Irrespective of the choice of variables in the four input categories, for every abrasive finishing process it is possible to visualize four interactions between the abrasive product and the work material (Figure 3.7). Of these, the abrasive/workpiece interaction is the most critical, which in many respects is analogous to machining processes with cutting tools.
Figure 3.7.
Interactions in the grinding zones [5]
(1)
Abrasive/workpiece Interface
(2)
Chip/bond Interface
(3)
Chip/workpiece Interface
(4)
Bond/work Interface
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In ceramics grinding, from a tribological viewpoint, to consider the grinding wheel as a microcutting tool is an oversimplication [7]. The random distribution and geometry of abrasive grains on the active surface of the wheel are modified during the grinding process. The modifications are due to the simultaneous actions in the contact zone involving three different mechanisms: microcutting, plowing, and rubbing [8]. In addition to the effects on distribution and micro-geometry, the abrasive process is also affected by the type of bond, the nature of the workpiece material and coolant, and by the mechanical working parameters [9] and [10].
Tribometers with monograins [11] and [12] can model very simplified abrasion machining processes without indicating the type of wear of abrasion materials from tools for long life. For this purpose, a modern grinding machine with variable speed and high accuracy of depth of cut should be used [13] and [14]. Table 3.2proposes the correlation between the main tribological parameters and technological parameters specific to the grinding process.
Table 3.2.
Correlation Between Tribological and Technological Parameters [6]
Tribological parameters Technological parameters
Wear particle width bw[μm] Equivalent grinding thickness heq[μm]
Linear wear of workpiece Dhw[mm] Ground material thickness Dhw[mm]
Linear wear of wheel Dhs[mm] Radial wear Drs[mm]
Workpiece wear Vw[mm3] Material removal Vw[mm3]
Wheel wear Vs[mm3] Wheel wear Vs[mm3]
Wear time ts[s] Grinding time ts[s]
Workpiece wear rate Qw[mm3/s] Stock removal rate Qw[mm3/s]
Wheel wear rate Qs[mm3/s] Tool time T [h]
Wear ratio Φ [-] Grinding ratio G [-]
Load Fn Normal force Fn
Frictional force Ff Tangential force Ff
Friction coefficient μ[-] Force ratio μA[-]
Friction energy Wf[J] Specific grinding energy u [J/ mm3]
Table options
Tribological Interactions in Abrasive Finishing Processes
Grinding processes for brittle material include cutting (plastic deformation), plowing and rubbing/sliding. The last two mechanisms are characteristic of tribological processes. In order to understand the tribological interactions in brittle material grinding, it is necessary to investigate the interaction between the following:
•
Superabrasives-workpiece,
•
Workpiece-bond,
•
Chip-bond, and
•
Chip-workpiece.
The mechanism is complex, and it is necessary to consider the nature of the active layer of the wheel in order to understand the tribological aspects [15].
The relative motion between the wheel and workpiece constitutes a ‘friction-pair’ [16]. If grinding is to be considered as a tribological process the wear of the wheel and of the workpiece must be considered. The wheel wear is defined by Vs, and workpiece wear by Vw, which is the material removal. The grinding ratio G is a convenient indicator of the tool life and tool cost and also gives an indication of correct wheel selection for the performance required.
The dominant wear mechanism of the active abrasive layer appears to be by grain attrition, grain micro-fracture, and abrasion of the bond, rather than poor grit retention caused by weakening of the bond matrix.Figure 3.8 and Figure 3.9 show the wear flats and fatigue wear induced grain fracture caused by the large compressive stresses.
Figure 3.8.
The distribution of the grains on the active layer surface [1] and [6]
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Figure 3.9.
The distribution of the grains on the active layer surface [6] and [11]
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Figure 3.8a–c also shows a relatively uniform distribution of the grains on the active layer surface with a comet-tail effect during the grinding process. In this effect, the bond remaining behind each grain provides a buttressing which has a positive effect upon the retention of the grains in the bond material.
Fatigue wear evidenced by grain and bond fractures provides automatic self-sharpening of the wheel which helps prevent burn due to the development of wear flats. The effect is evident in Figure 3.8 and Figure 3.9. The wear flats can be seen in Figure 3.8g, h. These figures also indicate a degree of bond erosion evidenced by loss of grains.
In Figure 3.8 and Figure 3.9, we observe signs of adhesive wear, also known as transfer wear. Small particles are transferred from the work-piece surface to the top of the super-abrasive grain surface. This process is similar to the three body abrasive process but is not a predominant wear mechanism for brittle material grinding.
There are only a few previous studies that attempted to link the wear of diamond wheels with the quality of the obtained surfaces for both brittle materials and ductile material grinding.
3.3. Single point scratch tests
Single point scratch tests have been performed by pin-on-disc sliding, fly-milling, scratching and ploughing, etc. in order to understand the removal mechanisms of ceramics machining and the frictional behavior of diamond abrasives during the machining processes.
Pin-on-Disc-Sliding [17]
The friction measurement, as well as the abrasive grain wear test, is shown in Figure 3.10. The friction force is measured by a strain gauge mounted on the thin part of the steel holder. Thus, both the variation of the friction force with sliding distance and the coefficient of friction under steady state conditions can be measured. Diamond abrasive grains with a grit size of 2.0 mm are used in the test. The disc (100 to 200 mm in diameter) materials are different ceramics. The vertical load is either 2.5 or 3.5 N and the sliding speed can be varied between 5.0 and 15 m/s. A scanning electron microscope is used to examine the worn surface of the diamond abrasive grains. An abrasive grain slides in its own particular track during the measurement of friction, and each separate grain has its own track. Scatter in the coefficient of friction is small and, therefore, is determined as the average for three individual grains. The wear volume of the diamond abrasive is determined from its loss of height and the worn flat area. Wear tests on the diamond abrasives are carried out at the same speeds and normal loads as in the friction tests, where the sliding distance is almost 4000 m, but the volumetric wear is based on the average of about six experiments due to the increased scatter of the data. The groove volume of wear for the disc is also measured, using a surface profilometer.
Figure 3.10.
Measurement of frictional force [17]
Figure options
Fly-Milling [18]
Overcut fly-milling tests on diamond abrasive grains are carried out with a surface grinder and with the ceramic workpiece slightly inclined, as shown in Figure 3.11. The method is similar to that of Stanislao (1969) and Brecker (1973). When making a measurement, the metal wheel is lowered until it just touches the workpiece surface at its lowest end. As the workpiece is fed past the wheel, the depth of the groove is automatically increased, due to the slope of the work surface (but it may cycled up and down if the abrasive grain chips). Each subsequent cut increases in depth, until the grain fractures, while attrition wear of the tip occurs. The measurement of attritious wear is made by tracing across the grooves with a stylus instrument to determine the decrease in depth of cut between successive grooves. The conditions for the fly-milling tests can be chosen in accordance with surface grinding parameters. The scratched grooves and worn diamond grains can be examined by a scanning electron microscope.
Figure 3.11.
Set-up for fly-milling test with an abrasive grain [18]
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Scratching and Ploughing [19]
Figure 3.12 diagrams an apparatus used for scratching experiments. The sample S is moved horizontally underneath the diamond D by the micromanipulator Mi, which is driven by the motor Mo. The diamond is attached to the arm A of a balance. This arm is connected to the frame by a leaf spring LS, so that it can be swung up and down but remain stiff horizontally. The balance is brought to equilibrium and displaced vertically until the diamond just touches the ceramic sample; the desired load L is then applied. The tangential force on the sample is measured by the strain gauges SG on the bending element BE (Figure 3.12b). In experiments, diamonds were ground to the shape of a square pyramid; the scratches were made in the one plane as in Figure 3.13. The scratching speed was 1 μm/s.
Figure 3.12.
(a) Diagram of the apparatus. The sample S is displaced underneath the diamond D by the motor Mo and the micromanipulatorMi. The diamond is attached by the bending element BE to the balance arm A, which can be swung up and down but is stiff horizontally. After the balance has been brought to equilibrium and adjusted vertically the load L is applied. C, F coarse and fine vertical adjustment. V counterweight and damper. LS leaf springs, (b) The bending element BE with the diamond D and strain gauges SG for measuring the tangential force [19]
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Figure 3.13.
Geometry of diamond and groove; vertical longitudinal section and plan view. The diamond is a square pyramid (half apex angle θ) that moves with one plane leading. F is the force the diamond exerts on the sample, Ft is the tangential component and Fn the normal component, b is the groove width. (‘Tangential’ and ‘normal’ relate to the surface of the sample, not to the leading plane of the diamond). In the ploughing model in its simplest form is perpendicular to the