Chapter 9
ELID Grinding with Lapping Kinematics
Ahmed Bakr Khoshaim*, Zonghua Xu? and Ioan D. Marinescu?
*The University of Mecca, Saudi Arabia; ?MIME Dept., University of Toledo,
Toledo, Ohio; ?The University of Toledo, Toledo, Ohio
9.1 INTRODUCTION
Grinding is one of the most important abrasive processes. It is used to perform smooth and precise dimensions and surfaces. Usually, the material removal rate from the workpiece is low in grinding operations compared to that in milling or other cutting operations. However, it is considered effective for brittle materials. Finishing operations have a long history. Since the Stone Age, people were rubbing stones to create sharp edges. Also, the Egyptians had an amazing machining process for cutting large rocks, such as in the pyramids. Modern machining started in the nineteenth century [1].
In grinding, a grinding wheel with a number of abrasives adhered to it is used. The abrasives come in small pieces and in irregular shapes. In addition to the ability to resist chemical reaction caused by the lubricating fluid, the bond material should be strong enough to overcome the grinding forces and temperature. Many types of grinding wheels offer different configurations for different grinding operation conditions [2]. Abrasives and Wheel Types
Abrasives can be either conventional or superabrasives. Conventional wheels are cheaper than superabrasives wheels. On the other hand, superabrasives wheels are made with more expensive materials, and therefore, only a small part of the wheel is made by the superabrasives material where the rest can be made with metal. Conventional abrasives can be aluminum oxide (Al2O3) or silicon carbide (SiC). On the contrary, superabrasives can be either cubic boron nitride (CBN) or diamond. These last two are the hardest materials, which give them the ability to machine other hard materials. The hardness can be cited as Knoop hardness. For example, diamond hardness is 6500 kg/cm3, and CBN is 4500 kg/cm3. These levels are high compared to SiC and Al2O3 at 2500 and
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ELID Grinding with Lapping Kinematics Chapter | 9395 1370–2260 kg/cm3, respectively [3]. The abrasives can be either small in size for better surface-finish roughness or large in size for higher material removal rate. The abrasives have the ability to fracture into pieces. This characteristic is called friability, which makes the abrasives self-sharpening. Very high friability and very low friability are undesirable. Very high friability makes the abrasives so soft that they are quite fragile. Low friability prevents the abrasives from breaking and sharpening themselves again, which will lead the abrasives to become dull. These abrasives have been made in the industry due to the amount of impurities in natural materials, which affects the reliability. The abrasives grain sizes are to be considered small compared
to machining tools or inserts [4].
The six grinding wheel properties include the type of coating, the kind of dressing, the method and precision of balancing, the type of resin, the abrasives grit size, and the grain formula [5].
Grinding wheels come in a variety of types and sizes. They can be produced in many shapes, including straight, cylinder, straight cup, flaring cup, depressed center, and mounted. Both conventional and superabrasives wheels use different bond types. The two sheared bond types are vitrified and resinoid. A vitrified bond consists of ceramic bond mixed with the abrasives and shaped to the desired grinding wheel shape. Then, it is heated gradually to 1300 °C to allow the ceramic to create strong structure. Finally, it is cooled slowly and tested. Consequentially, in resinoid bonding, organic compounds are used and mixed with the abrasives, compressed, and shaped at a lower temperature of about 175 °C [3].
Metal-bonded grinding wheels have some advantages over the resin- and
ceramic-bonded wheels, because metal-bonded wheels have high hardness and possess high heat conduction. Also, metal-bonded wheels have a high bondage capability, which gives the wheel a high holding ability for abrasives [6]. Sometimes, if the operation undergoes vibrations, but at the same time metal bond is needed for its electrical conductivity, a metal-resin-bonded grinding wheel can be used. Usually, this line of grinding wheels has a mixture of metal and resin bonds with a ratio of metal to resin at 7:3 [7]. Other types of bonds, including silicate, rubber, elastic, epoxy, and oxychloride, are also on the market and are manufactured by select manufacturers.
Grinding wheels typically have a standard coding, or marking system, for wheels to show their properties. For instance, we may find this code on a grinding wheel: 30A 46 H 6 V XX. The A in the code indicates that the wheel is made out of aluminum oxide Al2O3 abrasives. Different letters indicate different abrasives (e.g., C = SiC, D = diamond, Z = zirconia). The number 46 is the grain size, which ranges typically from 4 (coarse) to 500 or more (fine). H is the grade, which ranges from A (soft) to Z (hard). The harder the wheel is, the better the finishing and stronger the abrasives holding, hence, the longer the wheel life. On the other hand, the softer the wheel, the faster the cutting ability. The number 6 is the structure or the number of spaces between the bond and the abrasives grains. The structure is on a scale of 1–15, where the grains are dense at 1 and open at 15, V is the bond type (vitrified) (e.g., M = metal, R = rubber,
396 Handbook of Ceramics Grinding and Polishing
S = silicate, E = elastic, B = resinoid, O = oxychloride, P = epoxy), 30 and xx are the manufactur er’s symbol for abrasives and the manufacturer’s private marking bond type [8].
Material Removal Mechanism
Generally, the materials can be classified into either ductile or brittle materi
als. Ductile materials are more electrical and thermal-conductive. Also, they usually have a low melting point and high density compared to brittle materials. They also show some differences in the fracture toughness, where it is far higher in ductile compared to brittle material. However, brittle materials usually have higher Young’s modulus and lower surface energy [9]. Also, the crack growth rate is bigger in brittle materials. The crack growth rate usually depends on the crack length, which will be explained later [10]. Brittle materials are also different from ductile materials in their atomic bonding. Atomic bonding in ductile materials normally is metallic. On the other hand, atomic bonding in the brittle materials can be either covalent or ionic, or both. Covalent bond has a higher thermal conductivity and a lower thermal coefficient of expansion than the ionic bond. Brittle materials with both types of bonding have different properties, depending on the ratio of the ionic/covalent bond in the material. For example, Al2O3 has a ratio of 6:4 ionic/covalent bond, where it is about 1:9 in SiC. This difference makes SiC preferable for use in higher-temperature applications, because the elevation of the temperature affects ionic bonding strength significantly [9].
Brittle materials are more challenging to machine than ductile materials, not only due to high hardness, but also due to easy fracture. The fracture most likely occurs in brittle materials in two ways: Either at the edges of the bond of the grains, which is called an intergranular fracture, or through the grains themselves, which also known as a transgranular fracture. Unlike brittle materials, ductile materials fracture mainly through the grains. However, another fracture type is called the chevron pattern. This type happens when the crack is spread over different levels of the material in a circular shape away from the impact. This crack can easily show the crack starting point and can be recognized by bare eyes or by microscopes [11]. Cracks in grinding ceramics can be either pulverization or microcracks. A consequence of one or both types of fracture – intergranular or transgranular – cause the pulverization crack. However, mi- crocracks can be also categorized into scattered and clustered. Highly brittle material usually is not affected by clustered microcracks [12].
During the grinding of a brittle material, the material from the workpiece is mainly removed by two general principles. When a grain on the grinding wheel comes into contact with the workpiece, a chip from the workpiece is formed. This formation can be in either a ductile mode or brittle mode. Ductile mode, which is also known as a quasiplastic cutting mechanism, is preferred over the brittle mode. During the ductile mode, the chip formation result in grooves with
ELID Grinding with Lapping Kinematics Chapter | 9397
no cracks, giving the grooves a smoother surface profile. On the other hand, brittle mode, which can be referred to as microbrittle mode, is not preferred. In this mode, the grooves created affect the surrounding area, forming surface fractures and cracks. These cracks can be one or more of the following types of cracks: lateral, radial, or median cracks. Some other cracks have also been reported, such as fork, branch, and chevron[13]. In a case of a brittle mode removal, a careful choice or modification of grinding parameters can turn this grinding process into the desirable ductile mode [10].
A stage in-between is called the brittle-ductile mode, semiductile mode, or
partial-ductile mode [14]. In this mode, the material is removed at brittle mode and does not affect the material by forming advanced cracks underneath the material workpiece surface. Still, at this stage, the surface of the workpiece reaches the limits
of high surface stresses. In order to reach this stage and not exceed it into brittle mode requires a careful choice of grinding parameters. A laser-assist can be used to preheat the surface of the workpiece before it enters the grinding process. This process helps reduce any brittle mode material removal and enhance the surface quality and roughness of the machined workpiece [10]. If the workpiece machining was in the brittle mode, this will produce a fractured surface that requires additional machining processes of lapping and polishing. Nonetheless, if the properties have been changed accordingly so that the machining process was in the semi- ductile mode, the surface of the workpiece will be partially fractured; thus, it will need direct polishing only. When we control the machining flawlessly so that the grinding is performed in the ductile mode, the finished workpiece material will undergo little or no polishing process [15].
It has been reported that the depth of cracks can be predicted. The experiment of the prediction involves grinding of a silicon wafer machined by a diamond grinding wheel. The depth of the crack is approximately half the grain size of the diamond grinding wheel. So it can be observed that smaller grain sizes have better surface finish quality [13].
Figure 9.1shows the most commonly identified cracks that can occur dur-
ing the brittle-mode cutting. Radial and lateral cracks are not as bad as a median crack. They both can increase the material removal rate and can be monitored. The median crack is critical, because it is more likely to be responsible for the failure of machinery parts in the industry when they propagate. The problem is that this crack is hidden under the surface and can be difficult to locate.
In grinding, each grain works individually to scratch the surface of the workpiece. The continuation of fast small scratches on the surface results in a material removal from the workpiece. For better removal rate, the scratches from the grains should be close to each other. One of the many factors that the scratching process depends on is the length of the tip of the abrasive grain, as well as how deep the plastic deformation applied on the workpiece is. Load and tip radius are other factors that affect the scratching on the workpiece. Inappropriate choice of these factors leads to undesirable scratches, which in
398 Handbook of Ceramics Grinding and Polishing
FIGURE 9.1 Radial, lateral, and median cracks [16].
turn lead to entering the brittle mode with either lateral or radial cracks, or both [17]. Finishing a workpiece by ductile mode with no surface fractures gives a great smooth workpiece surface that does not need any additional processes such as lapping or polishing. In order to achieve that, the depth of cut should be small so that only nanoparticles are removed, depending on the workpiece material properties. The energy required for removing a certain volume from a workpiece while in ductile mode is [18]:
p =E HV p
(9.1)
Where V p is the volume and H is the hardness. However, the energy required for removing material in the brittle mode is [18]:
E f =?A
f(9.2)
Where A f is the fractured area, and d is the crack surface energy per unit area. The hardness and d are typically the same for some materials. The estimated relationship of critical depth of cut for brittle materials with other parameters that cannot be exceeded to be in the free fracture ductile mode is [19]:
E H? ?K
H? ?2
d c ?? ?????c?(9.3)
ELID Grinding with Lapping Kinematics Chapter | 9399 Where d c is the critical depth of cut, E is Young’s modulus, and K c is the fracture toughness. By adding a constant to the formula so it can be more useful
[10]:
? ? E H? ?
K H? ?2
d c=0.15?????c?(9.3a)
Therefore, the maximum grit depth of cut or chip thickness should be less than the critical depth of cut [20]:
h max=2L s? ??V V w
c? ??a d e s(9.4)
Where L s is the distance between the adjacent grits, V w is the work speed (rpm), V c is the peripheral wheel speed (rpm), a e is the wheel depth of cut (m m), and d s is the diameter of the wheel (mm).
Though difficult to evaluate, the crack length can be calculated from equation (9.5) [9]:
2
?? E H?
?3
lc= ? ?( 0.034 cot?)23
? ??? ??? ?F23(9.5)
? ?K? ?i
? ??c
? ???
Where ? is the indenter angle, and F i is the indenter load. Then the estimated critical force applied before a crack forms is [9]:
√
K H
4
F critical=c
(9.6)
3
Where √ is a constant that depends on the indenter geometry, which can lead to calculate the depth of the indenter as [21]:
1?1?1
h= (cot?)3? ?E H2
? ?F i2(9.7)
??
However, the maximum chip thickness on ELID grinding, which will be explained in detail later, is disturbed by the bond strength of the grinding wheel. Consequently, it can be written as [19]:
h max=2kL s? ??
V V w c? ??a d e s(9.8)
400 Handbook of Ceramics Grinding and Polishing
Where k is the ELID dressing constant that is proportional to the input power, voltage, and current duty ratio:
,,k I VR p c
With the help of these expressions, the holding and grinding forces for a single grit can be obtained as [19]:
f h =k1?s
a g(9.9)
f g=k Sh 2
max(9.10)
Where f h is the holding force, k1 a constant related to the wheel topography, σs is the yield strength of the layer, a g is the holding area of grit, f g is the grind-ing force, k2 is a constant related to the material properties, and S is the sharpness factor.
With knowing the approximate number of grits per unit area, we can evaluate the total holding force as [19]:
=F NfA h
h g(9.11)
Where F h is the total holding force, N is the number of grits per unit area, f h is the holding force per grit, and A g is the grinding area (mm).
Furthermore, the total normal and tangential force can be anticipated from resolving the force per grit:
=F N n
?fA n g g(9.12)
t = F N?
fA t g g(9.13) Where a n is the normal force component of f g, and b t is the tangential force component of f g[19].
The normal force can be obtained also from its relationship with the actual depth of cut (ADOC) [22].
F F n = +0
Cd a a(9.14)
Where F0 is the break-in force while ADOC is zero, C a is a constant that depends on grinding conditions, and d a is the ADOC, which can be expressed in terms of the number of grinding passes. For the i th pass, the ADOC is [20]:
d a= d0? ?? ?1 ?
?k k w? ??i? ?? ?(9.15)
?w +k s?
ELID Grinding with Lapping Kinematics Chapter | 9401 Where d0is the wheel ADOC, k w is the cutting stiffness, and k s is the machine stiffness. By substituting the equation in the normal force equation, the normal force can be realized after the i th pass as [22]:
F F n = +0Ca a0? ?? ?
1 ? ?k k k w? ??i
? ?? ?(9.16)
?w +s?
This equation shows that with time and increasing the number of passes, the normal forces should stabilize and become constant. The same is true with increasing the number of passes, k [ /(+)]
becomes zero for k w and k s, not negative, which will make the final normal force when its becoming constant as:w k w k s
= +(9.17) As F0, C a, and d a are constant for individual grinding conditions [22].
F F n 0Cd a a
Grinding Mechanism
The abrasives are spread in the wheel randomly, and therefore have different rake angles, which is the angle of the face of the tool with the workpiece. These angles can be positive, zero, or negative. The rake angles usually are negative at about –60°. The wheel surface speed in conventional grinding is high compared to other operations, and it is typically from 20 to 45 m/s. However, in high- speed grinding, the speed can reach up to 150 m/s [23].
In grinding, each abrasive works as an individual cutter, as in single-point machining. However, they are different in many ways. For instance, the abrasive grains are arbitrarily separated, and each grain has an irregular shape. In addition to the highly negative rake angle and the high surface speed compared to the single-point machining during grinding, not all grains are active at all times
[2].
Several parameters of the grinding operation can be observed by the following relationships [2]:
Undeformed chip length:
=(9.18) Where D is the diameter of the wheel, and d is the depth of cut.
Here, we l Dd
are estimating this function, as d/D is small. In most operations, when d = 0.1D, the expression (9.19)will have an error of about 1%. Therefore, it is still a good approximation for the contact length [23].
Moreover, the undeformed depth of cut or chip thickness can be calculated as:
t=? ??4v VC r t n
? ??d D? ??? ??
(9.19)
402 Handbook of Ceramics Grinding and Polishing
Where v is the speed of the workpiece, V t is the rotational speed of the wheel, C n is the number of cutting points per unit area, r is the chip width over the aver-age undeformed chip thickness [2].
The general equation for the maximum undeformed chip thickness is [24]:
? ?0.548
? ?? ?
t m ax=?? ?E E1??4v VC r? ?
? d D? ???(9.20)
2t n
Where E1 and E2are the Young’s modulus of both the grinding wheel and the workpiece, respectively.
The parameter C n can be estimated in the range of 0.1–10 /mm2, which is
102–103/in.2). It can also be estimated as [24]:
4f
C n=(9.21)
( )2
d2g 4 ?/ 3f3
Where d g is the equivalent spherical diameter of diamond grains, f is the friction, and
is the volume fraction.
The parameter r can also be estimated between 10 and 20. h e is the chip f
equivalent and can be calculated as:
?f d
h e=
(9.22)
V t
Figure 9.2 illustrates these parameters for straight grinding wheel [2].
The tangential force of the grinding wheel is proportional to the grinding parameters, and it is smaller in comparison to the forces on other machining operations, due to the small active area of the grains. It is important to what energy is being dissipated during producing a grinding chip. This energy is called specific energy and is used to overcome the friction, plowing, and chip formation.
FIGURE 9.2 Grinding parameters [2].
ELID Grinding with Lapping Kinematics Chapter | 9403 This energy requirement can be found on tables as the required energy per unit volume of the material removed from the workpiece. Another two important parameters during grinding are the normal force and the cutting force. The normal force, which is also known as the thrust force (F n), depends on the speed and the material removal rate [21]:
.
Kv
Q
F n=
(9.23)
w
The combination of the sharpness of the grains and the material being machined gives a constant known as the abradability constant (K). This force can be estimated to be greater than the cutting force by 30%. The cutting force, which is tangential to the wheel, can be calculated using this formula [2]:
P
F c=
(9.24)
2 D Nn
2
Where P is the power and Nn is the speed. This function can be directed using the function:
=P T?(9.25) Where T is the torque and is equal to:
=T F c
D 2(9.26)
And ω is the rotational speed:
=? 2
Nn(9.27)
The rotational speed is measured in radians per minute [2].
By assuming that the grains are spaced equally in the wheel, we can calcu-
late the feed per grain, which is [21]:
=s v T.gL
(9.28)
Where T gL is the time of the grain first engages to the next engagement and can be calculated as [23]:
T gL=L V t
(9.29)
404 Handbook of Ceramics Grinding and Polishing
Where L is the spacing between the cutting edges. The feed per grain can be rewritten as:
=s L v V t(9.30) The calculated depth of cut is usually larger than the real depth of cut and that is due to the deflection of the wheel, workpiece, and the machine when the contact occurs. Therefore, we can calculate the depth of cut from the equation for the material removal rate (MRR). The material removal rate equation is [3]:
=..Q d b v(9.31)
w w
Where b w is the width. The depth of cut can be written as:
d=Q bv w
w(9.32)
In cylindrical grinding, if the workpiece speed is v y and the wheel wear is negligible, the depth of cut can be expressed as:
d v n y= =w
d w v v y(9.33)
Where n w is the rotational speed of the workpiece and d w is the diameter of the workpiece, and can lead to the expression of the material removal rate for cylindrical grinding [23]:
Q w = b vd
w f w(9.34) The number of passes can also be used to express the material removal rate as [22]:
Q w = b vd w f0? ?? ?1
? ?k k w? ??i? ?? ?(9.35)
?w +k s?
Finally, it has been determined that the total material removed by abrasive wear is also related to the lateral cracks size under the scratch line. A mathematical equation that describes the relation has been derived by Evans and Marshall [25]:
94
?8( / F E H)5
V l= c n
(9.36)
? ??5H
K8c
? ??
ELID Grinding with Lapping Kinematics Chapter | 9405
Where V l is the volume of material removed per unit scratch length, d c is
a constant that depends on the material, F n is the normal force or load, and E, H, and K c are Young’s modulus, hardness, and fracture toughness, respec-tively. Jahanmir and Hockin showed the importance of including the grain size and microstructure properties of a brittle material in the removal model, which should also include the lateral cracks influence underneath the surface. The relationship between the lateral cracks and the material removal rate also depends on several conditions. For example, the crack can be strongly influenced if the grain boundary is weak. On the other hand, for polycrystalline ceramics, the relationship has been proven to be obscure. By studying the removal for repeated single-point scratches, the mechanism of material removal rate for ceramics can be observed [25].
Grinding forces can be expressed from the cutting model of grain edge as a combination of cutting components and friction components for both the normal and tangential force [26]:
F n= cutting n+friction n =
?k a p m +k a n t(9.37)
F t = cutting t+friction t = k a
p m +∝k a n t(9.38)
Where k p, k n, l, m, a m, and a t can be defined, respectively, as the specific cutting force of the workpiece, the yield compression strength of the workpiece, the ratio of two components of force for the chip, the friction coefficient, cutting area of the grain edge, and the wear flatness area. The cutting area of the grain edge can be calculated as:
1?t
d
a m=(9.39)
n V?t e
Where n? is the density of grain edge [26].
A series of papers were published about fine grinding with mathematical modeling and experiments. One of the experiments has proved a predicted mathematical model of the grinding lines on a workpiece machined by a face-cup grinding wheel. It demonstrates a relationship between the ratio of the chuck, or spindle, and wheel speeds, and the distance between lines on the workpiece grounded. The relationship illustrates that increasing the ratio will increase the distance between lines. These lines are actually parallel curves, which are also affected by the ratio in their curvature [27].
Grinding Wheel: Close View
With a close view of the grinding wheel during operation, it was noticed that the grains interact with the surface of the workpiece mainly by cutting,
406 Handbook of Ceramics Grinding and Polishing
FIGURE 9.3 Grains intact with the workpiece
plowing, and rubbing. Plowing and rubbing are undesired outcomes and will
result in affecting the workpiece and increase the roughness. Plowing occurs
when a grain slides over the surface without forming a chip, causing a surface change. Rubbing occurs when a loaded workpiece material is on the grinding wheel. Even though the grinding wheels contain a much harder material than the workpiece, wear begins immediately after starting the operation (Figure 9.3).
The best results from grinding occur when the rate of material removed from the workpiece is maximized, and the wear of the grinding wheel is reduced. In other words, increasing the grinding ratio of the wheel is accompanied by smooth surfaces and precise dimensions.
The wear that occurs to the grinding wheel can be classified in three mechanisms: attritious grain wear, grain fracture, and bond fracture. The attritious grain wear is similar to the flank wear in cutting tools. It occurs when the grains become dull, resulting in a wear-flat, where the grain slides over the workpiece
without removing material and causes an undesired surface finish and high temperature. It is helpful to reduce the attritious wear by carefully choosing the material of the workpiece and the grinding wheel. Attritious wear becomes low if the workpiece and wheel materials are chemically inert. However, grain fracture can help the dull grains to be sharpened. The idea of grain fracturing is known as friability. Friability is useful, as long as it is in a moderate rate, so new grains are always and continuously presented. Bond fracture also plays an important character in the process of grinding. The bond needs to be chosen based on the material to be ground. For example, grinding a hard material needs a soft bond so it can reduce high temperature and residual stresses on the workpiece. Nevertheless, the bond should not be too soft or weak, which will make dislodging the grains too easy and result in an increase of the wheel wear rate. As a consequence, it will be hard to maintain dimensional accuracy
in addition to an increase of the workpiece production cost. On the other hand, grinding a soft material needs a hard bond to increase the material removal rate. Nonetheless, the bond should not be too hard, which prevents the dull grains from being dislodged and replaced with new grains. As a result, the grinding process would not be sufficient.
ELID Grinding with Lapping Kinematics Chapter | 9407
The Grinding Ratio
Because wheel wear is something that cannot be eliminated, the reliability of this wheel can be estimated by calculating the grinding ratio (G-ratio), which is the ratio of the volume removed from the workpiece to the volume of the tool wear [28].
V workpiece material removed
G (9.40)
V tool wear
The G-ratio can be low at 1 or high at 1000. Both are considered not good. For instance, the low grinding ratio indicates that the tool wear is too high, which can be expensive and economically inefficient. On the other hand, the high grinding ratio shows that the wheel is too hard for the workpiece material, which can cause an increase in the forces and lead to poor surface texture and vibration.
The value of the G-ratio depends on the grinding wheel, workpiece materials, and the fluid used. Using efficient fluid can increase the G-ratio significantly, which increases the wheel life and accordingly reduces the cost. Also, the G-ratio is not a parameter of the wheel, as the same wheel can have a high or low G-ratio. The G-ratio can be determined by the other parameters, such as speed of the wheel, depth of cut, and pressure, in addition to the workpiece material and fluid [2].
Grinding Types
Among the many types of grinding, the most common are surface grinding, cylindrical grinding, internal grinding, and centerless grinding. Surface grinding is used to grind flat surfaces. The grinding wheel can be horizontal or parallel (vertical). Vertical wheel type uses a rotary table and can grind multiple workpieces at the same time. Horizontal wheels can travel across the direction of the workpiece, which is called traverse grinding, or travel along a groove in the workpiece in an operation is called plunge grinding. Figure 9.4 demonstrates the possibilities [2].
Horizontal grinding wheels can also be used with rotary tables, where it can grind multiple workpieces at once. Also, vertical wheels can work with reciprocating tables [29].
The feed rate on the grinding operation can be either traverse or plunge. In the traverse feed mode, the grinding wheel feed occurs in steps at the end of each grinding wheel pass. Conversely, in the plunge mode, the feed is considered continuous along the pass of the grinding wheel. Figure 9.5 demonstrates the differences.
Additionally, cylindrical grinding is for producing parts used in the auto industry, such as crankshafts, spindles, or pins. The workpiece is mounted from its axial ends. Both the workpiece and the grinding wheel rotate at different speeds
408 Handbook of Ceramics Grinding and Polishing
FIGURE 9.4 Surface grinding types [2].
from two different motors. Cylindrical grinding can be straight, by mounting the workpiece parallel to the grinding wheel. Cylindrical grinding can also be curved or steep, and can be performed to produce different shapes [2].
Moreover, the internal grinding has the same phenomenon as cylindrical grinding, except that it is for internal rotary parts. The internal grinding is a high- speed operation, because the grinding wheel rotates at a speed of 30,000 rpm or FIGURE 9.5 Traverse and plunge feed modes [28].
ELID Grinding with Lapping Kinematics Chapter | 9409 even more. Internal grinding can be one of three types: traverse grinding, plunge grinding, and profile grinding [2].
Finally, centerless grinding is typically the same as cylindrical grinding. However, as implied by its name, the workpiece is not mounted by its axial centers. This method is recommended for mass production and when small workpiece diameters are desired, such as engine valves, camshafts, pins, and any other similar component [23].
Other grinding types such as creep feed grinding are also performed. This type has the same kinematics of surface grinding, but also has a unique distinction as it removes a high amount of material from the workpiece. To help achieve the high removal rate, the workpiece speed should be low with a high- power grinding machine [3]. Usually the surface finish of the workpiece is lower than the other types of grinding, as finishing is not as important as the amount of material to be removed. In creep feed grinding, the depth of cut can be high; in some applications it can be up to 15 mm [2].
If a good surface finish is desirable for a workpiece that is machined by creep feed, an additional operation can be done to improve the surface finish. Usually a method called sparkout is used. The sparkout method uses no depth of cut. It is performed by barely touching the workpiece. This operation uses no coolant fluid, because the heat is desired to melt the external surface to smooth it. Spar- kout operations become stable after three to four passes [28]. Some operations are used only for finishing operations. One of them is called belt grinding. Belt grinding can replace the traditional grinding in the finishing operations. It uses a belt with abrasives in a grit range of 16–1500 and can rotate at different speeds. To avoid vibrations and to achieve highly accurate dimensions, belt grinding machines necessitate being rigid [2].
Fine grinding is one of the grinding operations that uses the face
of the grinding wheel, but also uses the kinematics of a lapping operation. Fine grinding has a constant pressure between the workpiece and the grinding wheel [7]. This project will focus solely on this operation and will be explained extensively later on. Despite the fact that conventional grinding undergoes high speeds and thus high temperatures, fine grinding, or grinding with lapping kinematics, is performed with low speeds and a much lower temperature, which will prevent any workpiece surface thermal damages. Fine grinding is performed on a lapping machine with bonded grinding wheels, providing advantages over the lapping process, including more speed, better accuracy, and cleaner parts.
Fine grinding is faster than the lapping operation, which will reduce the operating cost. This operation can be considered lapping with bonded abrasives, too [30]. Reducing the operating time not only reduces the power consumption, but also the labor operating time. Fine grinding with the correct choice of wheel and grinding parameters can give a precise dimensional accuracy and reduce the finishing operation time. In addition, fine grinding uses coolant fluid for cleaning and removing swarf to prevent loading. It does not use loose abrasives or slurry during the operation, which gives a cleaner overall environment.
410 Handbook of Ceramics Grinding and Polishing
FIGURE 9.6 ELID grinding mechanism [31].
Moreover, fine grinding can be automated. Automation is becoming widespread to reduce the cost of labor and for more accurate results. Lapping will need additional cleaning operation, because these workpieces are not as clean as the ones from fine grinding [32]. Fine grinding can be either single side or double side. In this project, single side is used as shown in Figure 9.6.
9.2 FUNDAMENTALS OF ELID
The ELID process can eliminate the use of lapping or polishing operations as a final stage. In addition, ELID grinding can provide better accuracy even though the grinding wheel wear is considered high when machining ceramics [6]. However, flatness is good in lapping compared to grinding operations, while some researchers claimed that ELID grinding with lapping kinematics, whether single or double side, can give similar flatness and reduced waviness in lapping operations. This approach can prevent the disadvantage of lapping when loose particles are dosed exceedingly in the operation, which is not economically appropriate, especially when using superhard abrasives [33].
Machining operations consist of inputs and outputs. The output is more important; we tend to change the inputs in order to gain certain outputs. Surface integrity is one of the most important outputs. Surface integrity output can have multiple measurements including the surface finish and freedom from cracks, chemical change, adverse (tensile) residual stress, and thermal damages such as burn, transformation, or overtempering. Surface finish is by far the most important of them all. Surface finish roughness has many values including R a, R v, R t, and R q. The most important measurements are R a and R t, where R a is the average peak-to-valley distance, and R t is the maximum roughness signal of the profile [34].
History
This process of dressing method is used for metal-bond wheels, and it is considered a new technology discovered in 1985 by Murata. The process has been