Grinding operations on structural materials (e.g. metallic and ceramic workpieces) typically involves contacting the structural material workpiece with an abrasive article (e.g. grinding wheel) to remove material from and shape the workpiece. Such grinding operations generally involve the input of large amounts of energy (i.e. grinding energy) into the removal of material from the workpiece and often employ high rotating speeds for the abrasive article (e.g. grinding wheel) and/or the workpiece. In some grinding operations it is known to rotate both the grinding wheel and the workpiece. Where high material removal rates, workpieces that are especially tough or hard, high grinding wheel speeds and deep cuts are employed the amount of energy applied to the grinding operation can be and often is very high. This energy in large measure translates into heat that is mostly applied to the workpiece and grinding wheel. The heat often has a detrimental effect on both the grinding wheel and the workpiece. Excessive heat generated during grinding can and often does result in burning of metallic workpieces (ie the formation of a yellow brownish or dark brown to black discoloration on the ground surface of the workpiece). Burning of the metallic workpiece results in a scrapped part. Often the effects of excessive heat generated during grinding can be distortion of the workpiece, out of tolerance parts, changes in the surface appearance and properties of the ground part (e.g. surface hardening effects), excessive break down of the grinding wheel, loss of grinding performance and efficiency, loss of productivity and increase costs.
Creep feed, snagging and cut off grinding operations are high heat generating processes because of the desire for high metal removal rates (i.e. cubic inches of metal removed per unit of time). In snagging and cut off grinding operations the burning of the metal part due to the high generation of heat is not critical because the metal part is in a rough condition after the snagging and cut off operations and is subject to subsequent shaping and finishing steps. The creep feed grinding operation also generates large amounts of heat because of the desire for high metal removal rates in the shaping of the metallic workpiece. However burning of the metallic piece (i.e. the formation of a yellow brown, brownish or brownish black discoloration on the surface) during creep feed grinding operations is a very undesirable condition resulting in the scrapping of the workpiece or article. Additionally, excessive heat generated in a creep feed grinding operation can cause distortion of the part, alteration of the surface appearance and surface properties of the part (e.g. change the surface hardness of the part) and cause the production of an out of tolerance part. Typically in the creep feed grinding operation the metallic workpiece, article or part is fed into a rotating grinding wheel which remains in one location. The rate at which the workpiece is fed into the grinding wheel and the depth of cut are established to maximize the metal removal rate consistent with the desires to produce quality parts, reduce scrap, achieve high grinding efficiency and lower grinding operation costs. Thus the higher the metal removal rate, the greater the G-ratio (i.e. amount of metal removed per unit of grinding wheel lost) without burning the part the greater the efficiency and productivity and the lower the cost of the creep feed grinding operations. Creep feed grinding is used for example in the production of gears. In the production of gears, formed grinding wheels (i.e. wheels having a particular shape) are often used in the creep feed grinding process. It is therefore important that such shaped wheels retain their shape for as long as possible consistent with the other desirable conditions of the creep feed grinding operation (e.g. high metal removal rate, high G-ratio, low heat production and non-burning of workpiece). Although the burning of metallic workpieces and excessive heat generation are of major concern in creep feed grinding operations they are also important concerns in other grinding operations for shaping metallic workpieces to produce useful articles. Such other grinding operations include, for example, surface, internal, plunge and roll grinding operations. Thus it is important and highly desirable to have grinding wheels which produce or contribute to low heat generation during grinding and reduce or eliminate part burn or the risk of part burn while providing high grinding efficiencies and performance, long wheel life and high productivity to reduce grinding operation costs.
It is known to employ metalworking fluids (e.g. water based or oils) in grinding operations to improve grinding performance and efficiency. These fluids are, in many cases, known to reduce friction and remove heat during the grinding operation. Reduction of friction by the fluids can reduce the heat generated during grinding. The ability of these fluids to reduce friction (i.e. friction between the workpiece and the grinding wheel and/or components thereof) and remove heat during grinding can depend upon such factors as the composition of the fluid and the ability of the fluid to penetrate into the grinding zone or interface (i.e. the area of contact between the grinding wheel and the workpiece during grinding). Many metalworking fluids are known to be effective in many grinding operations and have been found to be of value in mild (i.e. low heat generating) grinding operations to improve grinding efficiency or performance. However in severe (i.e. high heat producing) grinding operations (e.g. creep feed grinding) they are often found to be of limited, if any, effectiveness in reducing or preventing part burn when high metal removal rates are sought. In such severe grinding operations it has been found that the metalworking fluids often exhibit poor penetration into the grinding interface, i.e., the region within which material removal occurs, to reduce friction and remove heat.
In the art it is known that different grinding operations (e.g. surface vs internal vs roll vs plunge vs snagging vs cut off vs creep feed grinding) involve different conditions. Such operations therefore often employ for example different forces, speeds, temperatures, infeed rates, metal removal rates and workpiece materials. Some grinding operations (e.g. finish grinding or surface grinding) may employ mild physical conditions involving low forces, low feed rates and low metal removal rates etc. Other grinding operations (e.g. creep feed, plunge and cut off grinding) may employ severe physical conditions involving high forces, high feed rates and high metal removal rates etc. Thus it is known to produce grinding wheels tailored to particular grinding operations and/or workpiece materials. Such wheels may differ in composition (i.e. amount and kind of abrasive grit, bonding material binding together the abrasive grit and additives) and/or structure depending upon their end use. The wheel structure may vary in the amount and type of porosity it contains. The porosity of a grinding wheel, particularly a vitreous bonded grinding wheel, can be of an open and/or closed cell structure. In the open cell porosity the cells or pores are interconnected much like the pores of a sponge or open celled foam. In the closed cell porosity the cells or pores are not interconnected and remain as separated totally enclosed voids much like closed cell foam. Closed cell, rather than open cell, porosity is generally found in resin bonded grinding wheels. The pore structure of a vitreous bonded grinding wheel can serve a number of functions including, for example, controlling the physical strength of the wheel, controlling the breakdown of the wheel to present fresh cutting edges, the elimination of swarf and providing means for getting metalworking fluid to the grinding zone. In a vitreous bonded grinding wheel having an open pore structure it is known to have an essentially random distribution of pore or cell sizes (i.e. some pores being large and other pores being small) and in some cases a random distribution of pores. Thus vitreous bonded grinding wheels can have a heterogeneous open pore structure with respect to pore size and in some cases pore distribution. Pore sizes larger than the abrasive grain average size may be found. Grinding wheels, particularly resin bonded grinding wheels, are known in the art to include thermally conducting particles (e.g. metal particles) to act as heat sinks and improve the dissipation of heat from the grinding wheel. In the case of resin bonded grinding wheels the dissipation of heat from the wheel by such thermally conducting particles serves to protect the poor thermally conducting resin bond from thermally induced breakdown and thus helps protect (i.e. preserve) the strength of the wheel during grinding.
In the grinding process and in particular a grinding operation under severe physical conditions, as are encountered in creep feed grinding operations, using an open cell porosity vitreous bonded grinding wheel, the open pore structure of the wheel can serve as a significant avenue or means by which metalworking fluid can penetrate into the grinding zone or interface and by which metalworking fluid can be captured by the wheel during grinding to reduce friction and remove heat generated during grinding. Such reduction in friction and dissipation of heat are significant factors in reducing or preventing grinding burn of the metallic workpiece, increasing performance and efficiency and lowering the power or energy needed for the grinding operation. These improvements in turn can lead to higher metal removal rates, increased productivity and lower grinding operation costs
Vitreous bonded grinding wheels in the prior art are known to be less than desirable in preventing or reducing grinding burn of metallic workpieces under severe physical grinding (e.g. high metal removal rate) conditions even when the grinding operation is carried out in the presence of a metalworking fluid. Thus grinding burn obtained with prior art vitreous bonded grinding wheels under severe physical conditions is known in the art. In many cases, in the art, grinding burn is overcome by reducing the severity of the physical grinding conditions (e.g. reducing metal removal rate and/or infeed rate and/or wheel speed etc.) leading to a loss of productivity and increased grinding costs. Additionally the excessive heat generated during grinding under severe physical conditions with prior art vitreous bonded grinding wheels is often known to lead to scrapped metal parts because of out of tolerance conditions and/or adverse changes in surface appearance and/or properties (e.g. reduction or increase in surface hardness) of the parts. Improvements in vitreous bonded grinding wheels, particularly for use under severe physical grinding conditions, which reduce or prevent grinding burn of metallic workpieces, reduce power or energy consumption during grinding, improve grinding performance and efficiency and increase grinding productivity therefore are needed and desirable. This invention seeks to overcome these and other problems of prior art vitreous bonded grinding wheels, particularly those vitreous bonded grinding wheels used under severe physical conditions in a grinding operation and provide vitreous bonded grinding wheels with improved grinding performance, and improved penetration of metalworking fluids into the grinding zone for reducing or preventing grinding burn of metal workpieces and in reducing the energy or power used in the grinding operation.