In the past Sun-Diamond powder was used to manufacture grinding wheels for tungsten carbides. Nowadays, it is widely used to manufacture grinding wheels for grinding cemented carbides, glasses, hard ceramics, marbles and granites.
Design of diamond wheels using Sun-Diamond powder is quite different from conventional grinding wheel. In diamond wheels, diamond grit locates in a relatively thin abrasive layer on the perimeter or face of the wheels. Diamond grit is embedded on some bonding material which constructs this layer. Normally, total diamond grit embedded is not more than 26% of the total mass of this layer.
Improper bonding may reduce the effectiveness of grinding wheel during the operation. If the bond material is not strong enough to retain the diamond, the grits will be pulled away bodily and the work piece will be rubbed only by the bond. On the other hand, if the bond holds the grit very firmly, the edges and points of the grit will eventually be rubbed flat or broken off, and the wheel becomes ineffective. A steady and continuous grinding process can be maintained only if the wear of the grits is accompanied by some wear of the bond at an appropriate rate. In this case worn grits will fall out and new unworn grits are uncovered to take their place.
There have been several bond methods to manufacture diamond wheels using Sun-Diamond powder. However, the most commonly used bonds are the resinoid and metallic bonds.
Resinoid bonds are usually based on phenolic or polyimide resins together with a filler material which can act in two ways. It may improve the flow characteristics of the heterogeneous mixture of grit and resin during the sintering process, and may also improve the resistance of the bond to the diamond action of the work piece and the grinding debris. Silicon carbide is commonly used as a filler, either alone or with some solid lubricant such as graphite to reduce friction and therefore the temperature of the wheel.
Some diamond wheel manufacturers use Sun-Diamond powder with a metallic filler such as powdered silver or copper in a resinoid bond. These metallic filler can act to some extend as a binder and thus help to hold the grit more firmly. Also, because of their high thermal conductivity they facilitate heat flow and help reduce temperature rises at the grinding surface. This kind of modifications, bring us all kind of possibilities for making diamond-wheels with a wide range of performance.
Design of the wheel to produce a good removal rate with low wear involves balancing the wear of the grit against the wear of the bond for the particular work piece in question. For example, the carbide group of materials is so hard and tough that the points and edges of the grits soon become blunted, so it is necessary to use a friable grit which breaks down by fracture and provides more sharp edges. In addition, the bond must not be too strong, so that worn out grits are moved and replaced by new grits revealed by the erosion of the bond. The most effective grinding wheel for carbides is the one made of friable diamond grits in a resinoid bond.
Metallic bonds are produced by sintering a mixture of grit and powder metal, generally a bronze alloy, although steels and carbides are sometimes used. The main characteristic of these bonds is that they are stronger than resinoid bonds and hold the diamond more firmly. A graphite filler mixed with a metallic powder gives the possibility of varying the strength of the bond by introducing some porosity into the metal matrix. Because higher temperatures are required for sintering then for curing resinoid bond, care must be taken that the diamond grits are not damaged by the heat.
Glasses are more easily abraded by diamond grit and off course less hard than carbides. Therefore, it is possible to use stronger grits embedded in a stronger bond to obtain much higher removal rates. The diamond wheels manufactured for glasses generally contain blocky single crystal grits in a metallic bond. On the other hand, ceramic materials are somewhere between glasses and carbides in terms of hardness. Therefore, various combinations of grits and bonds including vitrified bonds have been recommended for different ceramics.
There are several other bond methods to manufacture diamond wheels, using Sun-Diamond powder. Vitrified bond is another relatively common bond. Vitrified bonds are manufactured from fusible powdered glasses along with fillers such as graphite or copper. These bonds are harder than metal bonds, but not as strong. They have less friction and more free-cutting action. Vitrified bonds are brittle and therefore more susceptible to mechanical damage through careless handling, and their thermal conductivity is lower so there are more susceptible to thermal damage. It also costs more to manufacture uitrified diamond wheels.
In some cases Sun-Diamond powder has been used for electroplated bonds. In electro plated bonds a single layer of diamond particles is held in place by a thin layer of nickel electroplated on the rim of the wheel. Normally, the thickness of this layer is about 1-1.6 times of the diamond particles. This is a relatively inexpensive bond and is convenient for fabricating wheels in a variety of shapes and sizes. However, the life of the wheel is restricted by the single layer of diamond grits, and there is little scope in the plating process to vary the strength of the bond
Electroplated wheels tend to have a relatively high number of grits per square millimeter of wheel, giving less load per particle and less wear. Also, there is a greater clearance for the removal of debris. On the other hand, this type of wheel tends to give a rougher surface finish.
We are constantly researching to develop a new variants of bonds to give improved performance to Sun-Diamond powder. Development of grinding wheels for precision grinding of up to one micron for sapphire windows is our latest focus.
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Sun Marketing Group, 1700 Shattuck Ave 315, Berkeley, California 94709, U.S.A.