MQAR Metrology, Quality Assurance & Reliability Text Books : Engg. Metrology by R.K.Jain Reference Books : 1.Statistical Quality Control by M. Mahajan, 2.Reliabity Engg. by L.Srinath . 1.Need of Inspection 2 . Standards of Measurement 3. Angle Measurement 4. Inspection of Screw-thread elements 5. S.Q.C. 6.Reliability Data Analysis TOPICS TO BE COVERED
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Microsoft Office Excel 2007.lnk
MQAR Metrology, Quality Assurance & Reliability
Text Books : Engg. Metrology by R.K.Jain
Reference Books : 1.Statistical Quality Control by M. Mahajan,
2.Reliabity Engg. by L.Srinath .
1.Need of Inspection
2 . Standards of Measurement
3. Angle Measurement
4. Inspection of Screw-thread elements
5. S.Q.C.
6.Reliability Data Analysis
TOPICS TO BE COVERED
Microsoft Office Excel 2007.lnk
from greek “metron” (measure) and –logy.
metrology is the science of measurements and that to measure is to
compare with something (a unit) which is taken as the basis for
comparison. (Measurement standard)
includes all theoretical and practical aspects of measurement.
Quality:
a product’s fitness for use.
the totality of features that bear on a product’s ability to satisfy a given
need.
the ability of a system or component to perform its required functions
under stated conditions for a specified period of time.
– Failure: the inability of an equipment to perform its required
function
– Reliability: the probability of no failure throughout a prescribed
operating period.
Metrology:
Reliability
Microsoft Office Excel 2007.lnk
This is the set of actions taken to develop primary standards of
measurement for the base units and the derived units of the International
System of Units (SI).
Legal metrology
Scientific metrology
It is that part of metrology which treats units of measurement, methods of
measurement and the measuring instrument, in relation to the statutory,
technical and legal requirements.
It assures security and appropriate accuracy of measurement.
Industrial metrology
The function of industrial metrology is mainly the proper calibration, control
and maintenance of all measuring equipment used in production,
inspection and testing. The purpose is to guarantee that
the products will comply with quality standards.
For convenience, a distinction is often made between the several fields of
application of metrology
Microsoft Office Excel 2007.lnk
Process of Measurement
set of operations having the objective of determining a value of a quantity
Measurand: particular quantity subject to measurement
Reference/Standard of Measurement:
Comparator: Fixed Gauges / Measuring Instrument:
Needs of Inspection To ensure that part and components are confirmed to
required standards.
To meet the need of Interchangeability of parts.
To maintain good customer relationship by ensuring that No
faulty product reaches the customer.
The result of inspection are forwarded to the manufacturing
department, thus helps in improving the quality.
It helps to purchase good quality raw material, tool and
equipment.
It led to development of precision measuring instruments.
Microsoft Office Excel 2007.lnk
High Quality Product
performs its functions reliably
performs its functions for a long time
performs its functions conveniently
Low Quality Product
does not perform its function reliably
fails or breaks after short time of use
is difficult to use
GOAL
Continuous Quality Improvement
(functionality, reliability, durability, …)
Inspection (Measurement)
What? When? How?
Microsoft Office Excel 2007.lnk
Inspection specific to PRODUCTS
Electronic parts (circuits, chips, etc.)
Machine elements (engines, brakes, gears, etc.)
Heat and thermodynamic components (engines, fuel injectors, etc.)
Medical and Bio-related products (implants, dental devices, surgical
SENSITIVITY OF MEASUREMENT Smallest difference in a dimension that an instrument can distinguish or
detect. It may be defined as the rate of displacement of the indicating
device of an instrument, w.r.t the measured quantity.
In other words, sensitivity of an instrument is the ratio of the scale
spacing to the scale division value.
For example, if on a dial indicator, the scale spacing is 1.0cm and the
scale division value is 0.01cm, then sensitivity is 100.
It is also called as amplification factor or gearing ratio.
Environmental changes affect instruments in two main ways, known as
zero drift and sensitivity drift.
Zero drift describes the effect where the zero reading of an instrument is
modified by a change in ambient conditions.
Sensitivity drift (also known as scale factor drift) defines the amount by
which an instrument's sensitivity of measurement varies as ambient
conditions change.
CALIBRATION
Calibration is the set of operations that establish, under specified
conditions, the relationship between the values of quantities indicated by
a measuring instrument and the corresponding values realized by
standards.
Calibration is the process of establishing the relationship between a
measuring device and the units of measure. This is done by comparing a
device or the output of an instrument to a standard having known
measurement characteristics.
When the instrument is made to give a null indication corresponding to a
null value of the quantity to be measured, the set of operation is called
zero adjustment .
Calibration can be called for:
with a new instrument
when a specified time period is elapsed
when a specified usage (operating hours) has elapsed
when an instrument has had a shock or vibration which potentially
may have put it out of calibration
whenever observations appear questionable
Calibration
Adjusting or setting of an instrument to
obtain accurate readings within a
reference standard.
Readability
Susceptibility of an instrument for having
its indications converted to a meaningful
number.
Precision
Degree of agreement in the
measurements of the same quantity.
Repeatability
Ability to do the same thing over & over.
Error between a number of successive
Attempts to move a machine to the same
position.
Terminology Accuracy
Degree of agreement of the
measured dimension with its true
magnitude.
Sensitivity
Smallest difference in a dimension
that an instrument can distinguish
or detect.
Resolution
Smallest dimension that can be
read on an instrument.
Reproducibility
Degree of agreement in the
individual results using the same
method and the same test
substance, but a different set of
laboratory conditions.
1 75
2 35
3 50
4 85
5 95
6 92
7 45
8 56
9 86
10 71
Mean 69.0
1 74
2 73
3 72
4 64
5 65
6 66
7 69
8 68
9 70
10 69
Mean 69.0
Standard Deviation A measure of the spread of a probability distribution, random variable, or
multiset of values.
More formally, it is the root mean square deviation of values from their
arithmetic mean.
In practice, it is often assumed
that the data are from
an approximately
Normally distributed
population.
According to this,
confidence intervals are:
σ: 68.26894921371% 4σ:99.99366575163%
2σ:95.44997361036% 5σ:99.99994266969%
3σ:99.73002039367% 6σ:99.99999980268%
Interchangeability An interchangeable part is one which can be substituted for similar part
manufactured to the same drawing.
The required fit assembly can be obtained in Two ways.
a)Universal or full interchangeability
b)Selective assembly Full interchangeability means any component will mat with any other
mating component without classifying Manufactured components into sub
groups or Without carrying out minor alteration for mating Purpose. It
requires precise machines or processes whose Process Capability is equal
or less than the manufacturing Tolerances
allowed for that part. So every component
produced will be with in desired tolerances
and capable of mating(Fitting) with any
other mating components to give the
required Fit.
Process capability of a machine is
defined as its ±3σ spread of
dimensions of components produced
by it.
Advantages of Interchangeability
1.Assembly time is reduced considerably.
2.There is an increased output with reduced production cost.
3.It facilitates production of mating components at different places
by different operator.
1.The replacement of worn out or defective parts and repair becomes
very easy.
2.The cost of maintenance and shutdown period is also reduced to
minimum.
Selective Assembly: In selective assembly components produced are classified into groups
according to their sizes by automatic gauging. This is done for both Holes
and Shafts and then corresponding parts will be matched properly.
It reduces chance of defective assembly and also the cost of assembly as
parts may be produced in wider tolerances.
Ex: Assembly of piston with cylinder bores.
Bore size = 50 mm
clearance required for assembly= 0.12 mm
Tolerance in both bore and piston = 0.04 mm
Dimension of bore diameter = 50 ±0.02 mm
Dimension of piston = 49.88 ±0.02 mm
By grading and marking the bores and pistons, they can be selectively
assembled as follows…
Cylinder Bore= 49.98mm 50 mm 50.02 mm
Piston = 49.86mm 49.88 mm 49.90 mm
Limits, Fits and Tolerances: 1.It is not possible to make any part precisely to a given dimension due to
variability of elements of production processes.
Man Machine Material
2. If by chance the part is exactly to a given dimension, it is impossible to
measure it accurately enough to prove it.
3. If attempts are made to achieve perfect size, the cost of production will
increase.
For a given system of Limits and fits to be successful following conditions
are to be satisfied:
It must be based on same standard so that every body alike
and a given dimension has the same meaning at all places.
The range of sizes covered by the systems should be sufficient
for most purposes.
Each basic size of hole and shaft must have a range of tolerance
values for each of the different fits.
Both unilateral and bi lateral methods of tolerances and hole
basis or shaft basis system should be acceptable.
The fundamental deviation required to give a particular fit must
increase with the basic size.
Size Designations Shaft: It refers not only to the
diameter of a circular shaft but Also
to any external dimension of a
component. (Male surface)
Hole: It refers not only to the diameter of a circular Hole but also
to any internal dimension of a component. (Female surface)
Basic Size or Basic dimension: It is the theoretical size worked
out by purely design consideration, from which limits of size are
derived by the application of allowances and tolerances.
Actual Size: is the measured size of the finished part.
Zero line: It is the straight line drawn horizontally to represent
the basic size. All the dimensions are shown w.r.t the Zero line.
Some Definitions Limit: Due to inevitable inaccuracy of manufacturing methods, it is
not possible to make a part precisely to a given dimension and may only be made to lie between to extremely permissible sizes called the limits for the actual size.
Upper/Lower limit: Largest/Lowest size permitted
Tolerance: The permissible variation in size or dimension of a part is called Tolerance. It is the difference between U.L and L.L of dimension.
It is the amount by which the job is allowed to go away from accuracy, with out causing any functional trouble.
Tolerance is always +ve.
Unilateral Tolerance: In this, the dimension
is allowed to vary only in one direction of
Basic Size, either above or bellow it.
Bilateral Tolerance: In this the dimension of part is allowed to vary in both
the sides of the basic size.
Deviation: It is the algebraic difference between the actual
size and the corresponding basic size.
Upper Deviation: It is the algebraic difference between the
upper (Max) limit and the corresponding basic size.
Denoted by “ES” for Hole and “es” for shaft.
+ve when UL> Basic size & -ve when UL< Basic size.
Lower Deviation: It is the algebraic difference between lower
limit and corresponding Basic size.
Denoted by “EI” for Hole and “ei” for shaft.
+ve when LL> Basic size & -ve when LL< Basic size.
So, Tolerance = IT
For Shaft: IT = es – ei For Hole: IT = ES - EI
Fundamental Deviation: (FD)
It is one of the two deviations (Either UD or LD) which is
conventionally choosen to define the position of tolerance
Zone in relation to the zero line.
It is one of the two deviations (Either UD or LD) which is
Nearest to the zero line for either hole or shaft.
When tolerance zone is above the zero line, LD is the FD.
When tolerance zone is bellow the zero line UD is the FD.
Maximum Metal Limit (MML): At this limit the part has
maximum possible amount of metal.
UL for Shaft and LL for Hole.
Least Metal Limit (LML): At this limit the part has minimum
possible amount of metal.
LL for Shaft and UL of Hole.
Basic Shaft (h)
It is the shaft whose upper deviation is Zero. UL= basic size.
Basic Hole (H)
It is the hole whose lower deviation is Zero. LL= basic size
Tolerance Zone: It is the zone bounded by two limits of size
of a part.
Tolerance grade (IT):
It is the degree of accuracy manufacture and is designated
by the letter IT followed by a number.
There are 18 grades of tolerances – IT01, IT0, IT1 to IT16
Larger the number, greater will be the tolerance. IT01 to IT4 - For production of gauges, measuring instruments
IT5 to IT 7 - For fits in precision engineering applications
IT8 to IT11 – For General Engineering
IT12 to IT14 – For Sheet metal working or press working
IT12 to IT14 – For Sheet metal working or press working
IT15 to IT16 – For processes like casting, general cutting work
Standard Tolerance Unit (i)
A unit, which is a function of Basic size and which is common
To the formula defining the different grades of tolerances.
It is denoted by letter “i” and expressed in Microns.
It serves as a basis for determining the standard tolerance (IT)
Of the system. (Micron)
where, D (mm) is the geometric mean of the lower and upper diameters of
a particular diameter step within which the chosen the diameter D lies.
Clearance:
This is the difference between the sizes of the Hole and shaft
before assembly when this difference is positive.
Maximum size of Hole-Minimum size of shaft=Max. clearance
Minimum size of Hole-Maximum size of shaft=Min. Clearance.
Size:
A number expressing the numerical value of a length in a
particular unit.
Allowance:
It is the prescribed difference between the dimension of two
mating parts (Hole and Shaft)
It is the intentional difference between lower limit of hole and
Higher limit of shaft.
Allowance= LLH-HLS
It may be +ve or –ve. +ve allowance = clearance
-ve allowance = Interference
Tolerance Allowance
• Permissible variation in dimension of a part.
• Tolerance= UL – LL • It is provided to the dimension
of a part. • It has Absolute value with out
sign.
• Prescribed difference between the dimension of two mating parts.
• Allowance = LLH - ULS • Provided on the dimension of
mating parts to obtain the desired type of fit.
• It may be +ve. or –ve.
“Go” limit and “NOGO” limit:
“GO” limit refers to UL of shaft and LL of Hole.
Thus it corresponds to MML.
“NOGO” limit refers to the LL of a shaft and UL of a hole.
Thus it corresponds to LML.
Fits:
It is the degree of tightness or looseness between two mating
Parts to perform a definite function when they are assembled
Together.
A fit may result either in a movable joint or a fixed joint.
Ex: Shaft in Bearing, Pulley on a Shaft.
Classification
Clearance fit
a) Slide Fit
b) Easy Slide fit
c) Running fit
d) Slack running fit
e) Loose running fit
Transition fit
a) Push Fit
b) Wringing fit
Interference fit
a) Force Fit
b) Tight fit
c) Shrink fit
Clearance fit:
In this type of fit Shaft is always smaller than the Hole i.e. UL of shaft is
smaller than LL of Hole.
Clearance fit exists when the shaft and the hole are at their MML.
The Tolerance zone of hole will be above the shaft tolerance.
Allowance is +ve.
Ex: Shaft can rotate or slide in a bearing with different DOF according to