Department of Mechanical Engineering

PROGRAM CODE: BE ME-201

Subject Name : Mechanical Measurement and Subject Code : MEY-202
Metrology

SURFACE ROUGHNESS MEASUREMENT

Introduction:-
With the more precise demands of modern engineering products, the control of surface texture
together with dimensional accuracy has become more important. It has been investigated that
the surface texture greatly influences the functioning of the machined parts. The properties
such as appearance, corrosion resistance, wear resistance, fatigue resistance, lubrication,
initial tolerance, ability 'to hold pressure, ,load carrying capacity, noise reduction in case of
gears are influenced by the surface texture.
Whatever may be the manufacturing process used, it is not possible to produce perfectly smooth
surface. The imperfections and irregularities are bound to occur. The manufactured surface
always departs from the absolute perfection to some extent. The irregularities on the surface are
in the form of succession of hills and valleys varying in height and spacing. These irregularities
are usually termed as surface roughness, surface finish, surface texture or surface quality. These
irregularities are responsible to a great extent for the appearance of a surface of a component
and its suitability for an intended application.

Factors Affecting Surface Roughness:-

The following factors affect the surface roughness:

 Vibrations

 Material of the workpiece

 Type of machining.

 Rigidity of the system consisting of machine tool, fixture cutting tool and work

 Type, form, material and sharpness of cutting tool

 Cutting conditions i.e., feed, speed and depth of cut

 Department of Mechanical Engineering

PROGRAM CODE: BE ME-201

Subject Name : Mechanical Measurement and Subject Code : MEY-202
Metrology

 Type of coolant used

Reasons for Controlling Surface Texture:-

(a) To improve the service life of the components

(b) To improve the fatigue resistance

(c) To reduce initial wear of parts

(d) To have a close dimensional tolerance on the parts

(e) To reduce frictional wear

(f) To reduce corrosion by minimizing depth of irregularities

(g) For good appearance

 Department of Mechanical Engineering

PROGRAM CODE: BE ME-201

Subject Name : Mechanical Measurement and Subject Code : MEY-202
Metrology

 If the surface is not smooth enough, a turning shaft may act like a reamer and the piston
rod like a broach.

However, as already explained perfectly smooth surface is not always required, the
requirement of surface texture depends upon the specific application of the part.

Orders of Geometrical Irregularities:-

As we Know that the material machined by chip removal process can't be finished perfectly due
to some departures from ideal conditions as specified by the designer. Due to conditions not
being ideal, the surface Produced will have some irregularities, these geometrical irregularities
can be classified into four categories.

First Order: The irregularities caused by inaccuracies in the machine tool itself are called as
first order irregularities. These include:

(a) Irregularities caused due to lack of straightness of guide ways on which the tool most moves.

(b) Surface regularities arising due to deformation of work under the action of cutting forces, and

(c) Due to the weight of the material itself.

Second Order: The irregularities caused due to vibrations of any kind are called
second order irregularities.

Third order: Even if the machine were perfect and completely free from vibrations some
irregularities are caused by machining itself due to the characteristics of the process.
 Department of Mechanical Engineering

PROGRAM CODE: BE ME-201

Subject Name : Mechanical Measurement and Subject Code : MEY-202
Metrology

Fourth Order: The fourth order irregularities include those arising from the rupture of the
material during the separation of the chip.

Irregularities on the surface of the part:-

The irregularities on the surface of the part produced can also be grouped into two categories:

(i) Roughness or primary texture, (ii) Waviness or secondary texture.

(i) Primary texture (Roughness):

The surface irregularities of small wavelength are called primary texture or roughness. These are
caused by direct action of the cutting element son the material i.e., cutting tool shape, tool feed
rate or by some other disturbances such as friction, wear or corrosion.
 Department of Mechanical Engineering

PROGRAM CODE: BE ME-201

Subject Name : Mechanical Measurement and Subject Code : MEY-202
Metrology

These include irregularities of third and fourth order and constitute the micro-geometrical
errors. The ratio lr / hr denoting the micro-errors is less than 50, where Ir = length along the
surface and hr = deviation of surface from the ideal one.

(ii) Secondary texture (Waviness):

The surface irregularities of considerable wavelength of a periodic character are called

secondary texture or waviness. These irregularities result due to inaccuracies of slides,
wear of guides, misalignment of centres, non-linear feed motion, deformation of work
under the action of cutting forces, vibrations of any kind etc.

These errors include irregularities of first and second order and constitute the macro-geometrical
errors. The ratio of Iw / hw denoting the macro-errors is more than 50. Where, Iw = length along
the surface and kw = deviation of surface from ideal one.

Elements of Surface Texture:-

The various elements of surface texture can be defined and explained with the help of fig which
shows a typical surface highly magnified.
 Department of Mechanical Engineering

PROGRAM CODE: BE ME-201

Subject Name : Mechanical Measurement and Subject Code : MEY-202
Metrology

Surface: The surface of a part 'is confined by the boundary which separates that part from
another part, substance or space. Actual surface. This refers to the surface of a part which is
actually obtained after a manufacture ring process.

Nominal surface: A nominal surface is a theoretical, geometrically perfect surface which does
not exist in practice, but it is an average of the irregularities that are superimposed on it.

Profile: Profile is defined as the contour of any section through a surface, Roughness. As
already explained roughness refers to relatively finely spaced micro geometrical irregularities. It
is also called as primary texture and constitutes third and fourth order irregularities.

Roughness Height: This is rated as the arithmetical average deviation expressed in micro-
meters normal to an imaginary centre line, running through the roughness profile.

Roughness Width: Roughness width is the distance parallel , to the normal surface
between successive peaks or ridges that constitutes the predominant pattern of the roughnes
Roughness Width cutoff: This is the maximum width of surface irregularities that is
included in the measurement of roughness height. This is always greater than roughness
width and is rated in centimetres.

Waviness: Waviness consists of those surface irregularities which are of greater spacing than
roughness and it occurs in the form of waves. These are also termed as moon geometrical errors
and constitute irregularities of first and second order. These are caused `due to misalignment of
centres, vibrations, machine or work deflections, warping etc.
Effective profile: It is the real canter of a surface obtained by using instrument
Laws: Flaws are surface irregularities or imperfections which occur art infrequent intervals
and at random intervals. Examples are: scratches, holes, cracks, porosity etc. These may be
observed directly with the aid of penetrating dye or other material which makes them visible
for examination and evaluatio
 Department of Mechanical Engineering

PROGRAM CODE: BE ME-201

Subject Name : Mechanical Measurement and Subject Code : MEY-202
Metrology

Surface Texture: Repetitive or random deviations from the nominal. Surface which forms the
pattern on the surface. Surface texture includes roughness, waviness, lays and flaws.

Lay: It is the direction of predominant surface pattern produced by tool marks or

scratches. It is determined by the method of production used. Symbols used to indicate
the direction of lay are given below:

| | = Lay parallel to the boundary line of the nominal surface that is, lay parallel to the line
representing surface to which the symbol is applied e.g., parallel shaping, end view of
turning and O.D grinding.

⊥= Lay perpendicular to the boundary line .of the nominal surface, that is lay perpendicular to
the line representing surface to which the symbol is applied, e.g. , end view of shaping,
longitudinal view of turning and O.D. grinding. X = Lay angular in both directions to the line
representing the surface to which symbol is applied, e.g.
traversed end mill, side wheel grinding.

M= Lay multidirectional e.g. lapping super finishing, honing.

 Department of Mechanical Engineering

PROGRAM CODE: BE ME-201

Subject Name : Mechanical Measurement and Subject Code : MEY-202
Metrology

C= Lay approximately circular relative to the centre of the surface to which the symbol is
applied e.g., facing on a lathe.

R= Lay approximately radial relative to the centre of the surface to which the symbol is
applied, e.g., surface ground on a turntable, fly cut and indexed on end mill.

Sampling length: It is the length of the profile necessary for the evaluation of the irregularities
to be taken into account. It is also known as cut-off length. It is measured in a direction
parallelogram general direction of the profile. The sampling length should bear some relation to
the type of profile. Evaluation of Surface Finish:

A numerical assessment of surface finish can be carried out in a number of ways. These
numerical values are obtained with respect to a datum. In practice, the following three
methods of evaluating primary texture (roughness) of a surface are used:

(i) Peak to valley height method

(ii) The average roughness

(iii) Form factor or bearing curve.

(1) Peak to valley height method:

This method is largely used in Germany and Russia. It measures the maximum depth of the
surface irregularities over a given sample length, and largest value of the depth is accepted
as a measure of

roughness. The drawback of this method is that it may read the same ℎ for two largely different
 Department of Mechanical Engineering

PROGRAM CODE: BE ME-201

Subject Name : Mechanical Measurement and Subject Code : MEY-202
Metrology

texture. The value obtained would not give a representative assessment of the surface.

To, overcomes this PV (Peak to Valley) height is defined as the distance between a pair of lines
running parallel `to the general ‘lay' of the trace positioned so that the length lying within the
peaks at, the top is 5% of the trace length, and that within the valleys at the bottom is 10% of the
trace length. This is represented graphically in Fig.

(a) The average roughness: For assessment off average roughness the following three
statistical criteria are used:
 Department of Mechanical Engineering

PROGRAM CODE: BE ME-201

Subject Name : Mechanical Measurement and Subject Code : MEY-202
Metrology

(a) C.L.A Method: In this method, the surface roughness is measured as the average deviation
from the nominal surface.

,
 Department of Mechanical Engineering

PROGRAM CODE: BE ME-201

Subject Name : Mechanical Measurement and Subject Code : MEY-202
Metrology
 Department of Mechanical Engineering

PROGRAM CODE: BE ME-201

Subject Name : Mechanical Measurement and Subject Code : MEY-202
Metrology

Centre Line Average or Arithmetic Average is defined as the average values of the ordinates
from the mean line, regardless of the arithmetic signs of the ordinates

The calculation of C.L.A value using equation (ii) is facilitated by the planimeter.

CLA value measure is preferred to RMS value measure because its value can be easily
determined by measuring. The areas with planimeter or graph or can be readily determined in
 Department of Mechanical Engineering

PROGRAM CODE: BE ME-201

Subject Name : Mechanical Measurement and Subject Code : MEY-202
Metrology

electrical instruments by integrating the movement of the styles and displaying the result as
an average.

(a) R.M.S. Method: In this method also, the roughness is measured as the average deviation
from the nominal surface. Root mean square value measured is based on the least squares.

R.M.S value is defined as the squre root of the arithmetic mean of the values of the squares of
the ordinates of the surface measured from a mean line. It is obtained by setting many
equidistant ordinates on the mean line ( 1, 2, 3 … . )and then taking the root of the mean of the
squared ordinates.
Let us assume that the sample length ‘L’ is divided into ‘n' equal parts and 1, 2, 3 ….are the

heights of the ordinates erected at those points.

 Department of Mechanical Engineering

PROGRAM CODE: BE ME-201

Subject Name : Mechanical Measurement and Subject Code : MEY-202
Metrology

(a) Ten Point Height Method: In this method, the average difference between the five highest
peaks and five lowest valleys of surface texture within the sampling length, measured from a
line parallel to the mean line and not crossing the profile is used to denote the amount of surface
roughness.
 Department of Mechanical Engineering

PROGRAM CODE: BE ME-201

Subject Name : Mechanical Measurement and Subject Code : MEY-202
Metrology

This method is relatively simple method of analysis and measures the total depth of surface
irregularities within the sampling length. But it does not give sufficient information about the
surface, as no account is taken of frequency of the irregularities and the profile shape. It is used
when it is desired to control the cost of finishing for checking the rough machining.
 Department of Mechanical Engineering

PROGRAM CODE: BE ME-201

Subject Name : Mechanical Measurement and Subject Code : MEY-202
Metrology

(a) Form factor and Bearing Curves: There are certain characteristic which may be used
to evaluate surface texture.

Form Factor: The load carrying area of every surface is often much less than might be thought.
This is shown by reference to form factor. The form factor is obtained by measuring the area of
material above the arbitrarily chosen base line in the section and the area of the enveloping
rectangle. Then,
Bearing Area Curve: The bearing area curve is also called as Abbot's bearing curve. This is
determined by adding the lengths a, b, c etc. at depths x, y, z etc. below the reference, line and
indicates the percentage bearing area which becomes available as the crest area worn away.
Fig. indicates the method of determining the bearing curve.
 Department of Mechanical Engineering

PROGRAM CODE: BE ME-201

Subject Name : Mechanical Measurement and Subject Code : MEY-202
Metrology

Conventional Method for Designing Surface finigh:

As per IS: 696 surface texture specified by indicating the following

(a) Roughness value i.e., Ra value in mm

(b) Machining allowance in mm.

(c) Sampling length or instrument cut-off length in mm.

(d) Machining production method, and

(e) Direction of lay in the symbol form as = ⊥, X, M, C, R
 Department of Mechanical Engineering

PROGRAM CODE: BE ME-201

Subject Name : Mechanical Measurement and Subject Code : MEY-202
Metrology

Measurement of surface finish surfaces texture:

The methods used for ensuring the surface finish can be classified broadly into two groups.

1. Inspection by comparison.

2. Direct instrument measurement

1. Inspection by comparison methods. In these methods, the surface texture is assessed by

observation of the surface. These are the methods of qualitative analysis of the surface texture.
The texture, e of the surface W be tested is compared with that of a specimen of known
roughness ~value and `finished by similar machining processes. Though these methods are
rapid, the results are not reliable because they can be misleading if comparison is not made
with\the surface produced by similar techniques. The various methods available for comparison
are:

(i) Visual Inspection

(ii) Touch Inspection

(iii) Scratch Inspection

(iv) Microscopicc Inspection

(v) Surface photographs

(vi) Micro-Interferometer

(vii) Wallace surface Dynomometer

 Department of Mechanical Engineering

PROGRAM CODE: BE ME-201

Subject Name : Mechanical Measurement and Subject Code : MEY-202
Metrology

(i) Reflected Light Intensity. Visual Inspection: In this method the surface is inspected by naked
eye. This method is always likely to be misleading particularly when surfaces with high degree
of finish are inspected. It is therefore limited to rougher surfaces.

(ii) Touch Inspection: This method can simply assess which surface is more rough, it cannot
give the degree of surface roughness. Secondly, the minute flaws can't be detected. 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

0.0125 mm can be detected. In modified method a tennis ball is rubbed over the surface and
surface roughness is judged thereby.

(iii) Scratch Inspection: In this method a softer material like lead, babbit, or plastic is
rubbed over the surface to be inspected. The impression of the scratches on the surface
produced is then visualised.

(iv) Microscopic Inspection: This is probably the best method for examining the surface
texture by comparison. But since, only a small surface can be inspected at a time several
readings are required to get an average value. In this method, a master finished surface is placed
under the microscope and compared with the surface under inspection. Alternatively, a straight
edge is placed on the surface to be inspected and a beam of light projected at about 600 to the
work. Thus the shadow is cast into the surface, the scratches are magnified and the surface
irregularities can be studied.

(v) Surface photographs: In this method magnified photographs of the surface are taken with
different types of illumination to reveal the irregularities.

If the vertical illumination is used 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 result is
assessed.
 Department of Mechanical Engineering

PROGRAM CODE: BE ME-201

Subject Name : Mechanical Measurement and Subject Code : MEY-202
Metrology

(vi) 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. The 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 bands.

(vii) Wallace Surface Dynamometer: It is a sort of friction meter. It consists of a pendulum in

which the testing shoes are damped to a bearing surface and a predetermined spring pressure
can be applied. The pendulum is lifted to its initial starting position and allowed to swing over
the surface to be tested. If the surface is smooth, then there will be less friction and pendulum
swings for a longer period. Thus, the time of swing is a direct measure of surface texture.

(viii)Reflected Light Intensity: In this method a beam of light of known 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.

2. Direct Instrument Measurement:

These are the methods of quantitative analysis. These methods enable to determine the numerical
value of the surface finish of any surface by using instruments of stylus probe type operating on
electrical principles. In these instruments the output has to be amplified and the amplified output
is used to operate recording or indicating instrument.
Principle, constructive and operation of stylus Probe type surface texture measuring instruments:

If a finely pointed Probe or stylus be moved over the surface of a workpiece, the vertical
movement of the stylus caused due to the irregularities in the surface texture can be used to
assess the surface finish of the workpiece.
 Department of Mechanical Engineering

PROGRAM CODE: BE ME-201

Subject Name : Mechanical Measurement and Subject Code : MEY-202
Metrology

Stylus which is a fine point made of diamond or any such hard material is drawn over the
surface to be tested. The movements of the stylus are used to modulate a high frequency carrier
current or to generate a voltage signal. The output is then amplified by suitable means and used
to operate a recording or indicating instrument.

Stylus type instruments generally consist of the following units:

(i) Skid or shoe

(ii) Finely pointed stylus or probe

(iii) An amplifying device for magnifying the stylus movement and indicator

(iv) Recording device to produce a trace and ~

(v) Means for analyzing the trace. Advantages:

The main advantage of such instruments is that the electrical signal available can be processed to
obtain any desired roughness parameter or can be recorded for display or subsequent analysis.
Therefore, the stylus type instruments are widely used for surface texture measurements inspite
of the following disadvantages.

Disadvantages:

(i) These instruments are bulky and complex.

 Department of Mechanical Engineering

PROGRAM CODE: BE ME-201

Subject Name : Mechanical Measurement and Subject Code : MEY-202
Metrology

(ii) They are relatively fragile.

(iii) Initial cost is high.

(iv) Measurements are limited to a section of a surface.

(v)Needs skilled operators for measurements.

(vi) Distance between stylus and skid and the shape of the skid introduce errors in
measurement for wavy surfaces.

The stylus probe instruments currently in use for surface finish measurement.
 Department of Mechanical Engineering

PROGRAM CODE: BE ME-201

Subject Name : Mechanical Measurement and Subject Code : MEY-202
Metrology

(a) Profilometer

(b) The Tomlinson surface meter.

(c) The Taylor Hobson Talysurf

(d) Profilograph.

(a) Profilometer:

Profilometer is an indicating and recording instrument used to measure roughness in

microns. The principle of the instrument is similar to gramophone pick up. It consists of two
principal units: a tracer and an amplifier. Tracer is a finely pointed stylus. It is mounted in the
pick up unit which consists of an induction coil located in the field of a permanent magnet. When
the tracer is moved across the surface to be tested, it is displaced vertically up and down due to
the surface irregularities. This causes the induction coil to move in the field of the permanent
magnet and induces a voltage. The induced voltage is amplified and recorded.
This instrument is best suited for measuring surface finish of deep bores.
 Department of Mechanical Engineering

PROGRAM CODE: BE ME-201

Subject Name : Mechanical Measurement and Subject Code : MEY-202
Metrology

b) The Tomlinson surface meter:

The Tomlinson surface meter is a comparatively cheap and reliable instrument.

It was originally designed by Dr. Tomlinson.
 Department of Mechanical Engineering

PROGRAM CODE: BE ME-201

Subject Name : Mechanical Measurement and Subject Code : MEY-202
Metrology

It consists of a diamond probe (stylus) held by spring pressure against the surface of a lapped
steel cylinder and is attached to the body of the instrument by a leaf spring. The lapped cylinder
is supported on one side by the probe and on the either side by fixed rollers. Alight spring steel
arm is attached to the lapped cylinder. It carries at its tip a diamond scriber which rests against a
smoked glass. The motions of the stylus in all the directions except the vertical one are prevented
by the forces exerted by the two springs.

For measuring surface finish the body of the instrument is moved across the surface by screw
rotated by asynchronous motor. The vertical movement of the probe caused by surface
irregularities makes the horizontal lapped cylinder to roll. This causes the movement of the arm
attached to the lapped cylinder. A magnified vertical movement of the diamond scriber on
smoked glass is obtained by the movement of the arm. This vertical movement of the scriber
together with horizontal movement produces a trace on the smoked glass plate. This trace is
further magnified at X 50 or X 100 by an optical projector for examination.

(c) The Taylor Hobson Talysurf:

 Department of Mechanical Engineering

PROGRAM CODE: BE ME-201

Subject Name : Mechanical Measurement and Subject Code : MEY-202
Metrology

Taylor-Hobson Talysurf is a stylus and skid type of instrument working on carrier modulating
principle. Its response is more rapid and accurate as comparred to Temlinson Surface Meter.
The measuring head of this instrument consists of a sharply pointed 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. In this instrument the stylus is made to trace the profile of the surface irregularities,
and the oscillatory movement of the stylus is converted into changes in electric current by the
arrangement as shown in Fig. 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. These two coils with other two resistances form an
oscillator. As the armature is pivoted about the central leg, any movement of the stylus causes
the air gap to vary and thus the amplitude of the original a.c. current flowing in the coils is
modulated. The output of the bridge thus consists of modulation only as shown in

Fig. This is further demodulated so that the current now is directly proportional to the
vertical displacement of the stylus only.

(d) Profilograph:

(i) Proyilograph : The principle of Working of a tracer type profilograph is shown in Fig. The
work to be tested is placed on the table of the instrument. The work and the table are traversed
with the help of a lead screw.
 Department of Mechanical Engineering

PROGRAM CODE: BE ME-201

Subject Name : Mechanical Measurement and Subject Code : MEY-202
Metrology

The stylus which is pivoted to a mirror moves over the tested surface. Oscillations of the tracer
point are transmitted to the mirror. A light source sends a beam of light through lens and a
precision slit to the oscillating mirror. The reflected beam is directed to a revolving drum, upon
which a sensitised film is arranged. This drum is rotated through two bevel gears from the same
lead screw that moves the table of the instrument. A profilogram will be obtained from the
sensitised film, that may be sub-sequently analysed to determine the value of the surface
roughness.
 Department of Mechanical Engineering

PROGRAM CODE: BE ME-201

Subject Name : Mechanical Measurement and Subject Code : MEY-202
Metrology

Problems:
 Department of Mechanical Engineering

PROGRAM CODE: BE ME-201

Subject Name : Mechanical Measurement and Subject Code : MEY-202
Metrology
 Department of Mechanical Engineering

PROGRAM CODE: BE ME-201

Subject Name : Mechanical Measurement and Subject Code : MEY-202
Metrology
 Department of Mechanical Engineering

PROGRAM CODE: BE ME-201

Subject Name : Mechanical Measurement and Subject Code : MEY-202
Metrology

ISI Symbols for Indication of surface Finish

The surface roughness is represented in figure. If the matching method is milling, .sampling
length is 2.5 mm, direction of lay is parallel to the surface, machining allowance is 3 mm and
the representative will be as shown in figure,

Representation of Surface Roughness:

(i) The limits of surface roughness can be represented as,

(ii) The surface roughness and sampling length can be represented as,

Here surface sampling length is 2.5 mm p

(iii) The surface roughness and lay can be stated as,

However, in most cases, one single piece of information is sufficient which is indicated as
follows,
 Department of Mechanical Engineering

PROGRAM CODE: BE ME-201

Subject Name : Mechanical Measurement and Subject Code : MEY-202
Metrology

The I.S.O has recommended as series of preferred roughness values and

corresponding roughness grade numbers to be used when specifying surface
roughness on drawings.