Conditioning
Because the grinding wheel will experience different kinds of wear during operation, we try to minimize wear by applying conditioning operations. Some of the operations are performed before starting the grinding process, and some are applied after the process proceeds. The operations that occur before the grinding process are called preparation and include ringing, mounting, and balancing. Other conditioning processes include cleaning-up, trueing, and dressing [23].
Ringing restores the integrity of the surface of the grinding wheel. The mounting process clamps and unclamps both the wheel and the workpiece to the grinding machine. Balancing makes sure that the wheel does not have any extra weight in any part of it that could generate a moment of inertia. The balancing process is especially recommended for new wheels [23].
Conventional Dressing
Dressing conditions the worn grains, especially grains that have attritious wear and dull grains. With conditioning, the grains gain new sharp edges, which increase their efficiency. Dressing is also applied when the wheel has loading. Loading occurs when part of the material removed sticks into the wheel affecting its accuracy, surface finish quality, and the heat generated during grinding. Loading usually happens from grinding soft materials or when grinding wheel type and material to be ground are not compatible. The dressing operation is performed using a hard diamond-point or diamond-cluster tool to remove a thin layer of the grinding wheel by traveling across the wheel while rotating. The diamond used in the dressing tool can be one of many types including the natural diamond (ND), broken natural diamond, synthetic diamond (SD), synthetic monocrystalline diamond (MCD), diamond chemical vapor deposition (CVD), and polycrystalline diamond (PCD) [23].
When a grinding wheel becomes dull or when it is loaded, it should be dressed by one of the dressing methods. The conventional method uses a diamond point or cluster dresser that travels across the wheel while the wheel is rotating, with a feed rate of Fd. The active area of the dresser width is dww. Because dressing plays a major role in the grinding operation, we can estimate the influence by calculating the overlap factor:
equation(9.51)
Turn MathJaxon
Increasing the overlap factor will decrease the removal rate, but will give better roughness [61].
If the wheel is typically used in wet grinding, wet dressing is required. However, if the wheel is used in dry grinding, dry dressing is recommended. Dressing hard wheels requires different methods including a set of star-shaped steel disks or one of the nonconventional techniques. Moreover, dressing metal-bonded diamond wheels require the use of one of the nonconventional technologies. The first contact of the dressing with the wheel is extremely important. Therefore, advanced methods are used, including acoustic-emission sensors, vibration sensors, and power monitors. Nowadays, the dressing can take place during grinding to keep the wheel in good shape continuously by using computerized machines that monitor the condition of the wheel and act accordingly [2].
Trueing is used when the wheel loses its round shape. The process will return the wheel to its original round shape. It is necessary to perform trueing if the wheel has been balanced. For hard wheels, both dressing and trueing can be performed together in one operation. Nonetheless, trueing and dressing soft wheels need to be done separately. These operations are called conventional technologies [2].
In addition to the conventional operations of wheel conditioning, several nonconventional technologies for wheel conditioning enhance the wheel condition and involve the wheel in electrical, chemical, or physical operations.
Nonconventional Dressing Technologies
Many nonconventional techniques have been discovered to enhance the grinding process. The nonconventional technologies include many types, such as electrodischarge dressing (EDD), laser dressing, electrochemical dressing, and electrolytic in-process dressing (ELID). These processes may take place during the grinding process and do not necessitate stopping the operation. Moreover, the nonconventional operations combine both trueing and dressing at the same time [23].
Conventional grinding wheel dressing using a diamond dresser is purely mechanical, but is not efficient because it does not give a precise dressing and because it has a low-life dresser due to its high wear, especially in dressing superabrasive wheels. Other dressing methods such as electric spark eroding are more precise and efficient. Still, these processes are more costly due to the need for extra equipments, and the complexity of the process makes it difficult to be extensively used [6].
Laser Trueing and Dressing
Laser trueing and dressing was first introduced on CBN wheels by Westkamper. The idea is to use laser irradiation on the grinding wheel, which will soften the bond of the wheel. Then, it can be easily removed by a regular dresser. However, this operation has a huge risk of thermal or grit damages including graphitization of the diamond grains. Also, in the case of metal grinding wheels, this process could cause bond resolidification, in addition to the high initial cost of the laser dresser. Using an Nd:YAG laser dressing has reportedly eliminated these problems [52].
Electrodischarge Dressing
Electrodischarge dressing (EDD) can eliminate the runout trueing. Several improvements of the process have been discovered including electrocontact discharge dressing (ECDD) and dry electrodischarge dressing (dry-EDD). In the ECDD process, two electrodes are used in contact with the grinding wheel to help reduce thermal erosion. Dry-EDD uses one electrode with direct contact with the grinding wheel. However, graphitization has been reported with these methods [52].
Electrochemical Dressing
Electrochemical discharge machining (ECDM) was introduced after the foundation of ELID dressing. They exhibit similar outcomes. Some researchers have reported the creation of the oxide layer by using two electrodes with AC power supply [52].
9.3. Kinematics of the ELID grinding with lapping kinematics
The study will be conducted on the Melchiorre Machine (210-3P). This machine has two lower wheels and one upper spindle. Because this experiment is for a single-side grinding, some modifications needed to be implemented.
Trajectory
Part of the research is to understand the path the grain of the grinding wheel. In order to check an arbitrary point on the workpiece, we need to draw the trajectory of these points relative to both the workpiece and the grains on the grinding wheel. We can calculate its position by calculating its absolute coordinates and its relative coordinates as a function of time. In the present project, both the grinding wheel and spindle are rotating at different speeds and directions. Taking each case separately will ease the understanding of the final trajectory. The dimensions of the grinding wheel used are 211 mm OD and 113 mm ID with a thickness of 39 mm. Figure 9.11 shows the dimensions.
Figure 9.11.
Grinding wheel dimensions