9. Surface Texture - PPT.pdf

1. MEASUREMENT OF SURFACE
TEXTURE
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2. • It was found that the functioning of machine parts, load
carrying capacity, tool life, fatigue life, bearing corrosion &
wear qualities of any component of a machine have direct
bearing with its surface texture.
• Hence, these effects made the control of surface texture very
important.
• Failure due to fatigue always starts to occur at sharp corners
where stress concentration occurs. The root of any surface
irregularity is the sharp corner & such part fails earlier.
2

3. • It has also been shown that the surface irregularity at non-
working surface also matters.
• Thus in order to increase the life of any part which is
subjected to repeated reversal of stress, both working & non-
working surfaces must be given good surface finish.
• Good bearing properties in any part is obtained when the
surface has large number of irregularities. If the surface is
perfectly smooth, then seizure would occur due to the
difficulty of maintaining the lubricating oil film.
3

4. • The hills in irregular surface (rough) reduce the metal-to-
metal contact & the valleys help to retain the film of
lubricating oil.
• In addition, the rate of wear is proportional to the surface
area in contact & the load per unit area.
• Thus it is seen that different requirements demand different
types of surfaces.
• Hence, it becomes essential to measure the surface texture
quantitatively & methods are devised for this purpose.
• Also, greater demands for high surface finish resulted in
refined processes viz. grinding, lapping, honing etc. 4

5. • It is not possible to produce perfectly smooth surfaces.
• The manufactured surface always departs from the absolutely
perfect surface to some extent.
• The imperfections on the surface are in the form of hills &
valleys, varying both in height & spacing.
• This departure from truly smooth surface may arise from a
variety of causes viz. machine tool used, cutting tool used,
material rupture, vibrations etc.
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6. MEANING OF SURFACE TEXTURE DEFINITIONS
• Usually the concept of surface roughness is sensory and it is
capable of being understood in an intuitive way.
• Generally the surface roughness has been experienced and
understood by the sense of light and touch.
• Therefore, to define surface roughness in terms of
measurable attributes presents great difficulties.
• The definition of surface roughness could probably be best
understood on a causative basis and independent of intuition.
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7. • Any material being machined by chip removal process can't be
finished perfectly due to some departure from ideal conditions
as envisaged by the designer.
• Due to conditions not being ideal, the surface produced will
have some irregularities; and these geometrical irregularities
are classified into four categories.
1. First order irregularities: It includes the irregularities arising
out of inaccuracies in the machine tool (defective machine
tool) itself. Ex.: lack of straightness of guideways on which tool
post/tool is moving.
2. Second order irregularities: Some irregularities are caused due
to vibrations of any kind such as chatter marks. 7

8. 8
3. Third order irregularities: Even if the machine was perfect &
completely free of vibrations, some irregularities are caused
by machining itself due to the characteristics of the process.
This also includes the feed marks of the cutting tool.
4. Fourth order irregularities: This includes the irregularities
arising from the rupture of the material during the
separation of the chip from the surface of the material.

9. 9

10. Further these irregularities of first to fourth orders can be
grouped under two major groups:
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1. 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/slide-ways and
non-linear feed motion. These errors are also referred to as
Waviness or secondary texture.

11. 12
2. 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 edge, 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.

12. 1. Lack of straightness of guide ways, deformation of work under
the action of cutting forces, the weight itself.
2. Due to vibrations of any kind , chatter marks.
3.Chararecteristics of the process, feed marks of the cutting tool.
4.Rupture of the material during the separation of the chip.
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13. 14
LAY: It represents the direction of predominant surface pattern
produced & it reflects the machining operation used to produce
the surface.
ROUGHNESS: It consists of surface
irregularities which result from the
various machining processes. These
irregularities combine to form
surface texture. Roughness is defined
as a quantitative measure of process
marks produced during the creation
of the surface.

14. 15
ROUGHNESS HEIGHT: It is the height of the irregularities with
respect to a reference line. It is measured in mm or microns. It
is also known as the height of unevenness.
ROUGHNESS WIDTH: It is the distance parallel to the nominal
surface between successive peaks/ridges which constitutes the
predominate pattern of the roughness. Measured in mm.

15. 16
WAVINESS: It refers to the irregularities which are outside the
roughness width cutoff values. Waviness is the widely spaced
component of the surface texture. It may be the result of
workpiece or tool deflection during machining, vibration or tool
runout. In short, it is a longer wavelength variation in the surface
away from its basic form.
WAVINESS HEIGHT: It is the peak to valley distance of the surface
profile, measured in mm.
FORM: This is the general shape of the surface, ignoring variations
due to roughness & waviness. Deviations from the desired form
can be caused by many factors (part being held too firmly or not
firmly enough, inaccurate slides/guideways etc.).

16. 17
COMPARISON OF ROUGHNESS, WAVINESS & FORM:

17. TERMS USED IN CONNECTION WITH SURFACE
TEXTURE
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18. SURFACE ROUGHNESS RANGE
Ra
μm
0.025 0.05 0.1 0.2 0.4 0.8 1.6 3.2 6.3 12.5 25 50
N-Grade N1 N2 N3 N4 N5 N6 N7 N8 N9 N10 N11 N12
Triangular
Indication
Finish
Lapping/Super
finishing
Ground Finishes Smooth Turned Medium Turned Rough Machined
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19. Primary Texture
(Roughness)
Secondary Texture
(Waviness)
It is caused due to the
irregularities in the surface
roughness which result from the
inherent action of the production
process.
It results from the factors such
as machine or work deflections,
vibrations, chatter, heat
treatment or warping strains.
These are deemed to include
transverse feed-marks and the
irregularities within them.
Waviness is the component of
surface roughness upon which
roughness is superimposed.
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20. TERMINOLOGY AS PER INDIAN STANDARDS
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.
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.
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21. FLAWS:
Flaws are irregularities which occur at one place or at relatively
infrequent or widely varying intervals on 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' generally
determined by the production process used.
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22. TRAVERSING LENGTH:
It 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.
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23. SAMPLING LENGTH (l):
It 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 with regard to the measuring instrument.
It is measured in a direction parallel to the general direction of the
profile. It is very difficult to specify any value for spacing (i.e.
the length over which the surface profile is to be considered).
However, for majority of engineering work, value of 0.8 mm is
generally considered to be quite satisfactory for instrument cut-
off and upper limit of 25 mm is commonly accepted as suitable
for most waviness measurements.
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24. MEAN LINE OF THE PROFILE:
It 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) for the effective
points on the profile and the mean line is a minimum.
CENTRE LINE OF THE PROFILE:
It 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/same.
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25. 30
• It may be noted that true repetitiveness in any
manufacturing process is impossible and as such some
difference in mean line & centre line of the profile is bound
to exist.
• But, however, in view of its insignificance of this error in
relation to other errors or measurement of surface geometry,
the mean line and the centre line of the profile may be
considered to be equivalent for all practical purposes.

26. MEAN LINE OF THE PROFILE, m:
• It is a line with a shape of the geometrical profile (perfect
geometric line) & it runs parallel to the profile.
• Denoted by “m”. The mean line of the profile is determined so
that the “sum of the squared deviations from this line is the
smallest”. Or, “the surface area above & below the mean line
of the profile is the same”.
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27. 32

28. 33

29. PARAMETERS USED TO QUANTIFY SURFACE
ROUGHNESS
ARITHMETICAL MEAN DEVIATION FROM THE MEAN
LINE OF PROFILE (RA):
It 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 (+ or -), i.e.
0
1 l
a
R y dx
n
 
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30. Approximately : 0
n
i
a
y
R
n


where, n is the no. of divisions over the sampling length, l.
Ra readings serve well for surface finish control in most instances.
It may be mentioned that Ra provides an average reading and
several different surfaces can have same average, i.e. although Ra
readings may be approximately same, the surfaces will function
quite differently. Thus, more knowledge of the surface texture is
required.
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31. For instance, measurement of peak heights may be needed in
painting, plating and glass applications, to control the pitting of gear
teeth, improve seals between surfaces and increase the stiffness of
press fits.
Ra – Average Roughness.
Also known as: Arithmatic Average (AA),
Centre Line Average (CLA),
Arithmatic Mean Deviation of Profile.
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32. The Ra measurement does not give a true picture of the real surface
profile/texture. Even with the same Ra value the two surfaces may
have different texture.
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33. • Arithmatic mean deviation (Ra) is the most widely recognized
& used parameter for surface roughness characterization. Ra is
the arithmetic mean deviation of all the measured values in
the assessed profile (LM) from the mean line of profile.
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34. TEN POINT HEIGHT OF IRREGULARITIES, RZ:
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35. 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.
Hence,
   
2 4 6 8 1
1 3 5 7 9 0
5
z
R R R R
R
R
R R R R R
    
   

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36. ROOT-MEAN-SQUARE ROUGHNESS, Rq:
Also called as RMS Value.
2
0
1
L
q
R h dx
L
 
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37. METHODS OF MEASURING SURFACE FINISH
There are two methods used for measuring the finish of
machined part :
(i) Surface Inspection by Comparison Methods.
(ii) Direct Instrument Measurements.
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38. SURFACE INSPECTION BY COMPARISON
METHODS
1. Touch Inspection,
2. Visual Inspection,
3. Scratch Inspection,
4. Microscopic Inspection,
5. Surface Photographs,
6. Micro-Interferometer,
7. Wallace Surface Dynamometer and
8. Reflected Light Intensity.
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39. 44
• In touch inspection the degree of surface roughness can not be
assessed. Also the minute flaws can not be detected. The
method can simply tell us which surface is more rough.
• In visual inspection by naked eye is always likely to be
misleading particularly when surfaces having high degree of
finish are inspected. Results of inspection vary from person to
person.
• In scratch inspection, softer material like lead, babbit or plastic
is rubbed over the surface to be inspected. By doing so it carries
the impression of the irregularities/scratches on the surfaces
which can be easily visualized.

40. 45
• In microscopic inspection, a master finished surface is placed
under the microscope and compared with the surface under
inspection. In another method a straight edge is placed on the
surface to be inspected & a beam of light projected at about 60 °
to the work. Thus the shadows cast into the surface scratches
are magnified & the surface irregularities can be studied.
• In surface photographs, magnified images/photographs of the
surface are taken with different types of illumination. In case we
take vertical illumination, then defects like irregularities and
scratches will appear as dark spots & flat portion of the surface
appears as bright area. Photographs with different illumination
are compared & the results assessed.

41. 46
• In micro interferometer, an optical flat is placed on the surface
to be inspected & illuminated by a monochromatic source of
light. Interference bands are studied through a microscope.
Defects on the surface appear as interference lines extending
from the dark bands into the bright bands.
• Wallace surface dynamometer, is a sort of friction meter &
consists of a pendulum in which the testing shoes are clamped
to a bearing surface & a predetermined spring pressure can be
applied. The pendulum is lifted to its initial starting position &
allowed to swing over the surface to be tested. If the surface is
smooth, then there will be less friction & the pendulum swings
for a longer period. Thus, time of swing is the measure of finish.

42. 47
• In reflected light intensity method, a beam of light of known
quantity is projected upon the surface. This light is deflected in
several directions as beams of lesser intensity & 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.

43. STYLUS PROBE INSTRUMENTS:
Consists of the following units:
i. 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.
ii. A stylus or probe which moves over the surface with the
skid.
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44. iii. An amplifying device for magnifying the stylus movement
and an indicator.
iv. 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
give the actual picture of surface roughness but a distorted
trace obtained.
v. A means for analysing the trace is obtained. The analysis can
be done separately or some automatic device may be
incorporated in the instrument for analysis
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45. THE TAYLOR-HOBSON “TALYSURF”
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46. 52

47. 53
• Talysurf is an electronic instrument working on carrier
modulating principle.
• It is more accurate and rapid. It records the static
displacement of the stylus.
• The measuring head of this instrument consists of a diamond
stylus of about 0.002 mm tip radius and skid/shoe which is
drawn across the surface by means of a motorized driving
unit (gear box), which provides three motorized speeds
giving respectively × 20, × 100 horizontal magnification & a
speed suitable for average reading.

48. 54
• A neutral position in which the pick-up can be traversed
manually is also provided.
• The arm carrying the stylus forms an armature which pivot
about the centre piece of E-shaped stamping as shown in
figure.
• On two legs of E-shaped stamping there are coils carrying an
AC current. These two coils with other two resistances from
an oscillator.
• As the armature is pivoted about the central leg, any
movement of the stylus causes the air gap to vary & thus the
magnitude of the original AC current flowing in the coils is
modulated.

49. 55
• The output of the bridge thus consists of modulation only as
shown in figure. This is further demodulated so that the
current now is directly proportional to the vertical
displacement of the stylus only.
• The demodulated output is caused to operate a pen recorder
to produce a permanent record & a meter to give a numerical
assessment directly.

50. INDICATING SURFACE ROUGHNESS ON DRAWINGS:
Roughness
grade number
Roughness value
Ra
(m)
Roughness
symbol
N12 50
~
N11 25

N10 12.5
N9 6.3

N8 3.2
N7 1.6
N6 0.8

N5 0.4
N4 0.2
N3 0.1

N2 0.05
N1 0.025 56

51. INDICATING LAY ON DRAWINGS:
Straight (Vertical, Horizontal)
Criss-Cross Straight
Criss Cross Arcuate
(shaped like a bow; curved)
Circular
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52. Parallel to the plane of projection of the view in
which this symbol is used.
Approximately radial relative to the centre of the
surface to which the symbol is applied.
Approximately circular relative to the centre of the
surface to which the symbol is applied
Multi-directional.
Crossed in two slant directions with regard to the
plane of projection of the view in which the symbol is
used.
Perpendicular to the plane of projection of the
view in which the symbol is used.
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53. 59
SPECIFICATION OF SURFACE TEXTURE CHARACTERISTICS:
• Design & production engineers should be familiar with the
standards adopted for specification of surface texture
characteristics.
• Symbols are used to designate surface irregularities such as
lay of surface pattern & roughness value.
• Following table provides the symbolic representation of the
various types of lays.

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55. 61
SPECIFYING SURFACE FINISH:
As per IS: 3073, indicating the following main
characteristics of surface texture on drawing is shown
schematically:
• Roughness value, Ra
• Sampling length or cut-off length (mm)
• Machining or production method
• Machining allowance
• Direction of lay in the symbolic form

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57. 63
As an example, a cylindrically ground surface with 0.10 mm
machining allowance having Ra value of 0.2 microns, with
cut-off length of 3 mm and direction of lay as perpendicular
is represented as follows:

58. 1. 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), Rz
and R.M.S. value of the rough surface.
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59. 2. 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 10,000 and a horizontal
magnification of 100. and the areas above and below the
datum line were :
Above : 150 170 150 120 mm2
Below : 80 60 80 40 mm2
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60. The C.L.A. or Ra value is given by:
2
( ) 1000 1
( )
Sum of areas mm
Sampling length mm Vertical magnification Horizontal magnification
 
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61. 67
3. If a surface has heights and depths represented as shown in
figure, evaluate Ra, Rz and RMS value.

62. 4. If a surface has heights and depths represented as shown in
figure, evaluate Ra, Rz and RMS values.
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