool wear in diamond turning of ductile and non-ferrous materials is considerably low whereas it is very high in case of hard and brittle materials. Wear on the diamond tool deteriorate the machined surface quality. In general, flank wear is observed to be the most dominant wear in single crystal diamond tool. There are various effects of the tool wear on the machining parameters which are concluded in **Table 3**.

Tool wear in machining vary depending upon the tool and workpiece material combination. In ultraprecision diamond turning, the tool being used is diamond, and the workpiece material is the solely responsible for change in mechanisms. Diamond tool wear is different for different class of materials. In machining the class of materials are categorized in easy to machine and difficult to machine materials. The easy to machine materials are Al, Cu, brass, and Al & Cu alloys. For instance, the single crystal diamond tool is very useful in machining nonferrous and easy to cut materials such as aluminum, copper, gold, brass, electroless Nickel and plastics. In these materials, diamond tool does not wear out for a few hundreds of kilometers of cutting distance and still achieves a surface finish in the order of few nanometers. In these materials, diamond tool wear out slowly and abrasive wear take place more predominantly. The wear mechanisms become complicated while machining difficult to cut materials. The wear pattern consists of crater and flank wear. Impurities in alloys causes initial microchipping on the tool edge. The dominant wear mechanism in easy to machine materials is abrasive type. **Table 4** shows the wear pattern and wear mechanisms for easy to machine materials in diamond turning.

Wear mechanism of a single crystal diamond tool is highly complicated which may involve one or multiple mechanism types. These can be chemical, physical, thermal and mechanical interactions between the diamond tool and the workpiece. Depending on the tool and workpiece material combination, the wear mechanisms can vary. Diamond tool wear can be characterized into mechanical, chemical and physical wear. Mechanical wear comprise of abrasive wear, fatigue, and adhesive

