ELID Grinding Applications
ELID grinding has been used in experiments with many materials including ceramics, glass, and biomedical materials and with many grinding types. The advantage with ELID technology is that it can be applied to any machine. Therefore, many ELID types are presented, such as ELID surface grinding, ELID cylindrical grinding, ELID internal grinding, ELID centerless grinding, ELID CG curve-generator grinding, ELID aspheric grinding, ELID face grinding, ELID double-side grinding, and ELID lap grinding.
With surface grinding, it is essential to apply the ELID technology because it is the most common method of grinding. The ELID method can be applied to many surface grinding applications including the regular straight-type grinding wheel, with a rotary grinding system in a straight-type grinding wheel, and with in-feed grinding systems. Cylindrical grinding and internal grinding are the same idea except that cylindrical grinding is for external cylinders, where internal grinding is for internal cylinders. The ELID phenomena are the same for all grinding types. ELID has also spread to special cases as in the CG grinding. ELID aspheric grinding has been investigated for optical lenses and molds. ELID technique has even been extended to face grinding and double-side grinding. Finally, ELID lab grinding is similar to ELID double-side grinding except that it uses one wheel and a spindle. This operation is the main focus of this project and will be explained extensively later [51].
ELID Methods of Development
Many researchers have studied improvements of ELID technology use for special operation conditions. In previous investigations, ELID technology has proven that it can be applied with environmentally friendly operations, which implies a reduction of fluid supply using a mist spray. The method was tested and proven to work and achieve an oxide layer with a considerably environmentally friendly amount of fluid.
On the other hand, some applications including micro lens molds are using small grinding wheels. Other applications require the grinding wheel to be connected to the workpiece all over, all the time, as in honing operations. These cases are difficult to be dressed in process. A solution for these cases is suggested by performing the ELID dressing in intervals. In the first case, ELID predressing is performed prior to starting the grinding operation. After a while, the operation pauses and another ELID cycle is executed for another grinding interval because the grinding wheel is so small that it is challenging to attach a small electrode with the required gap to perform the ELID operation. In the second case where the wheel is connected continuously, attaching an electrode to the grinding surface area is impossible. This method of ELID is also known as ELID-II or ELID interval dressing.
Sometimes, the area where an electrode can be mounted on the grinding machine is limited. In this case, an electrodeless technique can be used with the workpiece as an electrode. This approach requires the workpiece to be made out of an electrical conductive material. By connecting the negative pole to the workpiece and with the presence of the coolant fluid, the ELID cycle completed. This method of electrodeless ELID is also known as ELID-III [51]. ELID-III has potential erosion caused by the sparks. A careful choice of electrical parameters such as small current duty ratio, low voltage, and low current can eliminate this problem [52].
Using an alternative current can improve ELID-III. A method called ELID-IIIA, which has no contact between the workpiece and the wheel, will improve the surface finish due to the elimination of any electrical discharge. The reason is that using an alternative current will create a thick oxide layer on the workpiece surface [53].
In some applications, a different method of ELID dressing without the use of an electrode is to have a conductive material on the tip of the grinding fluid nozzle. By connecting the positive and negative poles to the two conductive plates on the nozzle, ELID can be performed by an ion-shot method. In this method, the whole grinding wheel face is free and not covered by any electrode or connected to the positive pole [51].
In some cases, ED-trueing and ELID grinding can be performed at the same time. This process occurs by applying ED-trueing before the ELID grinding operation. Some researchers have reported that this idea gives the wheel a better and more accurate profile surface [52].
In a research paper, ELID technology also has been investigated on quartz blank, which is used in many telecommunication devices. ELID was used with wheel mesh sizes #325, #4000, and #8000.Wheel mesh size #325 was used by ELID I, and wheel mesh sizes #4000 and #8000 were used using ELID-II. The result from mesh size #325 was acceptable and near the desired condition. However, after further processing by ELID-II with mesh sizes #4000 and #8000 correspondingly, the results were positive with Rmax of about 0.060 μm [54].
Grinding with Lapping Kinematics in Certain Applications
This operation is also known as lap grinding, fine grinding, or flat honing. Fine grinding is usually used for superabrasives wheels while flat honing is more for conventional abrasives wheels. Grinding with lapping kinematics is helpful when the workpiece needs machined precision surfaces on a difficult-to-grind material such as hard ceramics [55]. It is recommended for use if a mirror finish is needed when it cannot be obtained with constant feed or by using wheels finer than JIS #10000 [23].
Lap grinding differs from regular grinding because it is considered a cool process with lower speed and no sparks. However, a flood of cooling fluid is necessary to control the temperature and to remove the swarf from the machined workpiece. If insufficient coolant is provided, wheel loading increases and surface finish will not be as expected, in addition to the possibility of thermal damage to the workpiece or the wheel. In the experiment, the proposed TRIM C270E, high-performance synthetic is initially used with a concentration of 5% with water. This coolant is provided from Master Chemical Corporation located in Perrysburg, Ohio. The TRIM C270E has a low ferrous corrosion inhibition with an electrical conductivity of 1.8 mS, refractometer factor of 3.3, and a pH of 9.0–9.3 (typically pH of 8.7–9.2) [56].
This process is often useful in parallelism and its use of two parallel grinding wheels. Nevertheless, this experiment contains one grinding wheel only. Usually, the stresses on the machined surface were easily relieved, and mirror surface finish can be achieved. The wheels of grinding with lapping kinematics are flat wheels made out of aluminum oxide (Al2O3) or silicon carbide (SiC). Aluminum oxide is cheaper but silicon carbide is better. Nonetheless, in some cases where the material to be machined is quite hard, such as in this experiment, a harder wheel is necessary, like cubic boron nitride (CBN) or diamond. These wheels become more expensive if the friability is higher, which will give new sharp grains faster. Friability is the ability of the grains to break down and expose new sharp edges, which will increase the material removal rate with more accurate tolerances [57].
Lap grinding also differs from conventional grinding because it has a constant pressure throughout the operation. The pressure of the wheel depends on the type of material machined. Harder workpieces require more pressure than softer ones. Lower than the recommended pressure is always better than going over the recommended pressure; it is only going to consume more time, whereas in overpressure cases, more heat can be generated, causing more damage to the wheel by pulling off grains, and indeed bad surface finish production [57].