Grinding

SURFACE FINISHING
PROCESS
PRESENTED BY
SEENIVASABABU B G
B.E.MECHANICAL ENGINEER

GRINDING
Grinding is a material removal process accomplished by abrasive
particles that are contained in a bonded grinding wheel rotating at very
high surface speeds.
The rotating grinding wheel consists of many cutting teeth ( abrasive
particles ) and the work is fed relative to the rotating grinding wheel to
accomplish material removal.

Applications of grinding
 Surface finishing
Slitting and parting
 De-scaling , De-burring
 Stock removal (abrasive milling) finishing of flat as well as cylindrical
surface
 Grinding of tools and cutters and re-sharpening of the same.
Advantages of grinding:
 Dimensional accuracy and good surface finish.
 Good form and locational accuracy.
 Applicable to both hardened and unhardened material.

Classification of Grinding Machine
Grinding machines are classified based on their
construction & type of surface produced.
Bench Grinding Machine
Cylindrical Grinding Machine
Surface Grinding Machine
Centerless Grinding Machine
Internal Grinding Machine
Special Purpose Grinding Machine

Bench grinder
Portable Grinder
Pedestal Type

Cylindrical Grinding
 It is used generally for producing external & internal cylindrical surfaces.
 The machine is very similar to a centre lathe.
 The grinding wheel is located similar to the tool post with an independent
power and is driven at a high speed suitable for the grinding operation.
There are four movements in a cylindrical grinding process.
i. Rotation of cylindrical work piece about its axis.
ii. Rotation of grinding wheel about its axis.
iii. Longitudinal feed movement of the
work past the wheel face.
iv. Movement of wheel into the work
perpendicular to the axis of the work to give depth of cut.

Classification of Cylindrical Grinding
Plain center type cylindrical Grinding Machine.
Universal cylindrical Grinding Machine.
Internal cylindrical Grinding Machine

Plain center type cylindrical Grinding Machine

Construction
Base:
The base or bed is the main casting that rest on the floor and supports the
parts mounted on it. On the top of the base are precision horizontal ways set at
right angles for the table to slide on the base.
Tables:
There are two tables, lower table and upper table. The lower table slides
on ways on the bed and provides traverse of the work past the grinding wheel.
It can be moved by hand or power within desired limits.
The upper table that is pivoted at its centre is mounted on the top of the
sliding table.
It has T-slots for securing the head stock and tail stock or foot stock and
can be positioned along the table to suit the length of the work.
The upper table can be swiveled and clamped in position to provide
adjustment for grinding straight or tapered work as desired.

Head stock:
The headstock supports the work piece by means of a dead
centre and drives it be means of a dog, or it may hold and drive the
work piece in a chuck.
Tail stock:
The tail stock can be adjusted and clampen in various positions
to accommodate different lengths of work piece.
Wheel head:
The wheel head carries a grinding wheel and its driving motor is
mounted on a slide at the top and rear of the base.
The wheel head may be moved perpendicularly to the table
ways, by hand or power, to feed the wheel to the work.
The grinding wheel is fed to the work by hand or power as
determined by the engagement of the cross-feed control lever.

Universal cylindrical Grinding Machine
These grinders, in addition to the features offered by plain grinders, are
provided with a swiveling headstock and a swiveling wheel head.
This permits the grinding of taper of any angle, much greater than is
possible in plain grinder.
Universal machines are available to handle parts requiring swings up
to 450 mm and centre distance of 1800mm.
This allows grinding of any taper on the work piece. Universal grinder
is also equipped with an additional head for internal grinding.

Internal cylindrical Grinding Machine
Internal grinding is employed chiefly for finishing accurate holes in
hardened parts, and also when it is impossible to apply other more productive
methods of finishing accurate hole, for example, precision boring, honing etc.
Two Methods:
 With a rotating work piece.
 With the work piece held stationary.

Internal cylindrical grinding machine
With a rotating work piece:
The first method is used in grinding holes in relatively small work pieces,
mostly bodies of revolution, for example, the bores of gears and the inner surfaces
of ball bearing rings.
The work piece is held in a chuck or special fixture and rotated in the same
manner as in a lathe.
With the work piece held stationary:
The second method of internal grinding is used for grinding holes in large
bulky work pieces (housing-type parts) that are inconvenient or even impossible to
clamp in a chuck of the grinder.
They are mounted on the table of a planetary grinding machine.

Surface Grinding
This machine may be similar to a milling machine used mainly
to grind flat surface. However, some types of surface grinders are also
capable of producing contour surface with formed grinding wheel.
There are basically four types of machines depending upon the spindle
direction and the table motion.
 Horizontal spindle and reciprocating table grinding machine,
 Vertical spindle and reciprocating table grinding machine,
 Horizontal spindle and rotating table grinding machine and
 Vertical spindle and rotating table grinding machine.

Horizontal spindle and reciprocating table
Grinding Machine
This machine with various
motions required for grinding
action. A disc type grinding wheel
performs the grinding action with
its peripheral surface.

Vertical spindle and reciprocating table
Grinding Machine
The grinding operation is similar to that of
face milling on a vertical milling machine.
This brings more grits in action at the same
time and consequently a higher material removal
rate may be attained than for grinding with a
peripheral wheel.

Horizontal spindle and rotating table
Grinding Machine
In principle the operation is same as that for facing on the lathe.
This machine has a limitation in accommodation of work piece and
therefore does not have wide spread use.
However, by swiveling the worktable, concave or convex or
tapered surface can be produced on individual parts

Vertical spindle and rotating table
Grinding Machine
The machine is mostly suitable for small work pieces in large quantities.
This primarily production type machine often uses two or more grinding heads
thus enabling both roughing and finishing in one rotation of the work table.

Centreless grinding
Center less grinding is a process for continuously grinding cylindrical
surfaces in which the work piece is supported not by centers or chucks but by a
rest blade.
The work piece is ground between two wheels. The larger grinding wheel
does grinding, while the smaller regulating wheel,
Center less grinding can also be external or internal, traverse feed or
plunge grinding. The most common type of center less grinding is the external
traverse feed grinding.

Centreless External Grinding Machine
This grinding machine is a production machine in which outside
diameter of the work piece is ground.
The work piece is not held between centres but by a work support
blade.
It is rotated by means of a regulating wheel and ground by the
grinding wheel.

Internal Centreless Grinding
The principle of external centreless grinding is applied to internal
centreless grinding also.
Grinding is done on the inner surfaces of the holes. In internal
centreless grinding, the work is supported by three rolls – a regulating roll, a
supporting roll and a pressure roll.
The grinding wheel contacts the
inside surface of the workpiece directly
opposite the regulating roll.
The distance between the contours
of these two wheels is the wall thickness
of the work.

Types of abrasives
These are the hard materials with adequate toughness so that they will
be able to act as cutting edges for a sufficiently long time.
They also have the ability to fracture into smaller pieces when the
forces increases, which is termed friability.
This property gives abrasives the necessary self-sharpening capability
in use.
The abrasives that are generally used are
1. Aluminium Oxide. (Al2O3)
2. Silicon Carbide. (SiC)
3. Cubic Boron Nitride. (CBN)
4. Diamond.

Types of abrasives
Aluminium Oxide (Al2O3):
 This is one of the natural abrasives found and is commonly called corundum and
emery.
 However, natural abrasives generally have impurities and as a result, their
performance is inconsistent.
 Hence, the abrasive used in grinding wheels is generally manufactured from the
aluminium ore, bauxite.
Silicon Carbide (SiC):
 Silicon carbide is made from silica sand and coke with small amounts of common
salt.

Types of abrasives
Cubic Boron Nitride (CBN):
 Cubic Boron Nitride is next in hardness only to diamond. It is
not a natural material but produced in the laboratory using a
high-temperature/high-pressure process similar to the making of
artificial diamond.
 This is very expensive, 10 to 20 times that of the conventional
abrasive such as aluminium oxide.
Diamond:
 Diamond is the hardest material that can be used as a cutting tool
material.
 It has very high chemical resistance along with low coefficient of
thermal expansion. Also it is inert towards iron.

Bond
Bonding material are used as binders to hold the abrasive particle in place.
The firmness with which the grains are held in wheel and the strength of
wheel itself in which large centrifugal forces are developed in rotation depend on
the bonding material.
The bonding material determines the force that is required to dislodge an
abrasive particle from the wheel which plays a major role in cutting action.
Types of bonds - Bonds are classified into two types:
1. Organic - Resinoid, Rubber, Shellac.
2. Non - Organic - Metallic, Vitrified & Silicate.

Grit size or grain size
It refers to the actual size of the abrasive particles. The grain size is
denoted by the number.
The grain size affects material removal rate and the surface quality of
work piece in grinding.
 Large grit : Big grinding capacity, rough work piece surface.
 Fine grit : Small grinding capacity, smooth work piece surface.

Grade
Grade or hardness indicates the strength with which the bonding material
holds the abrasive grains in the grinding wheel.
This means the amount of force required to pull out a single bonded
abrasive grit by bond fracture. It does not refer to the hardness of the abrasive
grain.
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.
The selection of a particular grade of wheel is largely governed by the
nature of work, its composition, size, and hardness.

Structure / Concentration of wheels
 This term denotes the spacing between the abrasive grains or in other words
the density of the wheel.
 Structure of the grinding wheel is designated by a number.
 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.

Specification of grinding wheel
A grinding wheel requires two types of specification:
1. Geometrical specification. 2. Compositional specification.
Geometrical specification:
 This is decided by the type of grinding machine and the grinding operation to
be performed in the work piece.
 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.

Specification of grinding wheel
Compositional specifications:
Specification of a grinding wheel ordinarily means compositional specification.
Conventional abrasive grinding wheels are specified encompassing the following
parameters.
 The type of grit material.
 The grit size.
 The bond strength of the wheel, commonly known as wheel hardness.
 The structures of the wheel denoting the porosity i.e. the amount of inter grit
spacing.
 The type of bond material.
 Other than these parameters, the wheel manufacturer may add their own
identification code prefixing or suffixing (or both) the standard code.

Indian standard marking system
W A 36 K 5 R 17
Where,
W - Manufacture’s symbol indicating exact kind of abrasive,
(optional use).
A - Abrasive type: A for Al2O3, C for SiC, D for Diamond.
36 - Grain size.
K - Grade.
5 - Structure.
R - Bond type.
17 - Private marking to identify the wheel, (optional use).

Grinding

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