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
 where
• 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
where
• 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:-
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
 Grade:-
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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