2. Syllabus (Machine Tool)
Unit-III
Grinding & Super finishing
(i) Grinding: Grinding wheels, abrasive & bonds, cutting
action. Grinding wheel specification.
Grinding wheel wear - attritions wear, fracture wear.
Dressing and Truing. Max chip thickness and
Guest criteria. Surface and cylindrical grinding. Centerless
grinding
(ii) Super finishing: Honing, lapping and polishing.
Limits, Fits & Tolerance and Surface roughness: Introduction
to Limits, Fits, Tolerances and IS standards, Limit-gauges,
and surface-roughness.
3. Types of Machining
A. Conventional ( e.g. boring, milling.. etc)
Cutting Abrasive
Axisymmetric (drilling) Bonded cutting (grinding)
Prismatic (milling..) Loose cutting (lapping..)
B. Non Conventional (e.g. WJM, EDM, AJM, AWJM.. etc)
4. Grinding
• Grinding is the most common form of abrasive
machining. It is a material cutting process which
engages an abrasive tool whose cutting elements are
grains of abrasive material known as grit.
These grits are characterized by
sharp cutting points, high hot
hardness, chemical stability
and wear resistance. The grits
are held together by a suitable
bonding material to give shape
of an abrasive tool.
Conventional -- Abrasive -- Bonded cutting
5. Major advantages and applications of grinding
Advantages
Grinding process has
Good dimensional accuracy
&
Good surface finish
Applications
• Surface finishing
• Slitting and parting
• Descaling, deburring
• Stock removal (abrasive milling)
• Finishing of flat as well as cylindrical surface
• Grinding of tools and cutters and resharpening of the same.
6. Mechanism of chip formation
• However, grits having large negative rake angle or
rounded cutting edge do not form chips but may rub
or make a groove by ploughing leading to lateral flow
of the workpiece material
Grit with favorable
geometry can
produce chip in
shear mode.
7. Effect of rake angle & clearance angle
As the grits do not have definite
geometry unlike a cutting tool
& the grit rake angle may vary
from +45 to -60 or more.
9. Basically there are four different
types of Surface grinding machines
characterized by the movement of
their tables and the orientation of
grinding wheel spindles as follows:
• Horizontal spindle and
reciprocating table
Disc type grinding wheel performs
the grinding action with its
peripheral surface.
10. In this machine a cup
shaped wheel grinds the
workpiece over its full width
using end face of the
wheel. This brings more grits
in action consequently a
higher material removal
rate attained.
Vertical spindle and
reciprocating table
11. Horizontal spindle
and rotary tableIn principle the operation is
same as that for facing on the
lathe. This machine has a
limitation in accommodation
of workpiece and therefore
does not have wide spread
use. However, by swivelling the
worktable, concave or convex
or tapered surface can be
produced on individual part
12. Vertical spindle rotary table
grinder
The machine is mostly
suitable for small
workpieces in large
quantities. This
primarily production
type machine often
uses two or more
grinding heads.
13. Cylindrical grinding machine
• Plain centre type cylindrical grinder
• Universal cylindrical surface grinder
• Centreless cylindrical surface grinder
14. Centreless grinding
This grinding machine is a production machine in which, outside diameter of
the workpiece is ground. The workpiece 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.
The regulating wheel revolving at a much lower surface speed than grinding
wheel, controls the rotation and longitudinal motion of the workpiece. The
regulating wheel is kept slightly inclined to the axis of the grinding wheel and
the workpiece is fed longitudinally.
15. Internal grinding machine
• This machine is used to produce internal cylindrical surface. The surface
may be straight, tapered, grooved or profiled. Broadly:
– Chucking type internal grinder
– Planetary internal grinder
– Centreless internal grinder
17. Centerless Grinding Advantages
• Amount of time that is saved
• Since parts can be loaded automatically, it is quicker and easier to load
parts into a centerless grinder compared to traditional grinding machines.
• Work that is smaller in size can be loaded into a centerless grinder in large
quantities.
• There is no axial thrust or end pressure imposed on the work during the
grinding process which makes it possible to grind longer pieces of brittle
material as well as distorted parts.
• The work is supported directly under the cut, as well as for the full length of
the grinding cut. This support stops deflection from taking place which
permits heavier passes than grinding between centers
18. Grinding Wheel representation Grain size: The grain
size of the abrasive
particle is an
important parameter
in determining surface
finish and material
removal rate.
Small grit sizes-better
finishes
larger grain - larger MRR
abrasive grains are classified on basis of mesh size. (larger numbers
and vice versa)
Grain sizes in grinding wheels range between 6(very coarse )-600.
19. Wheel grade
Wheel grades indicates the wheel bond strength. It is measured on a
scale ranging from soft to hard. Soft wheels loose grains easily and are
used for low material removal rates and grinding of hard materials.
Harder grades are preferred for high productivity and grinding of
relatively soft materials,
Abrasive
materials
20. Structure
The wheel structure indicates spacing of the abrasive
grains in the wheel. It is measured on a scale that ranges
from open to dense.
Open structure means more pores and fewer grains per
unit wheel volume, and vice versa. Open structure is
recommended for work materials that tend to produce
continuous chips, while denser structure is used for better
surface finish and dimensional precision.
21. Bonds : The bonding material holds the abrasive grains and establishes the
shape and structural integrity o;f the grinding wheel. Desirable properties
of the bond material include strength, toughness, hardness, and
temperature resistance.
Bonding materials commonly used in grinding wheels include the
following:
Vitrified bond: (Mostly used) chiefly Ceramic materials. They are strong
and rigid, resistant to elevated temperatures, and relatively unaffected by
cutting fluids & most flexible of the bonding materials.
Rubber bond: rubber is the most flexible of the bonding materials. It is used
as a bonding material in cutoff wheels.
Resinoid bond: This bond is made of various thermosetting resin materials.
They have very high strength and are used for rough grinding
Shellac bond: shellac-bonded grinding wheels are relatively strong but
not rigid.
Metallic bond: metal bonds, usually bronze, are the common bond
material for diamond and CBN grinding wheels.
23. Grinding wheel wear mechanisms
Three mechanisms are recognized as the principal causes of wear in grinding wheels: Œ
Grain fracture, •
Attritious wear Ž
Bond fracture.
Grain fracture occurs when a portion of the grain breaks off but the rest of the grain
remains bonded in the wheel. The edges of the fractured area become new sharp cutting
edges on the grinding wheel. This makes the grinding wheel self-sharpening, a unique property
of a cutting tool.
Attritious wear involves dulling of the individual grains, resulting in flat spots and rounded
edges. Attritious wear is analogous to tool wear in a conventional cutting tool.
Bond fracture occurs when the individual grains are pulled out of the bonding material.
Bond fracture usually occurs because the grain has become dull due to attritious wear and the
resulting cutting force is excessive. Sharp grains cut more efficiently with lower cutting forces;
hence, they remain attached in the bond structure.
24. Dressing and Truing
After prolonged use, the grinding wheel gets
LOADED (pores filled with chip) & GLAZED (blunt)
Such grinding wheels can be reconditioned by
DRESSING & TRUING
• Dressing is the conditioning of the wheel surface which ensures that
grit cutting edges are exposed from the bond and sharp enough to
penetrate the workpiece material, for free cutting.
• Truing is regenerating the required geometry on the grinding wheel.
Both operations are performed with a tool called an abrasive wheel
dresser.
• For softer wheels, truing and dressing are done separately, but for
harder wheels (such as CBN), both are done in one operation.
Reconditioning of
Grinding Wheels
25. Dressing Truing
• produces the macro-geometry of
the grinding wheel.
• also required on a new
conventional wheel to ensure
concentricity with specific
mounting system.
• produces micro-geometry.
• Dressing is cutting the face of a
grinding wheel to restore its
original cutting qualities.
• Dressing is necessary when
excessive wear dulls the wheel
(called glazing) because of the
shiny appearance of the wheel
surface) or when the wheel
becomes loaded.
26. Super finishing : Honing, lapping and polishing
To ensure reliable performance and prolonged service life of
modern machinery, its components require to be manufactured
not only with high dimensional and geometrical accuracy but also
with high surface finish. The surface finish has a vital role in
influencing functional characteristics like wear resistance, fatigue
strength, corrosion resistance and power loss due to friction.
Unfortunately, normal machining methods like turning, milling or
even classical grinding can not meet this stringent requirement.
Therefore, superfinishing processes like lapping, honing, polishing,
burnishing are being employed to achieve and improve the high
surface finish properties in the machine component.
27. Lapping
Lapping is basically an abrasive process
in which loose abrasives function as
cutting points finding support from the
laps. Figure below schematically
represents the lapping process. Material
removal in lapping usually ranges from
0.08 to 0.25mm, but many reach .003 to
.03 mm.
28. Abrasives of lapping:
• Al2O3 and SiC, grain size 5~100μm
• Cr2O3, grain size 1~2 μm
• B4C3, grain size 5-60 μm
• Diamond, grain size 0.5~5 μm
Cast iron, soft steel, copper, brass,
hardwood as well as hardened steel and
glass are the mostly used lap material.
Vehicle materials for lapping
• Machine oil
• Rap oil
• grease
Lapping is performed
•Manually
•Machine
Classification
• Manually
Hand lapping
• Machine
Flat surface
Cylindrical surface
Centreless lapping
29. Characteristics of Lapping process:
• Use of loose abrasive between lap
and the workpiece
• Usually lap and workpiece are
guided in contact with each other
• Relative motion between the lap
and the work change continuously,
i.e. abrasive grains of the lap does
not repeat and no Lay patten is
observed on the workpiece.
30. Honing • Honing is a superfinishing process, in which a tool
called hone carries out a combined rotary and
reciprocating motion while the workpiece does
not perform any working motion. Most honing is
done on internal cylindrical surface, such as
automobile cylindrical walls. The honing stones are
held against the workpiece with controlled light
pressure. The honing head is not guided externally
instead, floats in the hole, being guided by the
work surface. It is desired that
• Honing stones should not leave the work surface.
• Stroke length must cover the entire work length.
In honing rotary and oscillatory motions are
combined to produce a cross hatched lay pattern.
31. The honing stones are given a
complex motion so as to prevent
every single grit from repeating
its path over the work surface.
The critical process parameters
are:
• oscillation speed
• length and position of the
stroke
• honing stick pressure
• rotation speed
The important parameters that affect material
removal rate (MRR) and surface roughness (R)
are:
(i) unit pressure, p
(ii) peripheral honing speed, Vc
(iii) honing time, T
32. Polishing
Polishing is a super finishing operation in which the abrasive stone (tool)
is slowly fed in radial direction while its oscillation is imparted in the axial
direction.
End-face of a cylindrical
workpiece. In this both feeding and
oscillation of the superfinishing
stone is given in the radial direction.
In this case the abrasive stone
covers the section of the workpiece
requiring polished
33. Limits, Fits & Tolerance --- WHY?
• No two parts can be produced with identical measurements by
any man/machine.
• In any production process, regardless of how well it is designed
or how carefully it is maintained, a certain amount of natural
variability will always exist in manufacturing process. These
natural variations are random in nature and are the cumulative
effect of many small, essentially uncontrollable causes.
• Usually, variability arises from improperly adjusted machines,
operator error, tool wear, and/or defective raw materials.
34. Tolerance
• Tolerance can be defined as the magnitude of permissible
variation of a dimension or other measured value from the
specified value.
• It can also be defined as the total variation permitted in the size of
a dimension, and is the algebraic difference between the upper
and lower acceptable dimensions. It is an absolute value.
The basic purpose of providing tolerances is to permit dimensional
variations in the manufacture of components, adhering to the
performance criterion as established by the specification and design
The industry follows certain approved accuracy standards, such as
ANSI (American National Standards Institute) and ASME (American
Society of Mechanical Engineers), to manufacture different parts.
35. Tolerance
Unilateral Limits occurs when both maximum limit and the minimum
limit are either above or below the basic size.
e.g.
Basic Size = 25 00 mm
Upper Limit = 25.18 mm
Lower Limit Lower Limit = 25.10 mm
Tolerance = 0.08 mm
e g
Basic Size = 25.00 mm
Upper Limit = 24.90 mm
Lower Limit = 24.80 mm
Tolerance = 0.10 mm
36. Bilateral
Bilateral Limits occur when the maximum limit is above and
the minimum limit is below the basic size.
e.g.
Basic Size = 25.00 mm
Upper Limit = 25.04 mm
Lower Li it m = 24.96 mm
Tolerance = 0.08 mm
37. Allowance
It is Minimum clearance or maximum interference. It is the intentional
difference between the basic dimensions of the mating parts. The
allowance may be positive or negative.
38.
39.
40. Fit
Fits :- (assembly condition between “Hole” & “Shaft”)
Hole – A feature engulfing a component
Shaft – A feature being engulfed by a component
Clearance Fits
Interference fits
Transition fits
41. Transition fits
The transition transition fits may be tight fit and push fit, wringing wringing fit
(Gear, pulley on shaft), press fit.
43. Hole basis system
The hole is kept as a constant member (i.e. when the lower deviation of the
hole is zero)
Different fits are obtained by varying the shaft size then the limit system is said
to be on a hole basis.
44. Shaft basis system
When the shaft is kept as a constant constant member (i.e. when the upper
deviation of the shaft is zero)
Different Different fits are obtained obtained by varying varying the hole size
then the limit system is said to be on a shaft basis.
45. Why Hole Basis Systems are Preferred?
Holes can be finished by tools like reamers, drills, broaches, and
their sizes are not adjustable. The shaft sizes can be easily
obtained by external machining. If shaft basis system is used
considerable no of reamers and other precision tools are required
for producing different classes of holes for one class of shaft for
obtaining obtaining different fits which increases increases cost of
production. It is economical
46. Limits and Fits
Limits and fits comprises 18 grades of fundamental tolerances for both shaft
and hole, designated as IT01, IT0 and IT1 to IT16. These are called standard
tolerances. (IS‐919) But ISO 286 specify 20 grades upto IT18.
There are 25 (IS 919) and 28 (ISO 286) types of fundamental deviations.
Hole: A, B, C, CD, D, E, EF, F, FG, G, H, J, JS, K, M, N, P, R, S, T, U, V, X, Y, Z, ZA,
ZB, ZC.
Shaft : a, b, c, cd, d, e, ef, f, fg, g, h, j, js, k, m, n, p, r, s, t, u, v, x, y, z, za, zb,
zc.
A unilateral hole basis system is recommended but if necessary a unilateral or
bilateral shaft basis system may also be used.
47. Fundamental deviation
Holes are designated by capital letter:
Letters A to G - oversized holes
Letters P to ZC - undersized holes
Shafts are designated by small letter:
Letters m to zc - oversized shafts
Letters a to g - undersized shafts
48. Tolerance Designation (IS)
Tolerance on a shaft or a hole can be calculated by using
table provided.
T = K × i
Where, T is the tolerance (in μm)
Standard Tolerance unit or Fundamental tolerance unit
i = 0.45 + 0.001D μm
D = K (D1 and D2 are the nominal sizes marking the beginning
and the end of a range of sizes, in mm)
K = is a constant [For IT6 to IT16]
51. Standard designation
For hole, H stands for a dimension whose lower deviation refers to the basic
size. The hole H for which the lower deviation is zero is called a basic hole.
Similarly, for shafts, h stands for a dimension whose upper deviation refers to
the basic size. The shaft h for which the upper deviation is zero is called a
basic shaft.
A fit is designated by its basic size followed by symbols representing the limits
of each of its two components, the hole being quoted first.
For example example, 100 H6/g5 means basic size is 100 mm and the
tolerance grade for the hole is 6 and for the shaft is 5.
52. Calculation for Upper and Lower Deviation
For Shaft
(Lower deviationn) ei = es – IT
(upper deviationn) es = ei + IT
For Hole
(Lower deviationn) EI = ES – IT
(upper deviationn) ES = EI + IT
es = upper deviation of shaft
ei = lower deviation of shaft
ES = upper deviation deviation of hole
EI= lower deviation of hole
55. Limit Gauges
Plug gauge: used to check the holes. The GO plug
gauge is the size of the low limit of the hole while the NOT
GO plug gauge corresponds to the high limit of the hole.
Snap, Gap or Ring gauge: used for gauging the shaft
and male components. The Go snap gauge is of a size
corresponding to the high (maximum) limit of the shaft,
while the NOT GO gauge corresponds to the low
(minimum limit).
56. Taylor’s Principle
This principle states that
the GO gauge should
always be so designed
that it will cover the
maximum metal
condition condition
(MMC) of as many
dimensions dimensions as
possible possible in the
same limit gauges.
Whereas a NOT GO gauges tocover the
minimum metal condition of one
dimension.
57. Slip Gauges or Gauge blocks
• These are small blocks of alloy steel. Used in the manufacturing shops
as length standards. A typical set consisting of 88 pieces for metric units
58. Surfaces: No surface is perfectly smooth, but the better
the surface quality, the longer a product generally lasts,
and the better is performs. Surface texture can be
difficult to analyse quantitatively. Two surfaces may be
entirely different, yet still provide the same CLA (Ra) value.
Surface Roughness
Surface geometry can be
quantified a few different
Ways. Real surfaces are rarely
so flat, or smooth, but most
commonly a combination of
the two.
59. Roughness height: is the parameter with which generally
the surface finish is indicated. It is specified either as
arithmetic average value or the root mean square value.
Roughness width: is the distance parallel to the nominal
part surface within which the peaks and valleys, which
constitutes the predominant pattern of the roughness.
Roughness width cut‐off: is the maximum width of the
surface that is included in the calculation of the
roughness height.
60. Surface
Measurement
Determined by the height of the waviness and its width.
The greater the width, the smoother is the surface and thus is more
desirable.
Waviness: refers to
those surface
irregularities that
have a greater
spacing than that of
roughness width.
61. Lay direction: is the direction of the predominant surface pattern produced
on the workpiece by the tool marks.
Flaw: are surface irregularities that are present which are random and
therefore will not be considered.
63. Surface roughness
measurement
1. Center Line Average (CLA) method
2. Maximum Peak to valley height, Rt or
Rmax
3. RMS Value, Rq
f = feed
ECEA = ψe SCEA = ψS
R = Nose radius
Peak to valley
height formula =
Hmax =
Centre line
avg. value (Ra) = Hmax
Representation of Surface
Roughness
