Concepts of Surface Metro logy, Introduction to Surface Texture, Terminology of Surface Texture, Analysis of Surface Traces, Peak to valley height of roughness ,The average roughness,Form factor & Bearing curve, Specification of Surface Texture Characteristics

1. MECHANICAL MEASUREMENT
& METROLOGY
Metrology of Surface Finish

2.  Branch :- Mechanical-4
 Division :- A
 Batch :- A3
 Group No :- A18

3. CONTENT:-
 Concepts of Surface Metrology
 Introduction to Surface Texture
 Terminology of Surface Texture
 Analysis of Surface Traces
1) Peak to valley height of roughness
2) The average roughness
3) Form factor & Bearing curve
 Specification of Surface Texture Characteristics

4. Concepts of Surface Metrology:-
 Surface metrology is the measurement of small-scale features on
surfaces, and is a branch of metrology. Surface primary form, surface
waviness and surface roughness are the parameters most commonly
associated with the field.
 The various manufacturing processes applied in industry produce the
desired shapes in the components within the prescribed dimensional
tolerances and surface quality requirements.
 The properties such as appearance, corrosion resistance, wear resistance,
fatigue resistance, lubrication, initial tolerance, ability to hold pressure,
load carrying capacity, noise reduction etc are influenced by surface finish.

5.  Basic industrial requirements with surface finish can be listed as,
 Low manufacturing cost
 Better properties
 Good appearance
 Quality of component/product
 Satisfactory performance etc.

6.  Now, the factors which affect the above requirements are,
> Material of the workpiece
> Type of machining process
> Vibrations of machines
> Cutting tool properties i. e. material, properties, sharpness, etc.
> Cutting conditions i. e. speed, feed, depth of cut.
> Type of coolant used.

7. Introduction to Surface Texture:-
 The characteristic 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 is a foremost characteristic among the surface integrity
magnitudes and properties imparted by the tools used in the processes,
machining mostly.
 The characterization and evaluation of engineering surface texture has
constituted a challenging metrological problem that has remained open so far,
especially when high-precision and/or functional performance requirements
exist.
 This fact is attributed to the usually complicated form of surface textures and the
need to obtain a satisfying description globally, as well as at various levels.

8.  Each manufacturing process produces a surface texture. The process is usually
optimized to ensure that the resulting texture is usable. If necessary, an
additional process will be added to modify the initial texture. i. e. grinding (abrasive
cutting), polishing, lapping, abrasive blasting, honing, electrical discharge machining (EDM),
milling, lithography, industrial etching/chemical milling, laser texturing etc.
 These differences in surface textures are apparent by many methods, i. e. Touch
Inspection, Visual Inspection, Scratch Inspection, Microscopic Inspection, Surface
Photographs, Micro-Interferometer, Reflected Light Intensity.
 Textures on the surfaces may be regular or irregular in character and may be
directional or non-directional.

9.  The geometrical irregularities can be classifies as follows,
Order Irregularities arise due to…
First Inaccuracies in machine tool.
i.e. straightness in guideways, deformation of work, weight of material etc.
Second Vibrations in machine.
i.e. due to cutting forces
Third Human errors in machining.
i.e. imperfect speed/feed/depth of cut.
Fourth Rupture of the material during separation from already finished surface of
the workpiece.

10. Terminology of Surface Texture:-
 Surface :- It is the surface limiting the body & separating it from surrounding.
 Actual surface :- It is the surface prescribed by the design or by the process of
manufacture.
 Nominal surface :- It is the surface prescribed by an average of irregularities
superimposed on it.
 Form error :- These are very widely spaced repetitive irregularities occurring
over the full length of the work surface.

12.  Roughness :-
 The surface irregularities o finest or short wavelength are known as
roughness or primary texture.
 It can also be said that closely spaced irregular deviations on a scale
smaller than that of waviness
 Basically, these irregularities are caused by direct action of the cutting
element on the material or by other disturbance such as friction, wear or
corrosion.
 It includes irregularities of first and third order.

13.  Waviness :-
 The surface irregularities of considerable wavelength of a periodic
character are known as waviness or secondary texture.
 These irregularities are caused due to misalignment of centres, lack of
straightness of guideways and non-linear feed motion.
 These includes irregularities of first and second order.
 Waviness errors are intermediate in wavelength between roughness and
form error.

14.  Flaws :- These are the surface irregularities which occur at on place or at
relatively infrequent or widely varying intervals on a surface. It includes random
irregularities such as scratches, cracks, holes, tears, inclusions, etc.
 Centre line :- It is an imaginary line about which the roughness is measured.
 Roundness :- It is the radial uniformity of a work surface measured from the
centre line of the workpiece.

15.  Lay :- It is the direction of predominant
surface pattern. It is produced by tool
marks or scratches. Symbols used to
indicate lay direction are as shown.

17.  Profile :- It is defined as the contour of any section through a surface.
 Effective profile :- It is the real contour of a surface obtained by using
instrument.
 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.
 Mean line profile :- It is the line having the form of the geometrical profile and
dividing the effective profile.

18. Analysis of Surface Traces:-
 To indicate degree of surface smoothness and roughness, a numerical
assessment may be assigned by different methods.
a. Peak to valley height method roughness,
b. The average roughness,
c. Form factor and Bearing curve.

19. Peak to valley height method 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.
 In this method, the maximum depth is accepted
as the measure of roughness. The disadvantage
of this method is that it may be read the same h
for two largely different texture.
 To overcome this lack of representation, the ten
point height average is used. This is determined
by drawing the line parallel to general lay of
trace.

20. The average roughness:-
Centre line average method:-
 It is the average height from the mean line of all ordinates of the surface.
 Surface roughness in this method can be determined by the average deviation from the nominal
surface.
 Centre line average value is given by,
C.L.A. =
ℎ1+ ℎ2+ ℎ3+⋯+ ℎ 𝑛
𝑛
& also C.L.A. =
𝐴1+ 𝐴2+ 𝐴3+⋯+ 𝐴 𝑛
𝐿
=
Σ𝐴
𝐿
where, ℎ1 , ℎ2 , ℎ3 , …ℎ 𝑛 are heights of ordinates,
𝐴1 , 𝐴2 , 𝐴3 , …𝐴 𝑛 are enclosed areas,
& L is the sampling length.

21. Root mean square value method:-
 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.
 It is the geometrical average of ordinates of profile about the mean line.
 Average areas above the mean line is approximately equal to areas below it.
 If we consider, ℎ1 , ℎ2 , ℎ3 , …ℎ 𝑛 are heights of ordinates, all parts are divided equally & L is the sampling
length then,
R.M.S. =
ℎ1
2+ℎ2
2+ℎ3
2+⋯+ℎ 𝑛
2
𝑛
or ℎ 𝑟𝑚𝑠 =
1
𝐿 0
𝐿
ℎ2. 𝑑𝐿
Generally R.M.S. value is greater than C.L.A. value for same
profile about 1.11 times.

22. 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,
Degree of fullness (K) =
𝐴𝑟𝑒𝑎 𝑜𝑓 𝑚𝑒𝑡𝑎𝑙
𝐴𝑟𝑒𝑎 𝑜𝑓 𝑒𝑛𝑣𝑒𝑙𝑜𝑝𝑖𝑛𝑔 𝑡𝑟𝑖𝑎𝑛𝑔𝑙𝑒
& hence,
Degree of emptiness (𝐾𝑝) = 1 − 𝐾

23. Bearing 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.

24. Specification of Surface Texture
Characteristics:-
 As per IS: 696 surface texture specified by indicating the following
 Roughness value i.e., Ra value in mm
 Machining allowance in mm.
 Sampling length or instrument cut-off length in mm.
 Machining production method, and
 Direction of lay in the symbol form as = ⊥, X, M, C, R

25. Symbol Meaning
Basic symbol, which is only be used when its meaning is explained by a note.
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.
It shows the production method used. Here, milled.
It shows the sampling length, here it is 2.5 mm.
If it is necessary to control the direction of lay, it is specified by a symbol added to the surface texture
symbol.
It represents the machining allowance, here its 2 mm.
Roughness value, 𝑅 𝑎 = 4 𝜇𝑚.

26.  If the machining method used is milling, sampling length is 0.25 mm with roughness
value 2.5 𝜇𝑚, direction of lay is perpendicular and machining allowance is 5 mm with
12.5 roughness grade then it can be shown as figure below.