UNIT-5
Comparators
Comparator is another form of linear measuring
method, which is quick and more convenient for
checking large number of identical dimensions.
During the measurement, a comparator is able to give
the deviation of the dimension from the set dimension.
Cannot measure absolute dimension but can only
compare two dimensions.
Highly reliable.
To magnify the deviation, a number of principles are
used such as mechanical, optical, pneumatic and
electrical.
Fig. Principle of a comparator
GATE – 2007 (PI)
Which one of the following instruments is a
comparator ?
(a) Tool Maker’s Microscope
(b) GO/NO GO gauge
(c) Optical Interferometer
(d) Dial Gauge
Mechanical Comparators
The Mikrokator principle
greatly magnifies any
deviation in size so that
even small deviations
produce large deflections of
the pointer over the scale.
Sigma Mechanical Comparator
The Sigma Mechanical Comparator uses a partially
wrapped band wrapped about a driving drum to turn a
pointer needle. The assembly provides a frictionless
movement with a resistant pressure provided by the
springs.
Sigma Mechanical Comparator
Mechanical Comparators
The Eden-Rolt Reed system uses a
pointer attached to the end of two
reeds. One reed is pushed by a
plunger, while the other is fixed. As
one reed moves relative to the other,
the pointer that they are commonly
attached to will deflect.
Optical Comparators
These devices use a plunger to rotate a mirror. A light
beam is reflected off that mirror, and simply by the
virtue of distance, the small rotation of the mirror can
be converted to a significant translation with little
friction.
Pneumatic Comparators
Flow type:
The float height is essentially proportional to the air
that escapes from the gauge head
Master gauges are used to find calibration points on
the scales
The input pressure is regulated to allow
magnification adjustment
Pneumatic Comparators
Measurement of Surfaces
Surfaces
No surface is perfectly smooth, but the better the
surface quality, the longer a product generally lasts,
and the better is performs.
Surface texture can be difficult to analyse
quantitatively.
Two surfaces may be entirely different, yet still provide
the same CLA (Ra) value.
Surface geometry can be quantified a few different
ways.
Real surfaces are rarely so flat, or smooth, but most
commonly a combination of the two.
Roughness height: is the parameter with which
generally the surface finish is indicated. It is specified
either as arithmetic average value or the root mean
square value.
Roughness width: is the distance parallel to the
nominal part surface within which the peaks and
valleys, which constitutes the predominant pattern of
the roughness.
Roughness width cut-off: is the maximum width of
the surface that is included in the calculation of the
roughness height.
Waviness: refers to those surface irregularities that have
a greater spacing than that of roughness width.
Determined by the height of the waviness and its
width.
The greater the width, the smoother is the surface and
thus is more desirable.
Lay direction: is the direction of the predominant
surface pattern produced on the workpiece by the tool
marks.
Flaw: are surface irregularities that are present which are
random and therefore will not be considered.
Lay
Diagram Symbol Description
Parallel lay: Lay parallel to
the Surface. Surface is
produced by shaping,
planning etc.
Perpendicular lay: Lay
perpendicular to the
Surface. Surface is produced
by shaping and planning
Crossed lay: Lay angular in
both directions.
Surface is produced by
knurling, honing.
Lay Contd..
Diagram Symbol Description
Multidirectional lay: Lay
multidirectional. Surface is
produced by grinding,
lapping, super finishing.
Circular lay:
Approximately circular
relative to the center.
Surface is produced by
facing.
Radial lay: Approximately
radial relative to the center
of the nominal surface.
Representation of Surface Roughness
Roughness Roughness Grade Roughness Symbol
Ra ( m) Number
50 N12 -
25 N11
12.5 N10
6.3 N9
3.2 N8
1.6 N7
0.8 N6
0.4 N5
0.2 N4
0.1 N3
0.05 N2
0.025 N1
Waviness height - the distance from a peak to a valley
Waviness width - the distance between peaks or
valleys
Roughness width cutoff - a value greater than the
maximum roughness width that is the largest
separation of surface irregularities included in the
measurements. Typical values are (0.003”, 0.010”,
0.030”, 0.100”, 0.300”)
Lay - the direction the roughness pattern should
follow
Stylus travel is perpendicular to the lay specified.
Evaluation of Surface Roughness
1. Centre line average (CLA) or arithmetic mean
deviation denoted as Ra.
2. Root mean square value (Rg) : rms value
3. Maximum peak to valley roughness (hmax)
4. The average of the five highest peak and five deepst
valleys in the sample.
5. The average or leveling depth of the profile.
Determination of Mean Line
M-System: After plotting the characteristic of any
surface a horizontal line is drawn by joining two points.
This line is shifts up and down in such a way that 50%
area is above the line and 50% area is below the line
Determination of Mean Line
E-System: (Envelop System) A sphere of 25 mm
diameter is rolled over the surface and the locus of its
centre is being traced out called envelope. This envelope
is shifted in downward direction till the area above the
line is equal to the area below the line. This is called
mean envelope and the system of datum is called E-
system.
Arithmetical Average:
Measured for a specified area and the figures are added
together and the total is then divided by the number of
measurements taken to obtain the mean or
arithmetical average (AA).
It is also sometimes called the centre line average or
CLA value. This in equation form is given by
L
1 1
Ra y ( x) dx
L0 N
y i
GATE-2016 (PI)
The roughness profile of a surface is depicted below.
The surface roughness parameter Ra (in μm) is _______
The other parameter that is used sometimes is the root
mean square value of the deviation in place of the
arithmetic average , This in expression form is
1
RRMS
N
2
yi
Fig. Surface roughness parameters
Methods of measuring Surface Roughness
There are a number of useful techniques for measuring
surface roughness:
Observation and touch - the human finger is very
perceptive to surface roughness
stylus based equipment - very common
Interferometry - uses light wave interference patterns
(discussed later)
Observation Methods
Human perception is highly relative.
To give the human tester a reference for what they are
touching, commercial sets of standards are available.
Comparison should be made against matched
identical processes.
One method of note is the finger nail assessment of
roughness and touch method.
Stylus Equipment
uses a stylus that tracks small changes in surface
height, and a skid that follows large changes in surface
height.
The relative motion between the skid and the stylus is
measured with a magnetic circuit and induction coils.
One example of this is the Brown & Sharpe Surfcom
unit.
Profilometer
Measuring instrument used to measure a surface's
profile, in order to quantify its roughness.
Vertical resolution is usually in the nanometre level,
though lateral resolution is usually poorer.
Contact profilometers
A diamond stylus is moved vertically in contact with a
sample and then moved laterally across the sample for
a specified distance and specified contact force.
A profilometer can measure small surface variations in
vertical stylus displacement as a function of position.
The radius of diamond stylus ranges from 20
nanometres to 25 μm.
Non-contact Profilometers
An optical profilometer is a non-contact method for
providing much of the same information as a stylus
based profilometer.
There are many different techniques which are
currently being employed, such as laser triangulation
(triangulation sensor), confocal microscopy and digital
holography.
Advantages of optical Profilometers
Because the non-contact profilometer does not touch
the surface the scan speeds are dictated by the light
reflected from the surface and the speed of the
acquisition electronics.
Optical profilometers do not touch the surface and
therefore cannot be damaged by surface wear or
careless operators.
Optical Flats
Optical-grade clear fused quartz or glass structures
lapped and polished to be extremely flat on one or
both sides.
Used with a monochromatic light to determine the
flatness of other optical surfaces by interference.
When a flat surface of another optic is placed on the
optical flat, interference fringes are seen due to
interference in the tiny gap between the two surfaces.
The spacing between the fringes is smaller where the
gap is changing more rapidly, indicating a departure
from flatness in one of the two surfaces, in a similar
way to the contour lines on a map.
When the fringes are perfectly straight and same fringe
width for dark and bright band we conclude that the
surface is perfectly flat.
For convex surface the fringes curve around the point of
contact.
For concave surface the fringes curve away from the point
of contact.
The distance of air gap between two successive fringes is given by
2
n
Distance of air gap of interference fringe of n order is
th
2
GATE-2016
Two optically flat plates of glass are kept at a small
angle θ as shown in the figure. Monochromatic light
is incident vertically.
If the wavelength of light used to get a fringe
spacing of 1 mm is 450 nm, the wavelength of light
(in nm) to get a fringe spacing of 1.5 mm is _______
For IES Only
Optical flat as a comparator
nl
h
2
Where l separation of edges
n number of fringes / cm
h The difference of height between gauges
wevlength of monochomatic light
Talysurf
It is based upon measuring the generated noise due to
dry friction of a metallic blade which travels over the
surface under consideration.
If the frictional force is made small enough to excite
the blade, and not the entire system, then the noise
will be proportional to surface roughness, and
independent of the measured specimen size and
material.
The specimen surface roughness was measured by a
widely used commercial instrument (Talysurf 10), and
the prototype transducer.
