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. 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.
Figure 9.6.
ELID grinding mechanism [31].
Figure options
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 Ra, Rv, Rt, and Rq. The most important measurements are Ra and Rt, where Ra is the average peak-to-valley distance, and Rt 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 extensively developed and enhanced since 1990 by Japanese researcher Hitoshi Ohmori [35]. This technology is useful when low roughness is needed or when machining hard materials using small grains to avoid cracks [23]. The idea of using electrical power for an in-process dressing came originally from a process called electrochemical grinding (ECG) [10]. Electrolytic in-process dressing (ELID) is a technology that enhances the grinding efficiency by increasing the removal rate and reducing the forces of the grinding process. This result is true when we continue the grinding operation for a period of time, because the material removal rate does not decrease significantly compared to that in conventional grinding. However, the reduction of the grinding forces occurs at the beginning of the process, but ELID can also provide constant forces as the process is being stabilized. The increase of the efficiency is essential machining ceramics because the cost of manufacturing them is relatively high with the low material removal rate, long dressing time, long machining time, and high tool wear [36]. Additionally, according to an experimental article entitled “High Efficiency ELID Grinding of Garnet Ferrite,” ELID grinding forces are lower than those in conventional grinding. The experiment has been conducted with the same parameters for both resin-bond diamond wheel and cast iron diamond wheel with ELID technique. The results show that ELID grinding with cast iron diamond wheel has about one-half the tangential and normal forces of the resin-bond diamond wheel [17]. ELID grinding also provides better surface topography than conventional grinding, which has been proven under the microscope experimentally [35].
A huge increase in the demand for ceramics in the industry can be attributed to their high temperature resistance, light weight, chemical stability, and their minimal lubrication. Ceramics can be also used in many applications including cutting tools, the auto industry, and aerospace industry. The problem is that with conventional grinding operations, the process cannot be continuous because the grinding process needs to be stopped for the wheel dressing. ELID technology has the ability to perform dressing for the wheel while it is in operation [36].
According to the article “Electrolytic In-Process Dressing (ELID) and Super-Precision Grinding for Ring Raceway of Ball Bearing,” ELID grinding has been investigated for the raceway of ball bearings. The results show that ELID grinding can replace or eliminate the use of the final finishing process of lapping or polishing. Elimination of this step is helpful because the final process of superfinishing with oilstone is complex and requires additional equipment [6]. In other research, ELID grinding has been compared with a non-ELID grinding for TiAIN. The results show that ELID grinding offers a slightly better average roughness (Ra), but a significant Ry, (Ry = Rt), improvement, whereas Ra and Ry are almost the same at ELID operation. However, the atomic force microscopy (AFM) 3D topography shows a huge difference between ELID and non-ELID grinding, where the workpieces with ELID were smoother with a greater surface finishing than non-ELID operation [37].
The ELID technique has been also applied to grinding X-ray mirrors. The experiment gave better roughness, but use of a pressurized method such as lap grinding was recommended [38]. ELID grinding with lapping kinematics produces no thermal damage or burn to either the grinding wheel or the workpiece due to the low speed of the operation [33].
The investigation on ELID technology also has been performed with acoustic emission. The acoustic emission technique helps in recognizing when the wheel is having any loading [39]. In other experiments, the acoustic emission was used to identify when the grinding wheel first touches the workpiece in face grinding[40]. One of the experimenters combined the ELID grinding with magnetorheological finishing process (MRF). The results show a better surface quality. Also, the outcomes show better efficiency: the MRF does not harm the surface as in conventional grinding, which will lead to a longer workpiece operational life [31].
In a paper by the Japanese researchers Itoh and Ohmori, an ELID lap grinding similar to the present project was performed with multiple wheels of varying mesh sizes including #1200, #4000, and #8000 and a coolant solution of 2% in water for silicon and tungsten carbide workpieces. The electrical characteristics were Eo 60 V, IP 30 A, and τ on/off 2 μs. The effect of ELID was obvious on two points. First, the improvement of the surface roughness and quality was excellent. And second, the removal rate was more stable and constant with time compared to a non-ELID operation. This paper also showed that using a complex material can reduce the resultant roughness in an insignificant manner [41].
Furthermore, Ohmori and Bandyopadhyay investigated the surface of a machined silicon nitride, and the results show a significant improvement of the surface after monitoring the workpieces with scanning electron microscopy (SEM) and AFM. One of the methods to ease the reading for these images is to coat the workpieces with Au-Pd. The images will show white spots as a sign of surface fragmentations and that the workpiece has been machined in brittle mode. If no spots are located, it is an indicator that the workpiece has been machined in ductile mode [36]. The results of that experiment show that grinding with ELID using #4000 mesh size or finer gives a continuous ductile mode machining in addition to the significant reduction of the bend strength [42].