List of Symbols
2.3 Process Parameters of SPDT
2.3.2 Cutting Tool Geometry
Figure 2.4 shows the tool geometry parameters employed in the machining of brittle and ductile material using SPDT process. These are rake angle, nose radius and cutting edge radius. Rake angle is the angle of inclination of rake surface from the reference plane. The rake angle can be of negative, neutral (zero) and positive value. A tool is said to have a negative rake angle when the wedge angle and clearance angle together makes an obtuse angle (Figure 2.5 (a). If both wedge angle and clearance angle together makes 90º, it will have zero rake angle (see Figure 2.5 (b)). When the wedge angle and clearance angle together makes an acute angle, the tool is said to have a positive rake angle (Figure 2.5 (c)). The cutting edge radius is the roundness of the corner made by rake face and clearance face. No tool can be made perfectly sharp as there will be always some bluntness at the cutting edge.
Nose radius is the curvature of the tooltip measured in the top rake plane of a cutting tool.
Figure 2.4 Schematic of the tool geometry of single point diamond tool
Figure 2.5 Schematic representations of positive, negative and zero rake angles of a tool Reported studies have shown that the cutting tool geometry has a significant effect on the machining of brittle materials. It affects the performance parameters such as ductile to brittle transition, machining force, and surface quality. Komanduri et al. (1998) shown that the high negative rake angle and large edge radius (relative to the depth of cut) of the tool provide necessary hydrostatic pressure beneath the tool for the formation of plastic deformation by suppressing the initiation of brittle fracture. Blackley and Scattergood (1991) performed cutting tests on silicon and germanium with 0º, −10º and −30º tools and found that
Edge radius
Clearance angle Nose radius
Rake face
Clearance face
Zero rake angle
Cutting velocity
Negative Rake Angle Tool
Workpiece Chip
Clearance angle (–ve)
zero
Cutting velocity
Zero Rake Angle Tool
Workpiece Chip
Clearance angle
Chip (+ve)
Cutting velocity
Positive Rake Angle Workpiece
Tool
Clearance angle
(a) (b) (c)
Reference plane Reference plane Reference plane
Rake
surface Rake
surface
Rake surface
Wedge angle Wedge
angle Wedge
angle
TH-2306_10610325
17 the necessary critical depth of cut for ductile machining increases with the increase in negative rake angle. Patten and Gao (2001) used both the rake face (–45°) and the clearance face (–85°) as rake angle for machining of silicon and found that a smooth ductile cut with no evidence of fracture. It was reported that hydrostatic pressure plays an important role in minimizing fracture and producing smooth surfaces. It was also observed that the negative rake angle tool produces high hydrostatic pressure in comparison to that of positive and zero rake angle tools. However, Fang and Venkatesh (1998) claimed that a zero degree rake angle tool perform better in producing high quality surface finish than –25° negative rake angle tools during cutting of silicon.
Figure 2.6 Schematic diagram for calculating the effective rake angle
The effective rake angle can be calculated with the help of tool geometry parameters as shown in Figure 2.6. It is given by equation (2.1) as suggested by Lai et al. (2012):
sin 𝛼𝑒 = 𝑟−𝑑𝑟 = 1 −𝑑𝑟 (2.1) where r is the cutting edge radius, d is the depth of cut and 𝛼𝑒 is the effective rake angle.
In addition to the rake angle, the cutting edge of the tool also plays an important role in ductile mode cutting of brittle materials. The relationship between the cutting edge radius of the tool and the undeformed chip thickness is instrumental in achieving ductile mode cutting of brittle materials. Studies by Fang and Zhang (2003) and Yan et al. (2002, 2009) suggested that the cutting tool of 0º rake angle can be negative if the cutting edge radius is more than the undeformed chip thickness. This negative effective rake angle helps in achieving ductile cutting of brittle material by providing the required hydrostatic pressure for the plastic deformation to occur. Asai and Kobayashi (1990) reported that during ultra- precision machining, to obtain a mirror-like surface, the undeformed chip thickness must be
zero
Cutting velocity
Tool
Workpiece Chip
Clearance angle Depth of cut, d
(r-d) d
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equal to or smaller than the cutting edge radius of the tool. Yan et al. (2002), Leung et al.
(1998), and Patten and Gao (2001) also reported the effect of cutting edge radius on the ductile cutting of brittle material. Leung et al. (1998) emphasized on the proper selection of negative rake angle to avoid the occurrence of extensive tensile cracking during diamond turning of brittle materials. Shibata et al. (1996) employed a diamond for turning of single crystal silicon with both –20º and –40º rake angle tools. Very fine surface finish was obtained when –40º rake tool at 100 nm depth of cut was employed.
Patten et al. (2005) performed experiments on 6H-SiC workpiece with a unique method of varying the tool rake angle by adjusting the center line of the tool cutting edge.
This created an effective negative rake angle between 0º to 90º. It was observed that the ductile cutting is more favorable when the rake angle is changed from 0 to –45º and the depth of cut is below 100 nm. Moreover, it was noted that the cutting mechanism changes from ductile to brittle as the depth of cut increases from 100 nm to 500 nm.
Fang et al. (2007) reported that the cutting edge radius (r) has significant influence on the ductile to brittle transition (DBT). If the depth of cut is less than the edge radius then an effective rake is created and the chips are formed via extrusion like phenomenon. In this situation, 0º rake produces a negative rake and a negative rake produces a more negative rake angle. Based on the experimental work by Arefin et al. (2007) and a molecular dynamics simulation by Cai et al. (2007a), it was reported that in order to obtain ductile-regime machining of silicon, the cutting edge radius must be below 807 nm and higher than maximum undeformed chip thickness.
A number of researchers [Yan et al. (2003), Li et al. (2005), Goel et al. (2013a), Singh et al. (2013)] attempted to investigate wear of diamond tools and reported that flank wear dominates the wear of diamond tool during machining of silicon. Goel et al. (2011, 2013a) reported the formation of silicon carbide at the tool-work interface during machining of silicon. This phenomenon provided information regarding the initiation of wear of the diamond tool. Further, they described the tribochemistry involved in the process which leads to diamond tool wear.
Observations:
In SPDT process, the influencing parameters can be classified into four groups namely process parameters, tool geometry parameters, workpiece–tool properties and machining condition. Understanding of the effect of these process parameters of SPDT on its performance parameters such as machining forces and surface roughness is important for TH-2306_10610325
19 improving the product quality and process efficiency. Literature reports significant research work on experimental studies of SPDT process on various materials. Most of these works have studied individual process parameters. Literature also reports some experimental studies on the effect of machining parameters on diamond tool wear. High capital and operating costs limit the extensive experimental studies of SPDT. After an extensive literature review on various aspects of SPDT process, it was found that very scant literature is available on numerical simulation of silicon and silicon carbide. Also, very scant research has been reported on the parametric study of the process to optimize the process conditions to improve the process efficiency and product quality.