Chapter9-ELIDGrindingwithLappingKinematics.pdf

Chapter 9 ELID Grinding with Lapping Kinematics

Ahmed Bakr Khoshaim*, Zonghua Xu† and Ioan D. Marinescu‡ *The University of Mecca, Saudi Arabia; †MIME Dept., University of Toledo, Toledo, Ohio; ‡The University of Toledo, Toledo, Ohio

9.1 INTRODUCTION Grinding is one of the most important abrasive processes. It is used to perform smooth and precise dimensions and surfaces. Usually, the material removal rate from the workpiece is low in grinding operations compared to that in milling or other cutting operations. However, it is considered effective for brittle materials. Finishing operations have a long history. Since the Stone Age, people were rubbing stones to create sharp edges. Also, the Egyptians had an amazing machining process for cutting large rocks, such as in the pyramids. Modern machining started in the nineteenth century [1]. In grinding, a grinding wheel with a number of abrasives adhered to it is used. The abrasives come in small pieces and in irregular shapes. In addition to the ability to resist chemical reaction caused by the lubricating fluid, the bond material should be strong enough to overcome the grinding forces and temperature. Many types of grinding wheels offer different configurations for different grinding operation conditions [2].

Abrasives and Wheel Types Abrasives can be either conventional or superabrasives. Conventional wheels are cheaper than superabrasives wheels. On the other hand, superabrasives wheels are made with more expensive materials, and therefore, only a small part of the wheel is made by the superabrasives material where the rest can be made with metal. Conventional abrasives can be aluminum oxide (Al2O3) or silicon carbide (SiC). On the contrary, superabrasives can be either cubic boron nitride (CBN) or diamond. These last two are the hardest materials, which give them the ability to machine other hard materials. The hardness can be cited as Knoop hardness. For example, diamond hardness is 6500 kg/cm3, and CBN is 4500 kg/cm3. These levels are high compared to SiC and Al2O3 at 2500 and

Handbook of Ceramics Grinding and Polishing. http://dx.doi.org/10.1016/B978-1-4557-7858-4.00011-X 394 Copyright © 2015 Elsevier Inc. All rights reserved. ELID Grinding with Lapping Kinematics Chapter | 9 395 1370–2260 kg/cm3, respectively [3]. The abrasives can be either small in size for better surface-finish roughness or large in size for higher material removal rate. The abrasives have the ability to fracture into pieces. This characteristic is called friability, which makes the abrasives self-sharpening. Very high friability and very low friability are undesirable. Very high friability makes the abrasives so soft that they are quite fragile. Low friability prevents the abrasives from breaking and sharpening themselves again, which will lead the abrasives to become dull. These abrasives have been made in the industry due to the amount of impurities in natural materials, which affects the reliability. The abrasives grain sizes are to be considered small compared to machining tools or inserts [4]. The six grinding wheel properties include the type of coating, the kind of dressing, the method and precision of balancing, the type of resin, the abrasives grit size, and the grain formula [5]. Grinding wheels come in a variety of types and sizes. They can be produced in many shapes, including straight, cylinder, straight cup, flaring cup, depressed center, and mounted. Both conventional and superabrasives wheels use different bond types. The two sheared bond types are vitrified and resinoid. A vitrified bond consists of ceramic bond mixed with the abrasives and shaped to the desired grinding wheel shape. Then, it is heated gradually to 1300 °C to allow the ceramic to create strong structure. Finally, it is cooled slowly and tested. Consequentially, in resinoid bonding, organic compounds are used and mixed with the abrasives, compressed, and shaped at a lower temperature of about 175 °C [3]. Metal-bonded grinding wheels have some advantages over the resin- and ceramic-bonded wheels, because metal-bonded wheels have high hardness and possess high heat conduction. Also, metal-bonded wheels have a high bondage capability, which gives the wheel a high holding ability for abrasives [6]. Sometimes, if the operation undergoes vibrations, but at the same time metal bond is needed for its electrical conductivity, a metal-resin-bonded grinding wheel can be used. Usually, this line of grinding wheels has a mixture of metal and resin bonds with a ratio of metal to resin at 7:3 [7]. Other types of bonds, including silicate, rubber, elastic, epoxy, and oxychloride, are also on the market and are manufactured by select manufacturers. Grinding wheels typically have a standard coding, or marking system, for wheels to show their properties. For instance, we may find this code on a grinding wheel: 30A 46 H 6 V XX. The A in the code indicates that the wheel is made out of aluminum oxide Al2O3 abrasives. Different letters indicate different abrasives (e.g., C = SiC, D = diamond, Z = zirconia). The number 46 is the grain size, which ranges typically from 4 (coarse) to 500 or more (fine). H is the grade, which ranges from A (soft) to Z (hard). The harder the wheel is, the better the finishing and stronger the abrasives holding, hence, the longer the wheel life. On the other hand, the softer the wheel, the faster the cutting ability. The number 6 is the structure or the number of spaces between the bond and the abrasives grains. The structure is on a scale of 1–15, where the grains are dense at 1 and open at 15, V is the bond type (vitrified) (e.g., M = metal, R = rubber,