Chapter 7 measurement of surface finish

Dr. Dayananda Pai
MIT, Manipal

Contents
 Introduction
 Definitions
 Symbols used in surface finish
 Analysis of traces
 Taylor – Hobson “Talysurf”
 simple numerical on surface roughness
Dayananda Pai, Aero & Auto Engg., Dept. MIT,
Manipal

Introduction
Functioning of machine parts, load carrying capacity,
tool life, fatigue life, bearing corrosion, and wear
qualities of any component of a machine have direct
bearing with its surface texture. Therefore, these
effects made the control of surface texture very
important
Manipal

Introduction
 The root of any surface irregularity acts as sharp corner
and such part fails earlier.
 Thus in order to increase the life of any part which is
subjected to repeated reversals of stress, the working
and non-working surfaces of that surface must be
given very good finish.
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Introduction
 Good bearing properties in any part are obtained when
the surface has large number of irregularities, i.e. a
large number of hills and valleys. The rate of wear is
proportional to the surface areas in contact and the
load per unit area.
 Thus it is seen that different requirements demand
different types of surfaces. Therefore, it became
essential to measure the surface texture quantitatively
and methods were devised for this purpose
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Introduction
 The departure from a truly smooth surface may arise
from a variety, of causes and may be of several kinds.
The texture or roughness {succession of minute
irregularities} on surface is influenced by the
machining process employed.
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Introduction
 Complete roughness is the resultant of irregularities of
various kinds.
 If the hills and valleys on a surface are very close, then
surface appears as rough. This is due to action of the
cutting tool and is referred to as primary texture.
 If the hills and valleys on a surface are far apart, it is
due to imperfection in the machine tool and is
referred to as secondary texture or waviness.
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Introduction
 This distinction between primary and secondary
texture is due to difference in wavelength.
 A surface actually is quite complex and consists of
many different wavelengths caused due to feed of the
tool, cutting action, vibrations, imperfections in
machine tools, etc.
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Meanings of Surface Texture and
Some Definitions
 First Order. This includes the irregularities arising out of
inaccuracies in the machine tool itself e.g. lack of
straightness of guide-ways on which tool post is moving.
 Second Order. Some irregularities are caused due to
vibrations of any kind such as chatter marks and are
included in second order.
 Third Order. Even if the machine were perfect and
completely free of vibrations, some irregularities are caused
by machining itself due to characteristic of the process.
 Fourth Order. This includes the irregularities arising from
the rupture of the material during the separation of the
chip.
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Some Definitions
 Further these irregularities of four orders can be
grouped under two groups:
 First group includes irregularities of considerable
wave-length of a periodic character resulting from
mechanical disturbances in the generating set-up.
These errors are termed as macro-geometrical errors
and include irregularities of first and second order and
are mainly due to misalignment of centres, lack of
straightness of guide-ways and non-linear feed
motion. These errors are also referred to as Waviness
or Secondary Texture.
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Some Definitions
 Second group includes irregularities of small
wavelength caused by the direct action of the cutting
element on the material or by some other disturbance
such as friction, wear, or corrosion. These errors are
chiefly caused due to tool feed rate and due to tool
chatter, i.e. it includes irregularities of third and fourth
order and constitutes the micro geometrical errors.
Errors in this group are referred to as Roughness or
Primary Texture.
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Surface Roughness
 The several kinds of departures are there on the
surface and these are due to various causes.
 Roughness or texture in the form of a succession of
minute irregularities is produced directly by the
finishing process employed.
 The characteristic roughness produced by the tool is
not the only cause of roughness in case of machining
operations, but the more openly spaced component or
roughness are also produced from faults in the
machining operation
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Surface Roughness
 Thus for the complete study of the surface roughness,
it is essential that the measurement and analysis of all
the component elements and an assessment of the
effects of the resulting combined texture be made.
 All this being very difficult and tedious job, in practice
all that is essential is that a practical method of
assessment be followed, the result of which can be
readily compared with a specified requirement of
quality, preferably on the numerical basis.
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Surface Roughness
 The measurement of surface roughness poses a
problem in three dimensional geometry, but for
simplification purposes, it is better to reduce it into
two dimensional geometry by confining individual
measurement to the profiles of plane sections taken
through the surface. The direction of measurement is.
usually perpendicular to the direction of the
predominant surface markings or 'lay'.
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Surface Roughness
 Surface roughness is concerned both with the size and
the shape of the irregularities e.g.. in certain profile the
height of departure from the nominal profile may be
same but the spacing of the irregularities may be wider
or closer, or the space of the irregularities may be of
various forms
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Terminology
 Real Surface : is the surface limiting the body and
separating it from the surrounding surface.
 Geometrical Surface: is the surface prescribed by the
design or by the process of manufacture, neglecting the
errors of form and surface roughness.
 Effective Surface: is the close representation of real
surface obtained by instrumental means.
 Surface Texture: Repetitive or random deviations from
the nominal surface which form the pattern of the
surface. Surface texture includes roughness, waviness,
lay and flaws.
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 Surface Roughness: It concerns all those irregularities which
form surface relief and which are conventionally defined
within the area where deviations of form and waviness are
eliminated.
 Primary Texture (Roughness) : It is caused due to the
irregularities in the surface roughness which result from the
inherent action of the production process. These are
deemed to include transverse feed marks and the
irregularities within them.
 Secondary Texture (Waviness): It results from the factors
such as machine or work deflections, vibrations, chatter,
heat treatment or warping strains. Waviness is the
component of surface roughness upon which roughness is
superimposed.
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 Flaws: Flaws are irregularities which occur at one place
or at relatively infrequent or widely varying intervals in
a surface (like scratches, cracks, random blemishes,
etc).
 Centre line: The line about which roughness is
measured.
 Lay. It is the direction of the predominant surface
pattern ordinarily determined by the method of
production used
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 Traversing length is the length of the profile necessary
for the evaluation of the surface roughness parameters.
The traversing length may include one or more
sampling lengths.
 Sampling length (l) is the length of profile necessary for
the evaluation of the irregularities to be taken into
account. This is also known as the 'cut-off' length in
regard to the measuring instruments.
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 Mean line of the profile is the line having the form of
the geometrical profile and dividing the effective profile
so that within the sampling length the sum of the
squares of distances (Y1, y2, ... Yn) between effective
points and the mean line is a minimum.
 Centre line of profile is the line parallel to the general
direction of the profile for which the areas embraced by
the profile above and below the line are equal. When
the waveform is repetitive, the mean line and the
centre line are equivalent
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 Spacing of the irregularities is the mean distance
between the more prominent irregularities of the
effective profile, within the sampling length. This
information is useful for measuring the wearing-in of
surfaces in relative motion, and assessing the electrical
and thermal conductivity between surfaces in contact.
Irregularity spacing and height parameters used in
combination are valuable for sheet-steel applications
and for friction and lubrication studies
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 Arithmetical mean deviation from the mean line of
profile (Ra) is defined as the average value of the
ordinates (y1, y2,....yn) from the mean line.
 The ordinates are summed-up without considering
their algebraic signs i.e.
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0
0
1
| |
| |
l
a
n
i
a
R y dx
n
y
R
n




n is the no. divisions
over the length l

 Ten point height of irregularities (Rz) is defined as the
average difference between the five highest peaks and
the five deepest valleys within the sampling length
measured from a line, parallel to the mean line and
not crossing the profile.
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5
10864297531 RRRRRRRRRR
Rz



 Maximum height of irregularity (Rmax) is defined as the
distance between two lines parallel to the mean line
and touching the profile at highest points within the
sampling length.
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Terms used to describe surface
roughness
 Average peak to valley height Rz: Average of single peak
to valley heights from five adjoining sampling lengths.
 Rt measurement. It is the maximum peak to valley
height within the assessment length. This
measurement is valuable for analysing finish to
provide guidance for planning subsequent metal-
cutting operations.
 Average wavelength =2πRa/Mean slope.
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 Bearing area. (Bearing area fraction). This is the
fraction of surface at a given height above or below the
mean line.
 Centre line. A line representing the form of the
geometrical profile and parallel to the general direction
of the profile throughout the sampling length, such
that the sums of the areas contained between it and
those parts of the profile which lie on either side of it
are equal
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 Depth of surface smoothness.
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max
0
1
( )
L
pR h h dx
L
 
Least-squares mean line. A reference line representing
the form of the geometrical profile within the limits of
the sampling length, and so placed that within the
sampling length the sum of the squares of the
deviations of the profile from the mean line is a
minimum.

 Measuring traversing length. The length of the
modified profile used for measurement of surface
roughness parameters. It is usual for the measuring
length to contain several sampling lengths.
 Waviness height. Separation of highest peak and lowest
valley of waviness over a waviness sampling length,
corrected for roughness.
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Methods of Measuring Surface
Finish
There are two methods used for measuring the finish of
machined part :
1. Surface Inspection by Comparison Methods.
(i)Touch Inspection, (ii) Visual Inspection, (iii) Scratch
Inspection, (iv) Microscopic Inspection, (v) Surface
Photographs, (vi) Micro-Interferometer, (vii)
Reflected Light Intensity
Manipal

Touch Inspection
 This method can simply tell which surface is more
rough. In this method, the finger-tip is moved along
the surface at a speed of about 25 mm per second and
the irregularities as small as 0.01 mm can be easily
detected.
 A modification of it is possible by using a table tennis
ball, which is rubbed over the surface and vibrations
from the ball transmitted to hand and surface
roughness judged thereby.
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Visual Inspection
 Visual inspection by naked eye is always likely to be
misleading particularly when surfaces having high
degree of finish are inspected. The method is,
therefore, limited to rougher surfaces and results vary
from person to person. More accurate inspection can
be done by using illuminated magnifiers.
Manipal

Scratch Inspection
 In this method, a softer material like lead babbit or
plastic is rubbed over the surface to be inspected. By
doing so it carries the impression of the scratches on
the surfaces which can be easily visualised.
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Microscopic Inspection
 In this method, a master finished surface is placed
under the microscope and compared with the surface
under inspection.
 This is probably the best method for examining the
surface finish but suffers due to limitation that only a
small portion of the surface can be inspected at a time.
Thus several readings are required to get an average
value.
Manipal

Surface Photographs
 In this method magnified photographs of the surface
are taken with different types of illumination.
 In case we use vertical illumination, then defects like
irregularities and scratches appear as dark spots and
flat portion of the surface appears as bright area. In
case of oblique illumination, reverse is the case.
Photographs with different illumination are compared
and the results assessed.
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Micro interferometer
 In this method, an optical flat is placed on the surface
to be inspected and illuminated by a monochromatic
source of light. Interference bands are studied through
a microscope.
 Defects, i.e. scratches in the surface appear as
interference lines extending from the dark bands into
the bright bands. The depth of the defect is measured
in terms of the fraction of the interference band.
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Reflected light intensity
 In this method a beam of light of know quantity is
projected upon the surface., This light is reflected in
several directions as beams of lesser intensity and the
change in light intensity in different directions is
measured by a photocell. The measured intensity
changes are already calibrated by means of reading
taken from surface of known roughness by some other
suitable method.
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Direct Instrument Measurements
These methods enable to determine a numerical value of
the surface finish of any surface. Nearly all instruments
used are stylus probe type of instruments
Stylus probe Instrument
(i) Profilometer
(ii) Tomlinson surface meter
(iii) Taylor Hobson talysurf
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Stylus probe Instrument
i. This type of instrument generally consists of the
following units
ii. A skid or shoe which is drawn slowly over the surface
either by hand or by motor drive
iii. A stylus or probe which moves over the surface with
the skid
iv. An amplifying device for magnifying the stylus
movement
v. A recording device to produce a trace
vi. A means for analysing the trace
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Skid
 A skid or shoe which is
drawn slowly over the
surface either by hand or
by motor drive. The
 skid when moved over
the surface, follows its
general contours and
provides a datum for the
measurements.
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Stylus probe
 A stylus or probe which
moves over the surface
with the skid. The stylus
for Ra measurement on
new instrument can have
a radius of 10 microns
±30%. Stylus should be
cone shaped with a
spherical tip.
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Amplifying
 An amplifying device for
magnifying the stylus
movement and an
indicator.
 Electronic or optical
magnification is
employed.
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Recording
 A recording device to
produce a trace or record
of the surface profile.
Usually the vertical
movement is magnified
more in comparison to
horizontal movement,
thus the record will not
Stylus give the actual
picture of surface
roughness but a
distorted trace obtained.
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Profilometer
 This is a dynamic instrument similar in principle to a
gramophone pick-up. A finely pointed stylus mounted
in the pick-up unit is traversed across the surface
either by hand or by motor drive. The instrument
records the rectified output from the pick-up which is
amplified further and operates an indicating device
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Tomlinson Surface meter
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 The diamond stylus on the surface finish recorder is
held by spring pressure against the surface of a lapped
steel cylinder. The stylus is also attached to the body of
the instrument by a leaf spring and its height is
adjustable to enable the diamond to be positioned
conveniently. The stylus is restrained from all motios
except the vertical one by the tensions in coil and leaf
spring. A light spring steel arm is attached to the
horizontal lapped steel cylinder and it carries at its tip
a diamond scriber which bears against a smoked glass.
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 When measuring surface
finish, body is traversed
across the surface by a
screw rotated by a
synchronous motor. Any
vertical movement of the
stylus caused by the
surface irregularities,
causes the horizontal
lapped steel cylinder to
roll.
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 By its rolling, the light arm
attached to its end
provides a magnified
movement on a smoked
glass plate. This vertical
movement coupled with
the horizontal movement
produces a trace on the
glass magnified in vertical
direction and there being
no magnification in
horizontal direction.
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Taylor-Hobson Talysurf
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 This instrument also gives the same information as the
previous instrument, but much more rapidly and
accurately.
 This instrument also as the previous one records the
static displacement of the stylus and is dynamic
instrument like profilometer.
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 The measuring head of
this instrument consists
of a diamond stylus of
about 0.002 mm tip
radius and skid or shoe
which is drawn across
the surface by means of a
motorised driving unit.
A neutral position in
which the pick-up can be
traversed manually is
also provided.
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 The arm carrying the
stylus forms an armature
which pivots about the
centre piece of E-shaped
stamping. On two legs of
(outer pole pieces) the E-
shaped stamping there
are coils carrying an a.c.
current.
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 These two coils with other
two resistances form an
oscillator. The amplitude
of the original a.c. current
flowing in the coils is
modulated because of air
gap between the armature
and E-shaped stamping.
This is further
demodulated so that the
current now is directly
proportional to the vertical
displacement of the stylus
only.
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 The demodulated output is
caused to operate a pen
recorder to produce a
permanent record and a
meter to give a numerical
assessment directly. In
recorder of this instrument
the marking medium is an
electric discharge through
a specially treated paper
which blackens at the
point of the stylus.
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Analysis of Surface Traces
 Maximum Peak to
Valley Height of
Roughness.
 This is the most common
measure of roughness
but is not by any means a
complete definition of
roughness. But, since
this is a relatively simple
method of analysis.
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 Root Mean Square
(R.M.S) Value
 R.M.S. value is defined as
the square root of the
mean of the squares of
the ordinates of the
surface measured from a
mean line.
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2 2 2
1 2 ......... n
rms
h h h
h
n
  


 Centre Line Average
(C.L.A) value.
 This is defined as the
average height from a
mean line of all
ordinates of the surface
regardless of the sign
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1 2 .......
. . nh h h
C L A
n
  


 Things can be much
simplified by using a
planimeter which can
find out the area of any
curve. Then C.L.A. value
Dayananda Pai, Aero & Auto Engg., Dept. MIT, Manipal
1 2 .......
. . nA A A
C L A
L
  

ionmagnificatVertical
1


 Calculate the CLA (Ra) value of a surface for which the
sampling length was 0.8 mm. The graph was drawn to a
vertical magnification of 10000 and a horizontal
magnification of 100. and the areas above and below
the datum line were :
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Above(mm2) 180 90 155 55
Below(mm2) 70 90 170 150

 Solution:
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 In the measurement of surface roughness, heights of 20
successive peaks and troughs were measured from a
datum and were 35, 25, 40, 22, 35, 18, 42, 25, 35, 22, 36,
18, 42, 22, 32, 21, 37, 18, 35, 20 microns.
 If these measurements were obtained over a length of
20 mm, determine the C.L.A. (Ra) and R.M.S value of
the rough surface.
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Chapter 7 measurement of surface finish

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