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Diamond and CBN are considered to be the hardest know grinding materials and are therefore often referred to as superabrasives. For this reason they are suitable for machining materials which are difficult or even impossible to grinding with conventional abrasives such as aluminum oxide and silicon carbide.
Diamond and CBN have the same crystal structure, with diamond consisting of pure carbon, whilst CBN is consist of two atoms boron and nitrogen.
Due to its extreme hardness, diamond is particularly suitable for machining of hard, brittle and short-chipping materials.
All carbide metal grades
Hard facing alloys
Glass fibre reinforced plastics.
Steel has a high affinity to carbon. Since diamond consist of pure carbon, it is not suitable for machining steel. The high temperatures produced in the grinding process cause the steel to extract carbon atoms from the diamond, eroding the diamond grinding grit.
CBN (Cubic Boron Nitride)
CBN is the second hardest abrasive, surpassed only by diamond. CBN consist of the elements boron and nitrogen. In contract to Diamond, CBN has no carbon atoms, making it suitable for machining steel. CBN is better suited for machining the following materials:
Hardened steel over approx.54 HRC hardness
High speed steel (HSS)
Nickel-based special alloys.
CBN have a higher thermal stability than diamond. Whereas diamond suffers a massive loss of hardness at above 700ºC, the hardness of CBN remain virtually unchanged at over 1000ºC.
Grit size – Diamond and CBN
|PEPA(um)||US Mesh||ISO R565/1972|
It is the grit size which
It is the grit size which largely determines the material removal rate of grinding tools and, in particular, the surface quality of the workpiece. Normally, coarser grit sizes will achieve a higher removal rate, but usually at the expense of the surface quality. Conversely, finer grit sizes achieve better surface qualities, at the expense of abrasive performance. This contrast between performance on the one hand and surface quality on the other means that many grinding operations still need rough grinding followed by finish grinding.
Concentration is the ratio of the weight of the Diamond or of the CBN in carat (1 carat = 0.2 g) to a cubic centimeter of grinding rim volume. A concentration 100 translates to into a diamond or CBN contents of 4.4 carat per cubic centimeter of grinding rim volume. All other concentrations are proportional. Concentration has a considerable impact on the cutting performance and the life of a grinding wheel, to some extent also on the price.
The common concentration designations are as follows: (1 carat = 0.2 g)
|Diamond||Diamond weight in carats per cm3 abrasive volume||CBN||CBN weight in carats per cm3 abrasive volume|
|C50||2.2 ct / cmm³||120V||2.2 ct / cmm³|
|C75||3.3 ct / cmm³||180V||3.3 ct / cmm³|
|C100||4.4 ct / cmm³||240V||4.4 ct / cmm³|
|C125||5.5 ct / cmm³||300V||5.5 ct / cmm³|
|C150||6.6 ct / cmm³||360V||5.5 ct / cmm³|
The basic value C 100 for diamond signifies that for every cm3 abrasive layer volume 4.4 carats (1 carat = 0.2 g) of grit were used (corresponding to 25% by volume). The basic value V240 for CBN indicates that 25% by volume CBN were used in the abrasive coating. These data do not apply to electroplated wheels.
The retention forces of the bond affect the grinding characteristics of the wheel. It is essential for the diamond or CBN particles to be embedded in the bond such that they are retained under the forces and temperatures occurring in the grinding operation. On the other hand the bond must also provide space for chip removal.
Resin bonds B
Metal bonds M
Vitrified bonds V
Eletroplated bonds G
Resin bonds - "B"
The usual bonding agents in resin bonds include phenolic and polyimide resins. The short-term thermal load capacity of these bonds is between 250°C and a maximum of 350°C. Resin-bonded grinding tools have an excellent cutting capacity and a cool cut, designed for use in a wide variety of applications both with Diamond and in CBN. Certain bonds can also be used for dry grinding.
Metal bonds - "M"
The bond agents used in metal bonds include various different bronze alloys. The thermal load capacity of these bonding systems can be as high as 600°C. Due to their superb grit retention power, metal-bonded grinding tools have an excellent stability of shape. Metal bonds tend to give particularly long tool life, but are generally inferior to synthetic resin bonds with regard to material removal rate.
Vitrified bonds -"V"
Vitrified bonds have an excellent dressing capacity, allowing these grinding tools to return to their original shape easily once they lose their grinding rim geometry. Another advantage of vitrified bonds is the controllable porousness of the grinding rim during production, ranging from a closed to a very open rim structure. No other bond system allows a similar range. High porousness enhances the coolant transport, improves chip removal and reduces and contact surface / the grinding force and therefore the process heat.
Electroplated bonds - "G"
A single layer of abrasive in bonded to the wheel body by an electrode position process.