Type of Grain
The type of grain used in the wheel plays a vital role in grinding. Diamonds is preferred to grind hard materials but the most used ones are aluminium oxide and silicon carbide. Aluminium oxide grains are preferred to grind materials like steels as silicon carbide grains dull rapidly, aluminium oxide grinding wheels are used on materials like hard stainless steels, annealed malleable iron, tough bronzes and use silicon carbide to grind grey iron, chilled iron, brass, soft bronze, hard facing alloys and cemented carbides.
Size of the grain is deterministic of the surface finish required. Small grains give finer finishes than large ones.
Bonding Type
Vitrified bond being the most common one which is hardened by baking. Silicate bond holds the grains loosely and the wheel behaves softer than the vitrified bonded wheel. Rubber bond is done with vulcanised rubber and is used to manufacture thin and flexible wheels. Shellac bonded wheels produce smooth finishes on hard surfaces and metallic bonded wheels are used with diamond, CBN as grains and they are not to be dressed the usual way but by processes as ELID or ECDD.
Resins like shellac and rubber as bonds have to be chosen for good surface finishes and vitrified for better material removal, when the surface speeds exceed 32 m/sec, vitrified bonded wheels are not to be chosen.
CVD(Chemical Vapour Deposition) is a special type of abrasive where poly-crystalline diamond films with sharp edges having micron sized diamond crystallites are also available these are referred to as bondless diamond grinding wheels.
Wheel Wear
The performance of grinding wheel has an important factor in grinding ratio which is the ratio of metal volume removed to the volume of grinding wheel wear. Grinding ratio can be elaborated upon on the effective explanation of the effect of thrust force on wheel removal patterns.
Coolants
The main qualities of cooling fluids or coolants are: (i) to rapidly remove heat from the grinding zone or the heat affected zone, (ii) to provide lubrication, and (iii) to flush away the ground chips.
The grinding fluids or coolants can be subdivided into three main categories as water miscible, water immiscible or oils, water composite fluids. Owing to their high viscosity index due to the paraffinic or naphthenic petroleum distillate origin, water immiscibles provide higher lubrication. But lower cooling properties relative to the other types which are formulated to be soluble in water [10, 26, 37–39]. Although their role as lubricants in some cases is considered more important, straight oils, because of the mist and the danger of fire propagation, are only used with special precautions and equipment where lubrication is critical for form and finish.
Synthetic fluids have also been extensively used since the last decade.
The type of grinding fluid used is determined on the application and changes depending on the place of application. The main characteristics of grinding fluids have to be low corrosivity, high thermal coefficient, high human compatibility, high aging resistance, higher washing capability and good lubricating capability.
Surface of the Wheel - Specific Energy
The ground surface and grinding forces are affected by the surface of the wheel. The wheel deteriorates due to grinding and should be dressed before the machined surface deteriorates beyond a quality limit of surface integrity as the deteriorated wheel will increase friction which in turn will increase the temperature at the wheel-work piece zone. In order to achieve the best wheel surface, dressing parameters must be set. The mathematical models use a simple energy method or slip-line field method to predict cutting forces.
Depth of cut
The lesser the depth of cut, yields small and non-continuous chips which gives a rough finish and the subsurface will be abound with micro cracks.
The increase in depth of cut has the capability of enhancing the amount of material removed and with that can govern the formation of cracks in the subsurface and there by an improvement in the surface finish. The grinding wheel will be spoiled and might crumble if the depth of cut is enhanced beyond a point and this point is the place yields the chip and the thickness is called critical chip thickness.
Feed rate
Higher feed rates are going to enhance the surface finish if the speed of the grinding wheel is high and the depth of cut is appropriate for the formation of chips and the thickness of which is going to be critical.
Lower feed rates are appropriate for lesser depth of cuts or if the material is hard and requiring a good surface finish.
Mathematical Formulae
To be edited later
Surface finish Ra according to Malkin [7a] has
the relationships with material removal rate
Q′w and wheel parameters as dressing leads Sd
and grinding wheel speed vs as
x K f d s vw Q' Ra 1/ 3
1 x vw Q a d d s R K S a ' 2 1/ 2 1/ 4
2 Ra, (dgy )
3 2 r a Cw R
4 where, δd is the dressing angle, ad is dressing
depth, dg is average grinding dimension, Cr is
the active grits per unit area, w is grinding
width, Kf and K2 are constants. The finding of
the exact number of active grits is the main
limitation of these formulae
Eranki et.al [24], formulated material removal
rate Q’w as a function of surface roughness Ra
and wheel grade g0 as
Q'w 35.0g00.78Ra0.43
5
And wheel grade as a function of grinding
ratio G and surface roughness Ra as
0.24
0 4.49 0.46 0.61
a R R g G a
6
Material removal rate Q′w as a relation of
surface roughness Ra and grinding ratio G
0.62 '
10.85 0.36 0.6 1
a R w R G Q a
7
And dressing leads Sd is related to surface
roughness Ra as
Sd 0.126Ra1.6
8
Fawcett and dow [25] formulated a relation
between surface roughness Ra and feed rate fr
as
Ra kfr2
9 where, k is a constant
A relationship between surface roughness Ra
and nose radius Nr was also given by them
[25] as
r a r N R f
12 5 2
10
This relation for surface roughness in contour
grinding is made easy as the variables being
used are less and can be easily found out
unlike finding out the number of active grits
on the wheel.
Metal
removal rate (Zw) and wheel removal rate (Zt)
can be expressed as
( ) 11
( ) 12
where w and t are workpiece and
wheel removal parameters [27] and these can
be taken as the volume of workpiece or wheel
removed per unit time per unit thrust force.
Metal and wheel removal rates can be shown
as
13
14
where dw is the diameter of the workpiece, ap
is the back engagement (width of cut), vf
is the
feed speed.
The above mentioned metal removal
parameter is expressed as below by [29] for
easy to grind materials and using a single point
diamond dressing as, being dependent on
wheel speed, workpiece hardness and wheel
dressing conditions as: ( ⁄ ) [ ( ⁄ )]
⁄
15
⁄
( ⁄ ) ( ⁄ )
( ( ⁄ ))
⁄
16
where vw is workpiece surface speed in
mm/min; vt
is wheel surface speed in mm/min;
ad is depth of dress in mm; fd is lead of
dressing mm/rev; de
is wheel diameter mm;
Rkc is Rockwell hardness number of the work
material (C scale); dg is the grain diameter in
mm; ka
is constant dependent on coolant and
wheel grain type; adf is down feed of grinding
mm/pass; Vb is percentage volume of bond
material in the wheel given as:
16a
where Hn is wheel hardness number; Sn is
wheel structure number.
Grinding ratio (G) is the main constraint when
considering the wheel performance expressed
as
17
Thrust force affects the grinding ratio as
shown in the Figure 5 given in [26]. Figure 10
shows that as there is a steep rise in Zt
the
grinding ratio is also is rising and applying
more force on the wheel to grind beyond this
point will degrade the wheel faster.
The total specific energy, u generated by the
grinding process as postulated by [7a] is
18
the specific energy of the chip is of the main
concern and is given as
18a
The
specific energy is related to the maximum
undeformed chip thickness hm as given by [7a] [
( ) ( ) ]
19
where ds
is the wheel diameter, C is active
grits per unit area of wheel surface, and β is
the semi-included angle of the chip crosssection.
Hwang and Malkin [55] modified the
undeformed chip thickness, taking into
account that the tip be taken as round to
decrease the specific energy which is stated by
[55] as:
20
√
( )
21
The plowing of workpiece if a pyramidal tool
with square base is used as done by Abebe
[54] is shown in the Figure 9 which shows
different heights as well as the angles during
grinding with a single point pyramidal grain.
The tool if taken to be pyramidal and having a
rounded tip as shown in the Figure 11 above
then the shape of the scratch is parabolic as
shown in the Figure 10 then the surface
roughness can be found out from the shape of
the scratch as:
22
where ( )
√
√
The value of z can be found out by trial and
error from experimentation.
The crack free and ductile grinding of optical
glasses which can be taken as hard and brittle
material can be done if we maintain
hydrostatic pressure and the maximum chip
thickness not surpassing critical material
specific chip thickness to avoid cracks being
formed. This critical chip thickness is
formulated as
(
) (
)
23
where E is the young’s modulus, HK is knoop
hardness and the Kc
is critical fracture
toughness of the material under consideration
for which critical chip thickness (hcrit) is to be
found. Bilfano et al. [12, 62–64,] found the
constant value to be approximately 0.15 after
extensive grinding experimentation on
different materials
Main Source Paper
Influence of Process Parameters on Grinding - A Review
P.V. Vinay1*, Ch. Srinivasa Rao2
1.Department of Mechanical Engineering, GVP College for Degree and PG Courses
(Technical Campus), Rushikonda, Visakhapatnam
2.Department of Mechanical Engineering, Andhra University College of Engineering (A),
Visakhapatnam
Trends in Mechanical Engineering & Technology, TMET (2013)
Volume 3, Issue 2, ISSN: 2231-1793, pp. 16-28
____________________
____________________
No comments:
Post a Comment