MESUREMENT TECHNIQUES
Lecture 13
OVERVIEW


Surface metrology examines the
deviation between one point (or
points) on a surface and another
points (other points) on the same
surface
Surface metrology is a concern of
many branches of science and
widely involved in the world of
commerce and manufacturing
Surface metrology


Surface metrology or surface topology
refers to the geometry and texture of
surfaces
The condition of surface is defined by its
characteristics:





Surface texture (finish)
Roundness (a function of geometry)
Material
Hardness
Surface metallurgy
Surface metrology (cont’)

Three forms of asperity
1.
2.
3.


Roughness
Waviness
Error of form
Vary according to the length
of spacing or wavelength
The fourth asperity is not distinguish by
wavelength; it is flaw
Lay is the direction of the asperities
which in most cases means that
roughness and waviness are
perpendicular to each other
Surface assessment
 Roughness – the finest of the
asperities
 Waviness - concern the more
widely spaced ones
 Flaw – surface defect
 Lay – The direction of the
asperities
* Asperity is defined as
unevenness of surface,
roughness
Various lay configuration
This chart categorizes the
various lay configurations and
shows the standardizes symbols
used on drawing
MAIN MEASUREMENT METHOD OF
SURFACE ROUGHNESS

Inspection and assessment of surface
roughness of machined workpieces can
be carried out by means of different
measurement techniques. These
methods can be ranked into the following
classes:
1.
2.
3.
4.
Direct measurement methods
Comparison based techniques
Non contact methods
On-process measurement
Direct Measurement Method





Direct methods assess surface finish by
means of stylus type devices.
Measurements are obtained using a stylus
drawn along the surface to be measured:
the stylus motion perpendicular to the
surface is registered.
This registered profile is then used to
calculate the roughness parameters.
Disadvantage- This method requires
interruption of the machine process, and
the sharp diamond stylus may make
micro-scratches on surfaces.
Comparison Based Technique



Comparison techniques use specimens of
surface roughness produced by the same
process, material and machining
parameters as the surface to be
compared.
Visual and tactile senses are used to
compare a specimen with a surface of
known surface finish.
Because of the subjective judgment
involved, this method is useful for surface
roughness Rq>1.6 micron
Non-contact Method





There have been some work done to attempt to
measure surface roughness using non contact
technique.
Here is an electronic speckle correlation method given
as an example (figure 8.1).
When coherent light illuminates a rough surface, the
diffracted waves from each point of the surface
mutually interfere to form a pattern which appears as a
grain pattern of bright and dark regions.
The spatial statistical properties of this speckle image
can be related to the surface characteristics.
The degree of correlation of two speckle patterns
produced from the same surface by two different
illumination beams can be used as a roughness
parameter.
Figure 8.1
On-process Measurement

Many methods have been used to measure surface roughness
in process. For example:



Machine vision.
In this technique, a light source is used to illuminate the surface
with a digital system to viewing the surface and the data being
sent to a computer to be analyzed. The digitized data is then used
with a correlation chart to get actual roughness values.
Inductance method. An inductance pickup is used to measure
the distance between the surface and the pickup. This
measurement gives a parametric value that may be used to give
a comparative roughness. However, this method is limited to
measuring magnetic materials.
Ultrasound. A spherically focused ultrasonic sensor is positioned
with a non normal incidence angle above the surface. The sensor
sends out an ultrasonic pulse to the a personal computer for
analysis and calculation of roughness parameters.
SURFACE EVALUATION



Surface roughness comparator
Microscope
Stylus method
Surface roughness comparator


The most common way to evaluate
surface finish is to compare it
visually and by feel with roughness
comparison specimens having
various surface finishes
It consist of composite set of
surface roughness specimen
standard
Surface roughness comparator (cont’)
Microscope


Examination of surfaces
by microscope can be
informative
But it does not usually
allow the heights of the
asperities to be
determined without
destroying the test part
by cutting a taper
through the surface
Stylus Instrument

Why we use stylus method??



It is the most familiar method for
practical shop work
It the best demonstrate the
fundamental principles of surface
metrology
Standard is based on the stylus method
STYLUS INSTRUMENT
Stylus instrument





The stylus instrument widely used technique for
measuring a surface profile
This technique uses a fine diamond stylus with tip size
approximately 0.1 to 10 µm to transverse the surface
As the stylus tracks the surface peaks and valleys, its
vertical motion is converted to a time varying electrical
signal that represent surface profile
Stylus instruments operate like a phonograph pickup:
the stylus is drawn across the surface and generates
electrical signals that are proportional to the changes in
the surface
The changes in height can be read directly with a
meter or on a printed chart
Two types of stylus instrument
1.
2.
True- datum or skidless
instruments
Surface- datum or skid type
instrument
True- datum instrument


With this instrument, we draw across the
surface in a very precise, mechanically
controlled movement
Advantages


The resulting graph is nearly a true
representation of the surface along that one
line showing roughness, waviness, errors of
form and flaws
Disadvantages

Very difficult to set the instrument up; must
precisely align the surface being assessed with
the path of the instrument
Figure 8.2
True-datum stylus instruments move the stylus across the part
along a reference datum established by the instrument
Surface datum instrument



Can easily set up because they use the
surface being assessed as the datum
A supporting slide (skid, a rounded
member fixed to the head, a shoe, a flat
pad) mounted in the head rests on the
surface and slides the stylus pickup along
Skids may be located, in front of, behind,
or on the opposite side of the stylus
Figure 8.3
Surface datum instruments create their own reference datum. This
is done by supporting the stylus, T, by a member S, that is
sufficiently wide to slide along the surface.
NUMERICAL VALUES FOR
ASSESSMENT
Arithmetic roughness average




This method is also known as roughness
average and by two earlier term; arithmetic
average (AA) and center-line average (CLA)
The roughness average is the arithmetic
average of the absolute values of the
deviation from the profile height measured
from the centerline along a specified
sampling length
Two method for determining the value
i.
ii.
Graphical method
Electrical averaging
NUMERICAL VALUES FOR
ASSESSMENT (cont’)
Other standardized assessment
methods
1.
Root-Means-Square roughness (Ra or RMS)




2.
Maximum Peak-Valley Roughness (Rmax or Rt)



3.
Closely related to the roughness average (Ra)
Square the distances, average them, and determine the
square root of the result
The resulting value is the index for surface texture
comparison
Usually 11% higher than the Ra value
Determine the distance between the lines that contact
the extreme outer and inner point of the profile
Second most popular method in industry
See figure A
Ten-Point Height (Rz)


Averages the distance between the five peaks and five
deepest valleys within the sampling length
See figure B
Other standardized assessment
methods (cont’)
4.
Average Peak-to-Valley Roughness (R or H or Hpl)



5.
Average Spacing of Roughness Peaks (Ar or AR)


6.
Average the individual peak-to-valley heights
See figure C
Use the height between adjacent peaks and valleys, not
measure from a center line to peak valleys
Average the distance between the peaks without regard
to their height
See figure D
Swedish Height of Irregularities (R or H)



Also known as Profiljup methos
Only standard in Sweden (H) and Denmark (R)
It assume that, in wear situation, the peaks are affected
by wear, but the valleys are not.
Other standardized assessment
methods (cont’)
7.
Bearing Length Ration (Tp and others)



8.
Leveling Depth (Rp and others)


9.
Create a reference line through some of the peaks
This line is at a predetermined height from the
mean line, and you have then divide the
subtended length through the peaks by sampling
length to arrive at the assessment value
See figure F
Measure the height between the highest peak and
the mean line
See figure G
Waviness Height (W)

Assess the waviness without regard to roughness
by determining the peak-to-valley distance of the
total profile within the sampling length
Example