This document discusses measurement of surface finish. It begins with basic concepts of surface roughness and factors that influence surface finish like machining variables, tool geometry, workpiece and tool materials. It describes different types of surface irregularities from first to fourth order. There are two main methods to measure surface finish - qualitative methods like visual inspection and quantitative methods using instruments like stylus profilometers. Key parameters to analyze surface roughness include Ra, Rz, and RMS. Surface roughness is important for functions like friction, wear and lubrication.

1. Measurement of Surface Finish
•Basic understanding of surface roughness.
•Indication of finish on drawing.
•Parameters of Surface roughness.
•Measurement methods: contact and non contact type.

2. INTRODUCTION
•Engineering components are manufactured by casting, forging, welding etc…
•These components are then subjected to various machining operations for getting required
geometrical surfaces .
•It is not practically possible to produce a component having a geometrically ideal surface.
•The surface finish after machining depends upon the material, vibration, deflection, speed, feed
and other working conditions.
•The surface finish requirements depend upon the functional requirements of the components to
be assembled.

3. Why Surface finish?
• Surface finish, also known as surface texture or surface topography, is the nature of the surface
as defined by characteristics such as lay, surface roughness and waviness.
• Surface texture is one of the important factors that control friction during sliding.
• To improve look and feel of the product.
• Fit between parts.
• To control the process that results in the surface needed.

4. Importance of surface finish
• Surface roughness, is important from the point of view of fundament problems such as friction
and wear, surface contact, lubrication, and tightness of joints.
• It affects conduction of heat, cleanliness and reflectivity of the surface.
• It also affects the positional accuracy of mating parts, resistance to corrosion.

5. Factors influencing surface finish
• The machining variables that includes:
a) Cutting speed
b) Depth of cut
c) Feed
• Geometry of tool.
• Work piece and tool materials, and their mechanical properties.
• Quality and type of machine tool used.
• Lubrication used.
• Vibrations between work piece, cutting tools and machine tools.

6. Classification of Geometrical Irregularities
The geometrical irregularities can be classified as
•First order
•Second order
•Third order
•Fourth order
1. FIRST ORDER:
These irregularities are formed in the machine, due to inaccuracies, this irregularity also contains lack of straightness
of guide ways, this is caused due to metal weight and cutting force.
2. SECOND ORDER:
Irregularities such as chatter marks, etc. are formed due to vibrations. This type of irregularities is called second-
order irregularities.
3. THIRD ORDER:
This is formed due to the machining process, even if the machine is perfect and vibration free are called third order
irregularities.
4. FOURTH ORDER:
This is formed due to the separation of the chip, this is caused due to rupture of material are fourth order irregularities.

7. Types of Surface Finish
Surface of a body is its boundary which separates it from another body. Generally there are two
types of surfaces namely:
Nominal Surface: A theoretically geometrically perfect surface is called as a nominal surface.
Rough Surface: A magnified view of an actual surface is shown. This surface has numerous small
peaks and valleys that deviate from the nominal surface. It is seen that the surface has a certain
degree of roughness and hence it is called a rough surface.
Wavy Surface: A surface having waviness as shown in the figure is called wavy surface. Waviness
is caused by:
1. Deflections of tools and work piece.
2. Uneven lubrication.
3. Vibrations

8. Nomenclature of Surface texture
1. Surface Roughness: It is the micro-irregularities in a surface resulting from the action of
production process.
2. Profile: It is the shape of any section through a surface.
3. Flaw: It is an irregularity that occurs in a surface due to cracks, blow holes, scratches, etc.,
Unless otherwise specified, the effect of flaws should not be accounted in the measurement of
roughness.
4. Sample or Cut-off length: The particular length that is taken for sample measurement on the
surface is called as cut of length. It is also known as roughness width cut off.
5. Lay: It is the direction of the predominant top surface grooves that are produced by machining.

9. 6. Waviness: Surface irregularities that are of greater spacing than roughness.

10. Analysis of surface finish
The assessment of average roughness is carried out by
a) Centre line average (CLA) value (Ra).
b) Root mean square (RMS) value (Rs).
c) Ten point method. (Rz)

11. Analysis of surface finish
a) Centre line average (CLA) value (Ra):
Ra is the average height from a mean line of all ordinates of the surface, regardless of sign. Let y1,
y2, …., yn be the height of the equally spaced ordinates at positions 1,2,…n.
In Centre Line Average (C.L.A.) method;
Centre Line Average= ( y1+y2+y3+…….+yn)/ n

12. Another way of calculating Ra is shown in the figure.
As per BIS, the surface roughness is assessed in terms of C.L.A. value and is denoted
by Ra

13. b) Root mean square (R.M.S) value (Rs):
R.M.S Rs is the square root of the mean of the squares of the surface measured from the mean line.
As y1 , y2 , ……yn are the heights of the equally spaced ordinates at positions 1,2,3,…n,
In Root Mean Square(R.M.S.) method;
Root Mean Square =
c) Ten point height method: The average difference between five highest peaks and five lowest
valleys of surface is taken and irregularities are calculated by

14. Direction of lay
The direction of lay is the direction of the predominant surface pattern ordinarily determined by the production
method and is shown in table.

16. Roughness grade and symbols
The principal criterion of surface roughness, Ra may be indicated by the corresponding
roughness grade number, as shown in Table

17. Roughness grade for various machining processes

18. Representation of surface roughness
The value or values defining the principal criterion of roughness are added to the symbol as
shown in figure
a = Roughness value, Ra in micrometres
b = Production method/ treatment of coating
c = Sampling length
d = Direction of lay
e = Machining allowance and
f = Other roughness values

19. Methods of measurement of Surface roughness
There are basically two methods for measuring surface finish, namely
1. Comparative or Qualitative Methods
2. Direct Measurement or Quantitative Methods
Comparative or Qualitative Methods
In these methods, the surface texture is assessed by observation of the surface. The surface to be
tested is compared with known value of roughness specimen and finished by similar machining
process. The various methods which are used for comparison are
1. Touch Inspection.
2. Visual Inspection.
3. Microscopic Inspection.
4. Scratch Inspection.
5. Micro Interferometer.
6. Surface photographs.

20. Touch Inspection
• The main limitation of this method is that the degree of surface roughness can’t be assessed.
• Also the minute flaws can’t be detected.
• 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.
Visual Inspection
In this method, the surface is inspected by naked eye and this measurement is limited to rough
surfaces .
Microscopic 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.
• In this method, a master finished surface is placed under the microscopic and compared with the
surface under inspection.
• In another method a straight edge is placed on the surface to be inspected and abeam of light
projected at about 60° to the work.
• Thus the shadows cast into the surface scratches are magnified and the surface irregularities can be
studied.

21. 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.
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.
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.

22. Direct Methods of Surface roughness
measurement
Instructor
Mr. Gaurav Bharadwaj
Assistant Prof.
Department of ME
GLA University

23. Direct Measurement or Quantitative Methods
Some of the direct measuring instruments are
1. Stylus probe instruments
2. Tomlinson surface meter
Stylus probe instrument
Principle
When the stylus is moved over the surface to be measured, the irregularities in the surface texture
are measured and it is used to assess the surface finish of the workpiece.
Working
• The stylus type instruments consist of skid, stylus, amplifying device and recording device.
• The skid is slowly moved over the surface by hand or by motor drive.
• The skid follows the irregularities of surface and stylus moves along
with skid.
• When the stylus moves vertically up and down, the stylus
movements are magnified, amplified and recorded to get a trace.
• Then the trace is analyzed by an automatic device.

24. Advantage:
1. Any desired roughness parameter can be recorded.
Disadvantages:
1. Fragile material cannot be measured.
2. High Initial cost.
3. Skilled operators are needed to operate.

25. Tomlinson Surface meter
Principle: This instrument uses mechanical-cum-optical means for magnification.
Construction:
•In this, the diamond stylus on the surface finish recorder is held by spring pressure against the
surface of a lapped cylinder.
• The lapped cylinder is supported one side by probe and other side by rollers.
•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 and the light spring steel arm is
attached to the Lapped cylinder.
• The spring arm has a diamond scriber at the end and
smoked glass is rest on the arm.

26. Working
• When measuring surface finish, the body of the instrument
moves across the surface by a screw rotation.
• The vertical movement of the probe caused by the surface
irregularities makes the horizontal lapped cylinder to roll.
• This rolling of lapped cylinder causes the movement of the
arm, So, this movement induces the diamond scriber on
smoked glass.
• Finally, the movement of scriber together with horizontal
movement produces a trace on the smoked glass plate and
this trace is magnified by an optical projector.

27. Thank you