Engineering materials are broadly classified as ductile and brittle mate- rials. The failure modes in these materials help us to understand the mechanism of material removal. Since one of the major objectives of machining is to remove material from the work surface and subsequently achieve desired qualities on the generated surface, it becomes important to understand the mechanism of material removal in various engineering TABLE 3.1
Key Characteristics and Capabilities of Deterministic and Random Finishing Processes
Deterministic Process Random Process Characteristics Path controlled Force controlled
Location of point of material
removal is controllable Material removal by area averaging Few process parametres Large number of process parametres
Faster process Slow and tedious process
Machine needs to be precise, as its signature is transferred to generated surface
Machine need not to be precise, as its signature is not transferred to generated surface
Lay pattern is generated on
finished surface No lay pattern is generated on finished surface
Capabilities Precisely controllable MRR Difficult to control MRR Complex surface generation is
possible Only generation of simple surfaces
like flat and spherical surfaces are possible
Size control is possible Size control is not possible Finish impaired by lay pattern Extremely high surface finish
31 Mechanism of Material Removal
materials. Material removal mechanism is generally affected by the fol- lowing factors:
• Work material and its properties
• Tool material and its properties
• Relative position between the work-piece surface and tool
• Relative motion between the work material and tool
Figure 3.3 depicts the process of material removal in ductile materials.
Numerous factors play a significant role in the material removal mechanism and subsequently affect the quality of the generated surface.
In the case of ductile material, when the material is compressed by the moving tool, it slides along the shear zone as indicated in Figure 3.3, gets converted into ‘chip’ due to plastic deformation and subsequently is removed from the parent material. Depending on the defect density of the shear zone, the specific cutting energy necessary to remove the material varies. As the uncut chip thickness reduces, the defect density in the shear zone also reduces, and this leads to the requirement of higher specific cutting energy to remove material as chip. During the material cutting process, the desired component shape is achieved by controlling the tool path. Table 3.2 sum- marises some of the factors affecting the ductile material removal process.
The specific cutting energy in machining ductile materials can be esti- mated by the following equation:
Specific Cutting Energy=G e(−2πW a/) (3.3) where G = modulus of rigidity, W = dislocation width and a = interatomic spacing.
Secondary zone of deformation Chip
Primary zone of deformation
Work-piece
Tool Direction of tool
movement
FIGURE 3.3
Ductile material machining mechanism.
For a given material, the most important factor affecting the cutting mech- anism is the tool nose radius. For a given tool edge radius r, the mechanism changes from cutting by shear to ploughing to rubbing, when the uncut chip thickness (tc) changes from above the threshold value of uncut chip thick- ness value to below the threshold value. Figure 3.4 shows the same schemati- cally and Table 3.3 shows the conditions of material removal mechanism for a variety of cutting edge sharpness.
For a given work material and cutting edge radius of the tool, when the uncut chip thickness is greater than critical chip thickness (tc), shearing by plastic deformation becomes predominant and chip is formed from the displaced work material [17] as shown in Figure 3.4b. When the uncut chip thickness is between critical chip thickness and the thickness that causes pure elastic deformation, elastic deformation becomes predomi- nant and ploughing action takes place as shown in Figure 3.4c and no material is removed from the work-piece. When the uncut chip thickness is less than the value corresponding to pure elastic deformation, rubbing of tool on the work surface takes place and no material is removed from the work-piece as shown in Figure 3.4d. At certain critical values of uncut chip thickness, the rake angle of the tool becomes positive to negative and it creates a dead metal zone leading to increasing cutting force. This increases the specific cutting force and specific cutting energy. This phe- nomenon of increasing specific cutting force or specific cutting energy with decreasing uncut chip thickness is known as size effect. Figure 3.4e shows the change in the mechanism with a/r ratio; when the ratio of a/r decreases for a given uncut chip thickness, the mechanism of mate- rial removal changes from shearing to ploughing and subsequently to rubbing.
TABLE 3.2
Factors Affecting Ductile Material Removal
Source Property Effect
Material Ductility Work hardening Grain size
Material flow back characteristic and rubbing of finished surface with tool Cutting tool Cutting edge radius Minimum achievable uncut chip
thickness Cutting tool angles/geometries/
orientations Hot hardness
Tool rubbing on the finished surface
Machining
parametres Uncut chip thickness Rubbing/ploughing/cutting
Machine Precision Minimum uncut chip thickness
Transfer of machine signature on finished surface
33 Mechanism of Material Removal
Figure 3.5 explains the cutting mechanism for brittle material machining.
When the indenter representing the cutting tool exerts pressure on the mate- rial (here it is shown normal to the machining surface), after certain penetra- tion the material generates lateral cracks and subsequently median cracks in the region of indentation as shown in Figure 3.5. When these lateral and median cracks meet, material removal takes place. This leads to generation of discontinuous chips in brittle materials [18,25]. Figure 3.6 schematically shows the formation of discontinuous chips in brittle material processing.
Maximum theoretical specific cutting energy in brittle material machining is given by the following equation:
Specific cutting energy=G/2π. (3.4)
Shearing Rubbing
Shearing Ploughing Rubbing
(b) (c) (d)
(e)
A A
Tool nose radius Rake
Flank Cutting edge
radius
(a)
t1 t2 t3
Section: A-A (not to scale)
a/r
a- uncut chip thickness r- cutting edge radius (sharpness) Ploughing
FIGURE 3.4
Effect of tool nose radius on cutting mechanism. (a) Tool nomenclature. (b) Shearing.
(c) Ploughing. (d) Rubbing. (e) Cutting mechanism for different a/r ratio.
TABLE 3.3
Effect of Cutting Edge Radius on Cutting Mechanism Decreasing
uncut chip thickness
Increasing cutting edge radius
r1 r2 r3
t1 Cutting Ploughing Rubbing
t2 Ploughing Rubbing –
t3 Rubbing – –
Note: where r1 < r2 < r3, t1 > t2 > t3
In general, material removal in both ductile and brittle materials can be schematically represented by the multigrain material removal process as shown in Figure 3.7. In multigrain machining, which takes place at the macro level, material is removed due to the failure line moving along the grain boundaries as shown in Figure 3.7. The resistance offered to the plastic defor- mation of the material is nearly equal to the shear strength of the material. In such cases, when the grain size becomes smaller, the number of grain bound- aries increases and results in increased resistance for material removal.
Work-piece Tool Chips
Work-piece Tool
Work-piece Tool
Work-piece Tool Initial
deformation
Crack formation
segmentChip
(a) (b) (c)
FIGURE 3.6
Formation of discontinuous chips in brittle material processing. (a) Initial deformation of mate- rial. (b) Crack formation. (c) Chip formation.
Length of median crack
from top surface Length of lateral crack
from top surface Ft
Tool
Median crack Lateral crack
Plastic deformation enclave
FIGURE 3.5
Brittle material machining.
35 Mechanism of Material Removal