Grinding is technically and economically comparable to cutting in many areas, and in some areas it is even the only method of machining. According to Salmon, the main reason for this is a lack of understanding of the principles of grinding and its inherent potential. The author’s purpose in writing this paper is to help those involved in the business community to properly understand and apply grinding technology.
Today’s manufacturing industry is eagerly looking for alternatives to grinding. Some of the “new” solutions being tested to improve part productivity include: hard cutting, dry cutting, wear-resistant coated tools and high-speed cutting.
It should be noted, however, that the word “high speed” is not new to grinding. The conventional operating surface line speed of grinding wheels reaches 1829m/min, the production of high-speed superabrasive grinding wheels reaches a practical speed of 4572-10668m/min, while the speed in the laboratory on special equipment for grinding can reach 18288m/min – only slightly below the speed of sound.
Part of the reason industry doesn’t like grinding is because it doesn’t understand it. Superabrasives and slow-feed grinding processes can rival milling, broaching, planing and, in some cases, turning, both from a technical and economic standpoint. However, there are many people in manufacturing companies whose knowledge is stuck at the level of traditional machining technology and who tend to take a rejectionist attitude towards grinding. However, with the advancement of new materials (e.g. ceramics, whisker-reinforced metals and reinforced polymeric materials, multi-layered metallic and non-metallic press-fit materials), grinding is often the only viable machining method.
With the use of appropriate bonding agents, it is possible to make the process of shedding and self-sharpening of abrasive grains during machining controlled. And the grinding wheel can be dressed on the machine when it becomes dull or when a chalk-like load appears. These advantages are difficult to achieve with other machining methods.
Grinding wheels allow machining surfaces to tolerances of tens of thousands ofths of an order of magnitude (micron level), while also allowing for optimal surface finish and cutting texture.
Unfortunately, grinding has long been viewed as an “art”. Until the last 40 to 50 years, researchers have continued to study the grinding process, developing new and improved abrasives, binder systems and various grinding fluids. These achievements have brought grinding into the realm of science.
Types of abrasives
Abrasives can be divided into two major categories: ordinary abrasives (such as alumina, silicon carbide, etc.) and superhard abrasives (diamond, cubic boron nitride, etc.).
cbn and gold zirconia are harder and more wear-resistant than ordinary abrasives, but very expensive. At the same time, superabrasives are excellent thermal conductors (diamond is 6 times more thermally conductive than copper), while ordinary abrasives are ceramic materials, so they are adiabatic.
Superabrasives also have high thermal diffusivity, which means they have the ability to dissipate heat quickly. This characteristic makes superabrasives “cold cutting” in nature. The abrasive resistance of superabrasives is also much better than that of ordinary abrasives, but these properties do not mean that they are suitable for all grinding processes.
Each type of abrasive has its own most appropriate area of use, so it is important to understand the characteristics of each abrasive. For example, alumina ceramic abrasives – sometimes referred to as crystal-gel (sg) abrasives or ceramic abrasives – generally have better wear resistance and shape retention than fused (normal) alumina. However, ceramic abrasives also have their most appropriate areas of use.
Aluminum oxide: al2o3 is the cheapest abrasive. It has excellent performance in grinding hardened steel and can also grind nickel-based superalloys under continuous dressing conditions. al2o3 is well adapted to various grinding conditions, such as soft and hard materials, light and heavy cutting, etc., and can grind to a high surface finish.
Ceramic alumina: Ceramic alumina is very strong, so it is most suitable for applications where the cutting force load per abrasive grain is high. Ceramic alumina is very effective in cylindrical grinding and large plane reciprocating grinding of hardened steel. However, it is not suitable for long cutting arcs where the loading force of a single grain is small, such as internal grinding and slow feed grinding.
But after “stretching” modified ceramic alumina grain, even in the case of long cutting arc, can be used for processing viscous stainless steel, high-temperature alloy, etc., when the shape ratio of the grain (length to width ratio) reaches 5.
The performance of ceramic alumina abrasive grains can be improved by adding part of the brittle fused alumina to form a composite abrasive grinding wheel. At this point, the length of the cutting arc of the grinding wheel on the workpiece needs to be known in order to target the grinding wheel ratios.
Silicon carbide: sic abrasives have a naturally sharp shape. It is suitable for grinding hard materials (such as cemented carbide). Because of its sharpness, it is also suitable for processing very soft materials such as aluminum, polymers, rubber and soft materials such as low strength steels, copper alloys and plastics.
Diamond: Both natural and synthetic diamonds can be used for grinding. Diamond is an ultra-hard form of carbon that forms rapid wear because of its affinity for iron (steel is an alloy of iron and carbon), so it is not suitable for processing ferrous materials, but diamond is particularly suitable for grinding non-ferrous materials, titanium, ceramics and metal ceramics.
Cubic boron nitride: cbn, like diamond, is a very expensive abrasive. The price of a superabrasive grinding wheel is more than 50 times more expensive than a wheel with ordinary abrasives, but its service life is more than 100 times higher than that of ordinary grinding wheels, and even when grinding the hardest steel, there is only slight wear.
The cbn is most suitable for processing ferrous materials, especially for applications that require the wheel shape to be maintained for a long time, such as the grinding of inner raceways of bearings. In addition, cbn is more suitable for processes where grinding wheels are not changed frequently, because small batches and grinding wheel changes, which are trimmed during installation, are the main factors leading to grinding wheel consumption. Because cbn has to react with water contact at high temperatures and accelerate wear, glycol or oil should be used.
The bonding agent for ordinary grinding wheels can be ceramic, resin or plastic, while the bonding agent for super-hard abrasives can be sintered metal matrix or covered to the grinding wheel by electroplating nickel layer. These wheels are impermeable to water and free of cavities.
The grinding fluid for metal bond and electroplated grinding wheels should be carefully selected to prevent the wheel from slipping. The huge dynamic load hydraulic pressure generated at the cutting arc during slippage can jack up the grinding wheel, leading to deterioration of the workpiece finish and accelerated grinding wheel wear.
The choice of binding agent and abrasive are closely related. For example, the use of cbn generally requires the grinding wheel to maintain its shape during use and not to be removed from the machine until it has been consumed. Since cbn has good thermal conductivity, it is more advantageous to use a metal bond. The combination of the two provides the conditions for cold cutting. This is because the cutting heat is conducted through the abrasive and grinding wheel and then carried away with the coolant much faster than into the workpiece.
Metal bonds come in two forms: electroplated and sintered. Electroplated grinding wheels are not dressed; they are made to the correct shape at the outset and are used until they are depleted. Sintered metal wheels are usually dressed with EDM and then mounted on the machine like electroplated wheels.
Sintered and plated wheels should have a radial runout of less than 0.0125 mm when mounted to the spindle. it is important to reduce spindle runout for metal bond wheels. Because of the small distance that the abrasive grains protrude from the bond, if the runout reaches 0.025mm, it can happen that one end of the wheel is overloaded and causes excessive wear, while the other end is lightly loaded and still very sharp.
Some electroplated wheels can be made with a very small contour radius (about 0.125mm). Generally, electroplated wheels are used for high-speed grinding, while metal sintered wheels are suitable for grinding ceramic materials.
The overall metal bond grinding wheel has a small range of adaptation to operating conditions such as vibration, runout and coolant flow. If the grinding machine, workpiece and fixture are not rigid, or if the bearings of the old machine are not in good condition and there is no balancing device on the machine, the use of electroplated grinding wheels under such conditions can lead to problems with wheel life, workpiece finish and surface texture. Depending on the vibration and stability of the machine, it is sometimes better to use resin-bonded grinding wheels. Resin bond has a high damping capacity for vibration. Of course, the equipment and time involved in the calibration and dressing of resin wheels adds to the cost.
Ceramic bond is the most commonly used. Because this bond has a grinding wheel with holes, it allows the cutting fluid to enter the grinding arc efficiently and has a large cavity to hold the abrasive chips. Also, ceramic bond grinding wheels can be easily reshaped to the correct shape and sharpened using diamond tools.
Attachment: The range of materials to be machined for common and superabrasives.
Aluminum oxide abrasives: hardened steel, nickel-based super alloys, high-temperature alloys, ferrous metals
Ceramic alumina abrasives: hardened steel, nickel-based superalloys, viscous stainless steel, high temperature alloys
Silicon carbide abrasives: carbide, aluminum and titanium, rubber polymers, copper alloys, plastics
Diamond abrasives: carbide, aluminum and titanium, ceramics, cermets
Cubic boron nitride abrasives: hardened steels, nickel-based superalloys, ferrous metals
Preparation of grinding wheels
Preparation of the grinding wheel includes: mounting, balancing, finishing and dressing. Poor preparation of the grinding wheel will be the root cause of many grinding problems in the future.
First, install the grinding wheel according to the grinding wheel manufacturer’s instructions to ensure that the wheel is in a good original balance and has minimal runout before dressing. Second, the installation should be done carefully to avoid bruising the bore of the grinding wheel. The bore of the grinding wheel is subjected to huge stress when rotating at high speed, and improper handling and installation is often the cause of bursting when the grinding wheel is started. Third, when installing a ceramic bond grinding wheel, paper washers must be used. Fourth, tighten the flange with a smooth torque and looseness.
After installation, the grinding wheel should be roughly balanced, trimmed, and finely balanced in turn before starting to grind. If the original condition of the grinding wheel is very unbalanced and the runout is large, additional dressing and rebalancing are often required.