Tech Talk
What are “Vitrified-Bonding Wheels”?
A vitrified bond is made of clay or feldspar which fuses at a high temperature to form a glass-like structure. During the firing operation, the clay or feldspar melts surrounding the abrasive grain, bonding each grain to the next, and forming a homogeneous structure. When the wheel cools, each grain is surrounded by a hard glass-like bond which has high strength and rigidity. This type of bond system is well suited to abrasive machining as it fractures readily when the grinding forces build up on the abrasive grain. The grinding forces will increase on the grain as it dulls; generally, it is the dull grains that we wish to be broken free from the wheel in order to expose the keener, sharper cutting grains. The amount of bond materials mixed in with the grain prior to firing will determine the strength of the grinding wheel and determine the grade of the wheel with respect to the hardness of the grinding wheel. The bond strength is the holding power of the bond to hold a grain in position under the grinding forces. In this instance hardness refers to the overall hardness of the grinding wheel, composed of the grain held into a bonding system. A balance may therefore be struck between the friability and toughness of the grain versus the strength and brittleness of the bonding agent. In one case the grain might be the first to fracture, while in another case the bond might be weak and the whole grain might be plucked or broken from the wheel periphery.
Grinding wheel hardness is determined by the grinding wheel manufacturer. One method of wheel hardness measurement is to take a hard spade-type drill with a constant thrust force and literally drill the grinding wheel. The depth of penetration of the drill is measured after a set period of time. The depth of penetration determines the wheel hardness and is the basis for a grinding wheel grade in the range of A through Z, with A being the softest and Z the hardest. The deeper the penetration over a given period of time, the softer the grinding wheel. Another method is to use an air/abrasive blast to break the grain from the bond system. After a blast at a given pressure for a given time using a known size of abrasive particle, the depth of erosion is measured and the wheel hardness determined. One other method utilizes a natural frequency of vibration measurement technique, a system called “Grind-O-Sonic”, developed in Belgium. The grinding wheel is supported on four equally spaced points on an isolated rubber pad, such that the wheel may vibrate when given a sharp blow with a hard rubber hammer or similar object. The frequency of vibration is detected and measured through a pickup. The numerical value is entered into a formula which relates the size, shape, and mass of the grinding wheel to the frequency of vibration, and determines a bulk Young’s modulus for grinding wheel. The Young’s modulus can then be related with surprising accuracy to the hardness and performance of the grinding wheel. This technique has proved to be a major contributor to hardness “balancing” of rotary honing stones.
A further property of a grinding wheel is its structure. The structure refers to the skeletal structure of the bond system. The structure is a measure of the density/porosity of the grinding wheel. Supposing a great deal of very fine abrasive grain were mixed with an equal amount of very strong bond material and pressed under high pressure, a dense, low porosity grinding wheel would result. If a small amount of grain were mixed with a small amount of bond material and another media (to space the grains apart), the result, once the spacing media were removed, would be a very open, highly porous structure grinding wheel. The latter method is used to manufacture the high porosity grinding wheels necessary for creep-feed grinding. The spacing media used to create the large and consistent porosity is paradichlorobenzene (moth ball crystals), which is removed from the wheel in its green state in a steam autoclave prior to firing. In the past, many different materials were used to increase the porosity of a grinding wheel; sawdust and walnut shells were quite common and usually left in the mix to burn out during the firing operation.
A vitrified grinding wheel is manufactured by selecting the correct abrasive and grain size, and thoroughly mixing the abrasive with the correct amount of bonding agent and porosity media, along with a little water. The mix is then packed and pressed into a grinding wheel old, with pressures varying from 10 to 675 bar (150 to 10,000 psi). The mold is then fully dried, forming a grinding wheel in a green state. Shaping or recessing the grinding wheel by machining is more easily performed in the green state. If there is a pore inducing media in the mix it is removed in a steam autoclave. The wheel is then dried and fired in a kiln, in a similar manner to firing a piece of pottery, at temperatures approaching 1400C (2500F) for several days, depending on the size of the grinding wheels and the charge. The wheels are then removed from the kiln and slowly cooled. They are then checked for distortion, shape, and size. After machining to a final size, the wheels are balance tested, balanced, and overspeed tested, generally at 1.5 times the rated Maximum Operating Speed (MOS), to ensure operational safety.
An alternative to pressing the grinding wheel mix to form a wheel in the green state is a method called puddling. A puddled wheel is typically mixed to such a consistency that the mixture can be poured into a shaped mold and allowed to set before firing in the kiln. This method of wheel manufacture allows a larger and more consistent porosity throughout the grinding wheel, particularly across the wheel width, where pressing tends to develop a wheel much harder at the edges than in the center. This method usually results in a very open and soft structure suitable for creep-feed grinding.
It should be understood that the vitrified bond system is hard and brittle. The great majority of wheel wear takes place by the mechanical action of stressing the bond with a high grinding force, breaking the bond bridges, and allowing the exposure of anew, sharper, grain deeper in the wheel’s structure.
There is an interesting note to be made with respect to silicon carbide and superabrasive wheels in a vitrified bond system as we have described it. A silica, glass like, vitrified bond media reacts adversely with a silicon carbide grain, so a porcelain/ceramic type bond system has to be used for silicon carbide wheels. Superabrasive grain can be bonded in a vitrified bond. However, diamond turns into graphite at 700C (1300F) and CBN begins to oxidize at 1000C (1850F) and completely oxidizes at 1900C (3500F). Therefore, lower temperature vitrified bonding systems had to be developed; remember, vitrification of A12O3 wheels takes place at 1400C (2500F).
