Grinding wheel ~ MECHTECH GURU

Grinding wheel

Grinding wheels:-
Grinding Machines are also regarded as machine tools. A distinguishing feature of grinding machines is the rotating abrasive tool. Grinding machine is employed to obtain high accuracy along with very high class of surface finish on the work piece. However, advent of new generation of grinding wheels and grinding machines, characterised by their rigidity, power and speed enables one to go for high efficiency deep grinding (often called as abrasive milling) of not only hardened material but also ductile materials.

Conventional grinding machines can be broadly classified as:-
1.     Surface grinding machine 
2.     Cylindrical grinding machine
3.     Internal grinding machine
4.     Tool and cutter grinding machine 
Grinding wheel consists of hard abrasive grains called grits, which perform the cutting or material removal, held in the weak bonding matrix. A grinding wheel commonly identified by the type of the abrasive material used. The conventional wheels include aluminium oxide and silicon carbide wheels while diamond and CBN (cubic boron nitride) wheels fall in the category of super abrasive wheel.

Specification of grinding wheel:-
A grinding wheel requires two types of specification

1.       Geometrical specification
2.      Compositional specification 

  1.Geometrical specification This is decided by the type of grinding machine and the grinding operation to be performed in the workpiece. This specification mainly includes wheel diameter, width and depth of rim and the bore diameter. The wheel diameter, for example can be as high as 400mm in high efficiency grinding or as small as less than 1mm in internal grinding. Similarly, width of the wheel may be less than an mm in dicing and slicing applications.

2. Compositional specifications 

Specification of a grinding wheel ordinarily means compositional specification. Conventional abrasive grinding wheels are specified encompassing the following parameters.

1.      the type of grit material 
2.      the grit size 
3.      the bond strength of the wheel, commonly known as wheel hardness 
4.      the structure of the wheel denoting the porosity i.e. the amount of inter grit spacing 
5.      the type of bond material 
6.      other than these parameters, the wheel manufacturer may add their own identification code prefixing or suffixing (or both) the standard code.

Standard marking system for conventional grinding wheel:-

The standard marking system for conventional abrasive wheel can be as follows:

51   A    60   K   5   V    05

• The number ‘51’ is manufacturer’s identification number indicating exact kind of abrasive used.

 • The letter ‘A’ denotes that the type of abrasive is aluminium oxide. In case of silicon carbide the letter ‘C’ is used.

 • The number ‘60’ specifies the average grit size in inch mesh. For a very large size grit this number may be as small as 6 where as for a very fine grit the designated number may be as high as 600.

• The letter ‘K’ denotes the hardness of the wheel, which means the amount of force required to pull out a single bonded abrasive grit by bond fracture. The letter symbol can range between ‘A’ and ‘Z’, ‘A’ denoting the softest grade and ‘Z’ denoting the hardest one.

 • The number ‘5’ denotes the structure or porosity of the wheel. This number can assume any value between 1 to 20, ‘1’ indicating high porosity and ‘20’ indicating low porosity.

• The letter code ‘V’ means that the bond material used is vitrified. The codes for other bond materials used in conventional abrasive wheels are B (resinoid), BF (resinoid reinforced), E(shellac), O(oxychloride), R(rubber), RF (rubber reinforced), S(silicate).

 • The number ‘05’ is a wheel manufacturer’s identifier.

Standard marking system for superabrasive grinding wheel:- 

Marking system for superabrasive grinding wheel is somewhat different as illustrated below

R   D   120   N   100    M   4

• The letter ‘R’ is manufacture’s code indicating the exact type of superabrasive used.

 • The letter ‘D’ denotes that the type of abrasive is diamond. In case of cBN the letter ‘B’ is used.

 • The number ‘120’ specifies the average grain size in inch mesh. However, a two number designation (e.g. 120/140) is utilized for controlling the size of superabrasive grit. The two number designation of grit size along with corresponding designation in micron is given in table 28.1.

 • Like conventional abrasive wheel, the letter ‘N’ denotes the hardness of the wheel. However, resin and metal bonded wheels are produced with almost no porosity and effective grade of the wheel is obtained by modifying the bond formulation.

• The number ‘100’ is known as concentration number indicating the amount of abrasive contained in the wheel. The number ‘100’ corresponds to an abrasive content of 4.4 carats/cm3. For diamond grit, ‘100’ concentration is 25% by volume. For cBN the corresponding volumetric concentration is 24%.

 • The letter ‘M’ denotes that the type of bond is metallic. The other types of bonds used in superabrasive wheels are resin, vitrified or metal bond, which make a composite structure with the grit material. However, another type of superabrasive wheel with both diamond and cBN is also manufactured where a single layer of superabrasive grits are bonded on a metal perform by a galvanic metal layer or a brazed metal layer.

Selection of grinding wheels:-
 Selection of grinding wheel means selection of composition of the grinding wheel and this depends upon the following factors:

1.      Physical and chemical characteristics of the work material
2.     Grinding conditions 
3.     Type of grinding (stock removal grinding or form finish grinding) 

 Type of abrasives 

 Aluminium oxide:
 Aluminium oxide may have variation in properties arising out of differences in chemical composition and structure associated with the manufacturing process.

Pure Al2O3 grit with defect structure like voids leads to unusually sharp free cutting action with low strength and is advantageous in fine tool grinding operation, and heat sensitive operations on hard, ferrous materials.

Regular or brown aluminium oxide (doped with TiO2) possesses lower hardness and higher toughness than the white Al2O3 and is recommended heavy duty grinding to semi finishing.

Al2O3 alloyed with chromium oxide (<3%) is pink in colour.

Monocrystalline Al2O3 grits make a balance between hardness and toughness and are efficient in medium pressure heat sensitive operation on ferrous materials.

Microcrystalline Al2O3 grits of enhanced toughness are practically suitable for stock removal grinding. Al2O3 alloyed with zirconia also makes extremely tough grit mostly suitably for high pressure, high material removal grinding on ferrous material and are not recommended for precision grinding. Microcrystalline sintered Al2O3 grit is the latest development particularly known for its toughness and self sharpening characteristics.

 Silicon carbide:-

Silicon carbide Silicon carbide is harder than alumina but less tough. Silicon carbide is also inferior to Al2O3 because of its chemical reactivity with iron and steel.

Black carbide containing at least 95% SiC is less hard but tougher than green SiC and is efficient for grinding soft nonferrous materials.

Green silicon carbide contains at least 97% SiC. It is harder than black variety and is used for grinding cemented carbide.

Diamond grit is best suited for grinding cemented carbides, glass, sapphire, stone, granite, marble, concrete, oxide, non-oxide ceramic, fiber reinforced plastics, ferrite, graphite.

Natural diamond grit is characterized by its random shape, very sharp cutting edge and free cutting action and is exclusively used in metallic, electroplated and brazed bond.

Monocrystalline diamond grits are known for their strength and designed for particularly demanding application. These are also used in metallic, galvanic and brazed bond.

Polycrystalline diamond grits are more friable than monocrystalline one and found to be most suitable for grinding of cemented carbide with low pressure. These grits are used in resin bond.

cBN (cubic boron nitride):-

Diamond though hardest is not suitable for grinding ferrous materials because of its reactivity. In contrast, cBN the second hardest material, because of its chemical stability is the abrasive material of choice for efficient grinding of HSS, alloy steels, HSTR alloys.

Presently cBN grits are available as monocrystalline type with medium strength and blocky monocrystals with much higher strength. Medium strength crystals are more friable and used in resin bond for those applications where grinding force is not so high. High strength crystals are used with vitrified, electroplated or brazed bond where large grinding force is expected.

Microcrystalline cBN is known for its highest toughness and auto sharpening character and found to be best candidate for HEDG and abrasive milling. It can be used in all types of bond.

Grit size:-
 The grain size affects material removal rate and the surface quality of workpiece in grinding. Large grit- big grinding capacity, rough workpiece surface Fine grit- small grinding capacity, smooth workpiece surface

The worn out grit must pull out from the bond and make room for fresh sharp grit in order to avoid excessive rise of grinding force and temperature. Therefore, a soft grade should be chosen for grinding hard material. On the other hand, during grinding of low strength soft material grit does not wear out so quickly. Therefore, the grit can be held with strong bond so that premature grit dislodgement can be avoided.

 Structure / concentration:-
The structure should be open for grinding wheels engaged in high material removal to provide chip accommodation space. The space between the grits also serves as pocket for holding grinding fluid. On the other hand dense structured wheels are used for longer wheel life, for holding precision forms and profiles.

Bond :-

vitrified bond:-
Vitrified bond is suitable for high stock removal even at dry condition. It can also be safely used in wet grinding. It can not be used where mechanical impact or thermal variations are like to occur. This bond is also not recommended for very high speed grinding because of possible breakage of the bond under centrifugal force.

Resin bond:-
Conventional abrasive resin bonded wheels are widely used for heavy duty grinding because of their ability to withstand shock load. This bond is also known for its vibration absorbing characteristics and finds its use with diamond and cBN in grinding of cemented carbide and steel respectively. Resin bond is not recommended with alkaline grinding fluid for a possible chemical attack leading to bond weakening. Fiberglass reinforced resin bond is used with cut off wheels which requires added strength under high speed operation.

Shellac bond:-
At one time this bond was used for flexible cut off wheels. At present use of shellac bond is limited to grinding wheels engaged in fine finish of rolls.

 Oxychloride bond:-
It is less common type bond, but still can be used in disc grinding operation. It is used under dry condition.

Rubber bond:-
Its principal use is in thin wheels for wet cut-off operation. Rubber bond was once popular for finish grinding on bearings and cutting tools.

Metal bond:-
Metal bond is extensively used with superabrasive wheels. Extremely high toughness of metal bonded wheels makes these very effective in those applications where form accuracy as well as large stock removal is desired.

Electroplated bond:-
This bond allows large (30-40%) crystal exposure above the bond without need of any truing or dressing. This bond is specially used for making small diameter wheel, form wheel and thin superabrasive wheels. Presently it is the only bond for making wheels for abrasive milling and ultra high speed grinding.

Brazed bond:-
This is relatively a recent development, allows crystal exposure as high 60-80%. In addition grit spacing can be precisely controlled. This bond is particularly suitable for very high material removal either with diamond or cBN wheel. The bond strength is much greater than provided by electroplated bond. This bond is expected to replace electroplated bond in many applications.

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