General Principles of Grinding with Lapping Kinematics
Even though different from the present project, double-side lap grinding usually takes place with two wheels where the workpiece is guided between them with a workpiece holder, and it is free to align itself. In addition, the flatness of the wheels is maintained precisely, and the pressure applied to the workpiece by the wheel causes the cutting action. A flood of coolant is always necessary to lubricate, cool, and clean the workpiece. Conversely, the present project uses the same principles except that the cutting medium is only one flat grinding wheel, and the workpiece is held stationary in the workpiece holder (see Figure 9.8). Lap grinding is useful to correct many properties including flatness, parallelism, surface roughness, and dimensional accuracy [57].
Figure 9.8.
Single side ELID grinding
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Lap Grinding Modes
Several modes of lap grinding are used in the industry such as batch-mode processing. Batch-mode processing is useful for mass production where the machine is loaded with the maximum number of workpieces. They are all machined at the same time, which ensures equal thickness machining from each workpiece, and almost the identical roughness and material removal rate. Another mode is called through-feed mode. In this mode, the workpiece passes through the wheels only once. This method is helpful for small workpieces. If more material needs to be removed, multiple passes can be applied [57].
In general, dressing takes place when a hard ceramic or diamond dresser abrades with the grinding wheel. Therefore, because the wheels used in this project are made of diamond and with small grid sizes, traditional methods cannot be used for dressing. Other methods can be used such as abrasive-jet, slurry dressing, and high-pressure water jet. Some of the new methods for dressing are performed during the grinding operation, such as electrodischarge machining or laser dressing, in addition to ELID [58] (Figure 9.9).
Figure 9.9.
Single-side grinding with lapping kinematics
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Cutting Fluids
In general, grinding fluids are used during grinding for three reasons. First, the most important reason is to lower the temperature, which is increased by the high speed of the grinding wheel. We tend to reduce the heat to keep it from burning the workpiece and affecting the tool life by increasing the wear. The second reason is for lubrication. It is better to lubricate the contact area between the wheel and the workpiece. The lubrication helps in averting any chips that might stick to the grinding wheel, and hence prevent chip loading. The third reason is cleaning, where the fluid helps to flush chips from the grinding area, and remove swarf from the workpiece. In addition to these reasons, grinding fluids also help in controlling the dust from the grinding operation, which is good for keeping the environment cleaner and safer for operators [8]. Cutting fluids also reduce energy consumption by reducing the friction between the tool and the workpiece.
Usually, high-speed operations are in need of more cooling-capability fluid. On the other hand, low-speed operations need more lubrication but do not need cooling capability [34].
Different grinding fluids are suitable for different grinding operations. Of the four types of grinding fluids, the first is water-soluble chemical, also called synthetic fluid. This type has great temperature reduction capability, but it lacks lubrication. It is useful for high-temperature grinding conditions and has great transparency. However, synthetic fluids can cause damage to the grinding wheel and workpiece due to rusting. The second type is straight or neat oil. This type is the opposite of water-soluble fluids. It has a great lubrication capability but lack of heat reduction. It is typically used for thread or heavy form grinding. The third type is water-soluble oil or emulsion, which combines the first two types. It has better lubrication than water-soluble chemical and better cooling than straight oil. It usually comes in a milky color and needs to be treated against bacterial growth. The last type is called semisynthetic grinding fluid. This new type uses a synthetic fluid with some additives. It has good lubrication and good cooling capability [34].
Some additives can be added to the semisynthetic, emulsion, or neat oils to enhance the performance of the fluid. For example, adding sulphur or chlorine to the fluid helps reduce the forces and tool wear by creating a low shear–strength film between the tool and the workpiece. They react with the tool material for this creation at high temperature [59].
Cutting fluids can be applied primarily in one of three areas (directions) in the high-speed orthogonal cutting. It can be applied on the back of the chip. This method, according to Taylor, can increase tool life up to 40%. In addition, cutting fluids can be applied on the area between the tool and the finished workpiece. This method is useful for heavy cuts. With last method, cutting fluids are applied between the tool and the chip. This method has less support than the other two even though it looks better for cooling the buildup chips area. Many researchers say that the best direction for cooling is in the direction of the tool, which is between the tool and the finished area. Figure 9.10 illustrates the possibilities [34].
Figure 9.10.
Cutting fluid directions [34].
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In modern industry, cutting fluid can be applied by cryogenic cooling. This method focuses on liquid nitrogen sprayed with two nozzles. Although environmentally friendly, this method is quite dangerous for the operator. Depending on the workpiece material and other parameters, the cryogenic cooling system can reduce the temperature by 34% and nearly 30% in cutting forces. This method sometimes uses CO2 instead of N2.
The increase of cutting with environmentally friendly operation guided some researchers to a new method called dry machining. In this method, no cutting fluids are presented. However, this method is limited to certain materials, and swarf control can be a problem. Some solutions were offered, but still the limitations exist [34].
Fluid application is accomplished by different methods including pouring the fluid, spraying it, or using high-velocity or low-velocity nozzles [8].
A newer application method, known as minimum quantity lubrication (MQL), uses nanoparticles of fluids and has a good success in milling and drilling. Nevertheless, it is still limited for grinding operations [60]. This method gives higher wheel wear and higher roughness [4].