Measurement of surface finish, surface texture, Methods of measuring surface finish, surface inspection by comparison, touch inspection, visual inspection, scratch inspection, microscopic inspection, surface photographs, Wallace surface dynamometer, reflected light intensity, Intersection method, microkarato surface roughness tester, Tomlinson surface roughness tester, Forster surface roughness tester, talysurf roughness tester, perthometer, numerical evaluation of surface texture, Indication of surface roughness symbol, symbols for direction of lay, position of the specification of roughness symbol,

1. Measurement of Surface Finish
PREPARED BY : RATNADEEPSINH JADEJA

2. Surface Texture
 The Surface texture may be classified as:
“The characteristics quality of an actual surface due to small departures from its
general geometrical form which, occurring at regular or irregular intervals, tend to
form a pattern or texture on the surface.”
 Surface texture vary according to the machining processes used in producing it.
 These differences are apparent by visual examination and can be felt readily by
passing a finger nail over the surface.

3. Surface Texture
 Any material being machined by chip removal process can’t be finished perfectly.
Due to conditions not being idle, the surface produced will have some
irregularities; and these geometrical irregularities could be classified into the
following four categories:
1. First order: due to inaccuracies in the machine tool itself (lack of straightness of
guideways on which tool post is moving)
2. Second order: due to vibrations of any kind such as chatter marks.
3. Third order: due to machining process itself. This also includes the feed marks
of the cutting tool.
4. Fourth order: due to the rapture of the material during the separation of the
chip.

4. Surface Texture
 These irregularities of four orders can be grouped into the following two groups:
1. Primary texture (or Roughness)
2. Secondary texture.
 Primary Texture: Irregularities of small wavelength caused by direct action of
cutting element on the material or by some other disturbance such as friction,
wear, or corrosion. These errors are mainly caused due to tool chatter i.e., it
includes irregularities of third and fourth order and constitutes that micro-
geometrical errors.

5. Surface Texture
 Secondary Texture: Irregularities of considerable wavelength of a periodic
character resulting from mechanical disturbances in the generating setup. These
errors are termed as macro-geometrical errors and include irregularities of first
and second order and are mainly due to misalignment of centers, lack of
straightness of guideways and nonlinear feed motion.
 Any surface could be considered to be combination of two form of wavelengths
(waviness and roughness)

6. Surface Texture

7. Methods of Measuring Surface Finish
 The surface finish of machined surface can be measured by the following two
methods:
1. Surface inspection by comparison methods
2. Direct instrument measurements.

8. Surface Inspection by comparison
Touch Inspection:
 One can feel the distance between the peaks of the surface irregularities by touching it
with finger, on conditions that this distance is greater than 3 mm.
 The surface having maximum depth of roughness of about 0.1 μm can be can be
compared when the sliding speed of the edge is between 3 and 5 cm/sec.
 The main limitation of this method is that the degree of surface roughness can’t be
assessed. Also minute flows cannot be detected.
 This method can simply tell which surface is more rough.

9. Surface Inspection by comparison
Visual Inspection:
 One can visualize and differentiate between surface having a roughness variation of not
less than 5 % due to the fact that the brightness of these surfaces varies proportionally.
 Visual inspection by naked eye is always likely to be misleading particularly when
surfaces having high degree of surface finish are inspected.
 The method is limited to rougher surfaces and results vary from person to person.

10. Surface Inspection by comparison
Scratch Inspection:
 A soft 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
visualized.
Microscopic Inspection:
 In this method a master finished surface is placed under the microscope and compared
with the surface under inspection.
Surface Photograph:
 Magnified photographs of the surface are taken with difference type of illumination.
 In case we use vertical illumination then defects like irregularities and scratched appear
as dark spots and flat portion of the surface as bright area.
 In case of oblique illumination, reverse is the case. Photograph with different
illumination are compared and the results assessed.

11. Surface Inspection by comparison
Micro-interference:
 An optical flat is placed on the surface to be inspected and illuminated by a
monocromatic source of light. Interference bands are studied through a microscope.
 Scratches in the surfaces appears as interference lines extending from the dark bands
into the bright bands. The depth of the defects is measured in terms of the fraction of
the interference band.

12. Surface Inspection by comparison
Wallace surface dynamometer:
 It is a short of friction meter. It consists of a pendulum in which the testing shoes are
clamped to the 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 time of swing is a direct measure of surface roughness.
Reflected light intensity:
 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 photo cell.
 The measured intensity changes are already calibrated by means of readings taken from
the surface of known roughness by some other suitable method.

13. Surface Inspection by comparison
Comparison with standard specimens:
 For qualitative assessment of the surface roughness, the roughness comparison
specimens are used. Roughness of the finished product is evaluated by visual or tactile
comparison.
 The specimens used for the comparison are available either in flat of segmented
cylindrical form. The flat specimens have the size: length = 25 mm; width = 16 mm and
thickness = 1.6 to 10 mm. cylindrical specimens will have either convex surface or
concave surface having a surface radius about 125 mm.

14. Direct Instrument Measurement
The quantitative measurement of surface roughness can be done by the following
methods:
1. Intersection method:
2. Interface method
3. Stylus method
4. Pneumatic measuring method
5. Perthen condenser method

15. Intersection Method
Cross-section method:
 A specimen is cut perpendicular to its surface after applying a protective layer of
approximately the same hardness is applied to the surface either by chromate plating or
by casting of a thin film layer of white metal on the surface.
 The specimen, after cutting, has to be polished and the actual surface texture can be
observed under a measuring microscope.
 The destruction of a component for measuring its surface hardness can be avoided by
making a surface replica from celluloid or gelatine. After drying the replica, it can be
sliced and observed under the microscope.

16. Intersection Method
Light-section method:
 A narrow light band is made to intersect the test surface
and intersected contour of the surface can be observed by
microscope.
 When i = o, then arrangement is known as bright field
 When i ≠ o, then arrangement is known as dark field

17. Microkarato surface
roughness tester
This instrument be directly applied for
measuring the roughness of flat
surfaces. It uses a stylus with a
mechanical magnification based upon
the microkator principle.
The instrument is supported on three
fixed skids and an adjustable one. The
diamond stylus traces the test surface.

18. Tomlinsion tester

19. Tomlinsion tester
 When surface finish is to be measured, the body is transversed 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.
 By its rolling, the light arm attached to its end provides a magnified movement on
smoked glass plate.
 This vertical movement equipped with horizontal movement produces a trace on the
glass magnified in the vertical direction and there is no magnification in the horizontal
direction.
 The smoke glass trace is then, further projected at ×50 or ×100 magnification for
examination.

20. Forster surface roughness tester
Forster surface roughness tester with cam
Forster surface roughness tester with
electric magnet

21. Forster surface roughness tester
 In this instrument the stylus is lifted and made to fall on the surface under test either by
a cam rotating at a constant speed or by an electric magnet.
 The nose radius of the stylus is 10 mm and the static measuring force is 1g so that
compressive stress of 6.2 KN/mm2 will be achieved.
 The impulse tracing method has the advantage that the frictional and traditional forces
on the stylus are minimized and grease layer that might cover the test surface does not
affect the measurement.
 This tester allows tracing a 100mm long surface but the weight of the component
should not exceed 3 kg. The distance moved by the component during two successive
impulses is about 1 mm and the measuring range is between 0 and 125 μm.

22. Talysurf surface roughness tester

23. Talysurf surface roughness tester
 A downward movement of the stylus results in decreasing the air gap of the primary coil
and in an equal increase of the air gap at the secondary coil. The impedance of the coils
will be changed according to the variation of air gap and an additional alternating
current flows in the secondary coil whose magnitude is governed by the variation of the
impedance of the coil and is proportional to the displacement of the stylus.
 The maximum length that can be traced is 10 mm.
 The measuring force of the stylus is 0.1 g.
 The vertical magnification can be varied in steps from 1000 to 50000 times.

24. Perthometer
The perthometer is equipped with different tracer systems that can be selected according to the
measuring conditions.
1. Pendulum type 2. Semi-rigid type 3. Free supported type with slip gauge
4. Free supported type with reference datum plane.

25. Perthometer
 The two inductive coils are connected in a Wheatstone bridge circuit which can be
electrically balanced when the iron plate, i.e., the stylus is in the middle position. The
displacement of the stylus results in upsetting the balance of the circuit and in
generating a current that flows in a diagonal connection of the Wheatstone bridge
circuit which can be amplified and recorded. The magnitude of this current is
proportional to the displacement of the stylus.
 Perthometer can be used for measuring all parameters of the surface texture. The length
of the transverse of the tracer head can be selected from 2 to 25 mm according to the
requirements. It is provided with six measuring ranges with six different amplification is
given below:
Measuring Range (mm) 25 10 2.5 1 0.25 0.1
Amplification 400:1 1000:1 4000:1 10000:1 40000:1 100000:1

26. Numerical Evaluation of
Surface Texture
C.L.A. = ‘Centre Line Average’
R.M.S. = ‘Root Mean Square’
n = Total number of ordinates
L = Length of pen record

27. Indication of surface roughness symbol
The basic symbol, consist of two legs of
unequal length included at approximately
60° to the line representing the surface
under consideration.
If the removal of Material by machining is
required, a bar is added to the basic
symbol.
If the removal of material is not permitted,
a circle is added to the basic symbol.
When special surface characteristics have to
be indicated a line is added to the longer
leg of any of the above symbols.

28. Symbols for
direction of lay
Parallel to the plane of projection of the view in which the
symbol is used
Perpendicular of the plane of projection of the view in
which the symbol is used
Crossed in two slant directions relative to the plane of
projection of the view in which the symbol is used
Multi-directional
Approximately circular relative to the centre of the surface
to which the symbol is applied
Approximately radial relative to the centre of the surface
to which the symbol is applied

29. Position of the
specification of
roughness in
symbol
The specification of surface roughness should be placed relative to the
symbol as shown in figure.
a = roughness value Ra in micrometres or roughness grade symbol N1
to N12;
b = production method, treatment or coating;
c = sampling length;
d = direction of lay;
e = machining allowances;
f = other roughness value (in brackets).