Chapter 4 – Grinding
• Osamu Ohnishi*
• Hirofumi Suzuki†
• Eckart Uhlmann§,
• Nikolas Schröer§,
• Christoph Sammler§
• Günter Spur
• Michael Weismiller**
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doi:10.1016/B978-1-4557-7858-4.00004-2
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Abstract
A material wherein abrasive grains are bonded together by a bonding material is known as a whetstone. The whetstone comprises abrasive grains, a bonding material, and pores. The abrasive grains play the role of a cutting edge, the bonding material fixes and supports the grains, and the pores act as a chip pocket to help discharge of cutting chips. In whetstones where the bonding material is a resinoid bond or metal bond, there are not normally any pores.
The process whereby a grinding wheel which is a circular whetstone is rotated, and the surface of a workpiece is gradually ground down by the abrasive grains on the grinding wheel, is referred to as grinding. Grinding can produce very high shape accuracy and dimensional precision even with hard workpieces like ceramics, or permit a surface with a satisfactory roughness to be obtained, and is therefore an extremely important processing technique.
Keywords
• Theory of Grinding;
• Characteristics;
• Grinding tools;
• Grinding Wheel Design;
• Bonds materials;
• Cores;
• Wheel Description;
• Diamond Grit Type;
• Cooling Lubricants
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4.1. Fundamentals of grinding
Osamu Ohnishi
Introduction
A structure in which abrasive grains are bonded together by a bonding material is known as a whetstone.Figure 4.1 shows the general structure of a whetstone. As can be seen from the diagram, the whetstone comprises abrasive grains, a bonding material, and pores. The abrasive grains play the role of a cutting edge, the bonding material fixes and supports the grains, and the pores act as a chip pocket to help discharge of cutting chips. Whetstones in which the bonding material is a resinoid bond or metal bond normally do not have any pores.
Figure 4.1.
General structure of whetstone
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When a grinding wheel (or circular whetstone) is rotated and the surface of a workpiece is gradually ground down by the abrasive grains of the grinding wheel, the process is referred to as grinding. Grinding can produce high shape accuracy and dimensional precision even with hard workpieces such as ceramics. It can also permit a surface with a satisfactory roughness to be obtained and is therefore an extremely important processing technique.
As shown in Figure 4.2, various types of grinding can be performed, such as cylindrical grinding of the outer surface of a cylindrical workpiece, internal grinding of the inner surface of a cylinder, surface grinding of a flat face, and centerless grinding of workpiece without using a chuck to hold the workpiece.
Figure 4.2.
Typical types of grinding. (a) Cylindrical grinding, (b) internal grinding, (c) surface grinding (d) centerless grinding
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When grinding is performed, as shown in Figure 4.3, grinding performance will deteriorate if the surface of the grinding wheel becomes clogged with chips (known as loading), the tips of the abrasive grains wear down (known as dulling), or excessive numbers of abrasive grains fall off the surface of the grinding wheel (known as shedding) [1]. When cutting performance has declined due to loading or dulling, the dressing is performed to remove chips from the clogged surface or worn abrasive grains and to recover the cutting performance of the abrasive grains. Also, when the grinding wheel is mounted on a spindle, truing is performed by adjusting the shape of the grinding wheel to eliminate run out of the grinding wheel surface. Dressing and truing are extremely important in order to perform grinding work with high precision. If the grinding conditions and grinding wheel are chosen appropriately, a self-dressing process will occurs [2].
Figure 4.3.
Undesirable grinding conditions. (a) Loading, (b) dulling, (c) shedding
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Theory of Grinding
Cutting and grinding are mechanical processes that remove unnecessary parts of a material by sinking the cutting edge of a tool into a workpiece. In cutting, machining is performed by a cutting tool that has a cutting edge of the desired shape, whereas in grinding, machining is performed by large numbers of abrasive grains scattered on the outer surface of the grinding wheel. These particles do not have orderly shapes, and as shown in Figure 4.4, the rake angle of the grain cutting edge has a large negative value. In addition, although the abrasive grains are fixed by a bonding material, they wear down, chip, and fall off during machining, so machining conditions are not constant. At the same time, grinding can make many fine cuts that can be made at high speed, but the work surface is easily damaged by heat as a result. To better understand grinding, we shall discuss some essential basic theory in the same way as many other works [3] and [4].
Figure 4.4.
Conditions of cutting edge. (a) Cutting, (b) grinding
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Figure 4.5 shows a model diagram of grinding. Here, let the circumferential speed of the grinding wheel be V, the workpiece speed be v, the depth of cut of the abrasive grains be a, and the diameter of the grinding wheel be D. Let us consider abrasive grains X1, X2 on the grinding wheel. Assume that after particle X1 has cut along an arc ABC, particle X2 cuts along an arc DEF. In a strict sense, the path of the abrasive grains is a trochoid curve (see Figure 4.6, but because V ≫ v, it may be considered as an arc. Assume the center of the arc ABC is O1 and the center of the arc DEF is O2. At this time, let g (= length of line CG) be the maximum grain depth of cut. Also, let l (= length of arc DEF) be the grain cutting length. The arc DE is fairly short, so the grain cutting length l may be calculated as arc EF.
Figure 4.5.
Grinding model
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Figure 4.6.
Locus of an abrasive grain
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If the angle EO2F is α:
equation(4.1)
cosα=(D−2a)/D
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If α is small:
equation(4.2)
cosα≈1−α2/2
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Substituting equation 4.2 into equation 4.1, and solving for α:
equation(4.3)
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Therefore, the abrasive grain grinding length l is given by:
equation(4.4)
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The volume (Vr) of chips discharged in unit time is:
equation(4.5)
Vr=vba
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Here, b is the grinding width. If the average grain distance is w, the number Ng of abrasive grains required to remove Vr is:
equation(4.6)
Ng=Vb/w2
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Therefore, the volume (Vs) of chips removed by one abrasive grain, from equation (4.5) and equation (4.6), is:
equation(4.7)
Vs=Vr/Ng=vaw2/V
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Therefore, the mean area of chip section Am is:
equation(4.8)
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The grinding force acting on one abrasive grain depends on the mean area of chip section. Consequently, when the mean area of chip section increases, the grinding force also increases, which leads to problems. When problems arise, we should therefore consider how to adjust the values of the parameters on the right-hand side of equation (4.8).
Characteristics of Ceramic Grinding
Among ceramics, fine or advanced ceramics that have superior engineering characteristics are used for various types of instruments and parts. Fine ceramics are extremely hard and difficult to machine. In grinding fine ceramics, the material is destroyed because it is brittle; that is, it is brittle mode grinding, so it is important to perform machining without giving rise to cracks in the material, and to machine with high performance.
We shall consider ductile mode grinding for removing even brittle material by plastic deformation without cracks [5] and [6]. To perform ductile mode grinding, the maximum grain depth of cut g should always be less than the critical depth of the cut.
Here, let us consider the maximum grain depth of cut with reference to Figure 4.5. The number of abrasive grains (Ns) on the same circumference is:
equation(4.9)
Ns=πD/w
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On the other hand, the workpiece speed (v) may be expressed as follows in terms of the feed (f) per abrasive grain, and the rotational speed of grinding (n):
equation(4.10)
v=Nsfn
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The circumferential speed (V) of the grinding wheel is:
equation(4.11)
V=πDn
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Solving (4.9), (4.10) and (4.11) for f results in:
equation(4.12)
f=wv/V
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The line O1O2 is equal to the value of f. If the angle EO2C is β.
equation(4.13)
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The following is also true:
equation(4.14)
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Normally, V ≫ v, D ≫ g, D ≫ a, so the maximum grain depth of cut g is given by the following equation:
equation(4.15)
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When we wish to make the value of g approach the critical value, we should consider how to adjust the parameters on the right-hand side of equation (4.15).
In rough grinding, because the presence of cracks arising in the workpiece is not a great concern, brittle mode grinding is performed at a high grinding speed. On the other hand, the need to avoid cracks in the workpiece and to reduce the surface roughness in finishing grinding indicates that ductile mode grinding may be performed.
4.2. Grinding tools