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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
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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.
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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?
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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
parts, etc.)
…
Inspection specific to PROCESSES
Chip removal processes (turning, milling, drilling, etc.)
Chipless manufacturing (casting, molding, forging, etc.)
Non-traditional methods (EDM, ECM, ultrasonics, etc.)
…
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Inspection AFTER production
costly production steps already complete
high cost of rejection or rework
difficult to test for all possible defects
difficult to identify responsibility for defect
Inspection DURING production
defects found early, at each production step
reduced cost of rejection or rework
facilitates continuous process improvement
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Measurement of DIMENSIONS
Linear measurements (length, thickness, etc.)
Angular measurements (taper, angle, etc.)
Measurement of surface texture (roughness, waviness, etc.)
Measurement of geometric shape (roundness, flatness,
squareness, etc.)
Measurement of screw threads and gears
Inspection for DIMENSIONAL ACCURACY
post-process (traditional)
in-process (modern trend)
DIMENSIONAL TOLERANCES
permissible variation in dimensions
directly affects product quality and cost
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SOURCES OF ERRORS
an error is defined as real (untrue, wrong, false, no go) value at the output
of a measurement system minus ideal (true, good, right, go) value.
Error = Ŧ(MV – TV)
Classification of errors:
1.Absolute Error:- It is the algebraic difference between the result of
measurement and the value of comparison.
(a) True absolute error: algebraic difference between result of
measurement and conventional true value.
(b) Apparent absolute error: if a series of measurements are made, the
difference between one of the measurement and the arithmetic mean.
1.Relative Error:- It is the ratio of absolute error and the value of
comparision used for measurement.
Relative error = (Absolute error/True value)
TYPES OF ERRORS
Static Error
Reading Error
Environmental Error
Characteristic Error
Dynamic Error
Systematic Error
Random Error
Instrumental loading Error
1. Static Error: these result from the physical nature of the various
components of the measuring system as the system responds to a
fixed Measurand input.
Due to intrinsic imperfections in the hardware and apparatus
compared to the ideal instrument.
(a) Reading Error:
i. Parallax Error: Possibility of Error due to parallax (Read out). Use of
mirror behind the read out or pointer virtually eliminates such type of
error.
ii. Interpolation Error: It can be tackled by increasing Optical
resolution by using a magnifier or using digital read out devices.
(a) Environmental Error: This error is due to the effect of surrounding
temperature, pressure and humidity on measuring system. External
influences also include Magnetic or electric fields, nuclear radiation,
vibration or shock etc… these factors affects both measuring system
and measurand.
(b) Characteristic Error: The deviation of the output of the measuring
system under constant environmental conditions from the theoretical
predicted performance or from nominal performance specification.
2. Instrumental loading Error: This result from the change in measurand
itself when it is being measured. It is thus the difference between the
value of the measurand before and after the measurement system has
measured.
1. Dynamic Error: (Related with time)
This error caused by time variation in the measurand and results from
the inability of a measuring system to respond faithfully to a time
varying measurand. Usually dynamic response is limited by inertia,
damping, friction or other physical constraints in the sensing, read out
or display system.
Systematic Errors Come from the measuring instruments.
Something is wrong with the
instrument or its data handling system,
or instrument is wrongly used by the
Experimenter.
The errors in temperature
measurements because of poor
thermal contact between the
thermometer and the substance.
Errors in measurements of solar
radiation astrees or buildings shade
the radiometer.
Random Errors Caused by unknown and
unpredictable changes in the
experiment.
May occur in the measuring
instruments or in the
environmental conditions
(humidity, temperature, etc.)
The errors in voltage
measurements because of an
electronic noise in the
circuit of electrical instrument.
irregular changes in the heat
loss rate from a solar collector
due to the wind.
A thermometer that always
Reads 3ºcolder than the
actual temperature
A thermometer that gives
random values within 3º either
side of the actual temperature
Systematic Errors Reproducible between
measurements.
In principle, they can be
eliminated partially or
completely.(Controllable error)
Accuracy is often reduced by
systematic errors, which are
difficult to detect even for
experienced researchers.
We must define their size
To estimate what confidence
We have in our measured
value.
Random Errors Not reproducible, but fluctuate in
magnitude and sign between
measurements.
We can only know the probable
range over which a random error
lies.
Precision is limited by the
random errors. It may usually be
determined by repeating the
measurements.
They can be estimated so
that the measured value
can be adjusted to allow
for them.
Accuracy and Precision
Precision is defined as the repeatability of the measuring
instrument. It shows how close the measured values are to
each other.
The precision of a measurement is the size of the unit you
use to make a measurement. Ex: 12 s and 12 day
The number of decimal places in a measurement also affects
precision. 10,10.1, 10.12, 10.1237…..
Accuracy is how close a measured value is to the actual
(true) value. The accuracy of a measurement is the
difference between your measurement and the accepted
correct answer. The bigger the difference, the less accurate
your measurement.
Mistake of 5 cm in measurement of 100 cm or 1000cm…
Difference between Accuracy and Precision
Low Accuracy
High Precision
High Accuracy
Low Precision
High Accuracy
High Precision
apply a
systematic
adjustment
need to change the
equipment or
methodology used
If the instrument measures in "1"s then
any value between 6½ and 7½ is
measured as "7"
If the instrument measures in "2"s then
any value between 7 and 9 is
measured as "8"
Degree of Accuracy
Accuracy depends on the
instrument you are
measuring with. But as a
general rule: The degree of
accuracy is half a unit each
side of the unit of measure
Factors affecting Accuracy:
1. Standard: ambient influence, stability with time, elastic property,
Position of use……
2. Work piece: ambient influence, cleanliness, surface condition,
Elasticity, support arrangement, defining datum.
3. Instrument: hysteresis, backlash, friction, zero drift, error in
Amplification, calibration error etc…..
4. Personal: Improper training for handling instrument, skill, sense of
Precision and accuracy, attitude……
5. Environmental: temperature, vibration, lighting, pressure….
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
purpose of mating part.
a)Slide Fit: Tail stock spindle of Lathe
b)Easy Slide fit: Spindle of lathe & dividing head, Pistons &
Slide Valves, Spigots etc.
c)Running fit: Gear Box Bearings, Shaft Pulleys
d)Slack running fit: Arm Shaft of IC Engine, Shaft of CF
Pump
e)Loose running fit: Idle Pulley on their shaft (Quick Return
Mechanism)
Interference fit In this type of fit, LL of shaft is larger than UL of Hole.
Thus, the shaft and holes are attached permanently and used as a solid
Component.
Elastic strains are developed during the process of assembly.
Allowance is –ve. (Interference)
Ex: Bearing bush, Small end in connecting rod, Gear in intermediate
shafts in trucks.
a)Force Fit: Gears on the shaft
b)Tight fit: Stepped pulley on drive shaft of a conveyor,
Cylindrical Grinding M/C.
A)Shrink fit/
Heavy Force fit: Metallic rim on the wheels of a cart.
Transition fit:
It lies midway between the clearance and interference fit.
In this, tolerance zone of hole and shaft overlap completely or
in part.
UL of hole > LL of shaft but LL of hole < UL of shaft.
Ex: Spigot in Mating parts, Coupling rings etc.
a)Push Fit: Change gears, Slip bushings
b)Wringing fit: Parts which can be replaced with out difficulty
during minor repairs.
Hole Basis System:
The size of hole is kept constant and shaft sizes are varied to
Give various types of fits.
In this, lower deviation of the hole is Zero i.e. LL = Basic size.
Hole basis system is commonly used as it is convenient to
make a hole of correct size due to availability of standard drills,
Reamers, with less cost.
Shaft Basis System:
The size of the shaft is kept constant and sizes of hole are
varied to get the required type of fit.
In this, Upper Deviation of the shaft is zero i.e. UL= Basic size.
This system is not suitable for mass production because it is
Time consuming and costly to make a shaft of correct size.
Recommendation for limits and fits for Engineering:
For universal Interchangeability it is essential to follow a
uniform standard Through out the world.
Indian standards (IS) are in line with ISO recommendations.
It consists of 25 Holes designated by capital letter
A, B, C, D, E, F, G, H, JS, J, K, M,N, P, R, S, T, U, V, X, Y, Z,
ZA, ZB, ZC
It consists of 25 shafts designated by small letter
a, b, c, d, e, f, g, h, js, j, k, m, n, p, r, s, t, u, v, x, y, z, za, zb, zc
Each of holes and shafts has a choice of 18 Grades of
Tolerances Designated as:
IT01, IT0, IT1, IT2, IT3, ……… IT15, IT16. IT01 – 0.3 + 0.008D IT0 – 0.5 + 0.012 D IT1 – 0.8 + 0.020D
IT2 – 2.7i; IT3 - 3.7i; IT4 – 5i; IT5 – 7i;
IT6 – 10 i; IT7 – 16i; IT8 – 25i; IT9 – 40i;
IT10 – 64i; IT11 – 100i; IT12 –160i;
IT13 – 250i; IT14 – 400i; IT15 – 640i;
IT16 – 1000i.
The value of IT for Hole and shaft
Using the value of “i” as
Where D=Geometric Mean Diameter of the lower and upper diameters of
A Particular diameter step in which diameter lies in mm.
The seven Tolerance grades IT01, IT0, IT1, IT2, IT3, IT4, IT5 covers diameter
Sizes up to 500 mm and rest eleven grades i.e. IT06 – IT16 covers diameter
Sizes up to 3150 mm.
Fundamental Deviation are obtained from Empirical Formula (Table/Given in
Question) for shaft and hole respectively up to 500 mm.
FD for Hole A – H are same as that of Shaft a – h but opposite in direction.
They provide clearance fit.
FD for hole “H” and shaft “h” are Zero.
Now IT = ES – EI (Hole)
IT = es – ei (Shaft)
Basic size followed by symbol Φ30 H7/h8
Hole with tolerance Grade IT7 = 16i
Shaft with tolerance grade IT8 = 25i.
If Hole basis system FD for hole = 0
FD for shaft can be found out from the table or given in the question.
Example #1
Evaluate limits and fits for a pair of – Diameter 6 H7/g6. The
size 6 mm lies in the diametral step of 3-6. Standard
tolerance for hole H7 is 16i and shaft g6 is 10i. Fundamental
deviation for g shaft is µ.
Calculate the limits of sizes for φ 25 S8/h7 and identify the fit.
The size 25 mm lies in the diametral step of 24-30. The
fundamental deviation S8 hole – IT7 + 0.4D. For grade 8 and
7 the standard tolerance is 25i and 16i respectively.
Calculate the limits of sizes for φ 60 H8/m6 and identify the
fit. The size 60 mm lies in the diametral step of 50-80 mm.
The fundamental deviation for m is IT7 – IT6. For grade 6
and 7 the multipliers are 10 and 16 respectively.
Calculate the limits of sizes for φ 32 S7/h6 and identify the
fit. The size 32 mm lies in the diametral step of 30-50 mm.
The fundamental deviation S is IT7+0.4D. For grade 6 and 7
the multipliers are 10 and 16 respectively.
GAUGES Gauges are scale less inspection tools at rigid design which are used to
check the dimensions of manufactured parts. Measurement by gauges is
Easy and rapid. So they are suitable in mass production. Instead of
measuring actual dimension of each part which is time consuming and
Costly, the conformance of part with tolerance specification can be
checked by gauges.
Measuring Instrument Gauges
1. They carry calibrated scales. 1. With out scales.
They are general purpose instrument.
They are made for a particular component.
Measures actual dimension of part.
Checks whether the dimensions of parts are with in the specified tolerance limit or not.
Time consuming and not suitable for mass production.
Easy and rapid, suitable for mass production.
Skilled labour to handle. No need of skilled labour.
Increased cost. Reduced cost.
Adjustment is required. No adjustment
PLAIN GAUGES Plain gauges are used to check plain, i.e. unthreaded holes and shafts.
Classification:
1.According to Type (a)Standard Gauge: If a gauge is made as an exact copy of the mating part
Of component to be checked, it is called standard gauge.
A standard gauge can’t be used to check interference fit.
It has limited application.
(a)Limit Gauge: Two gauges are used to check each dimension of the
Part i.e. upper and lower limit. These are “GO” and “NO-GO” gauges.
GO gauges check MML and NO GO gauges check LML.
These are widely used industries.
A part is considered to be good if the GO gauge pass through the work and
the NO GO gauge fails to pass under the action of its own weight. This
Confirms the actual dimension of part with in the specified tolerances.
If both the gauges fail, it indicates that hole is under size and shaft is
Oversize.
1. According to Purpose:
a) Workshop gauge
b) Inspection gauge
c) Reference or master gauge
d) Purchase inspection gauge
2. According to the form of the tested surface:
a) Plug gauges for checking holes
b) Snap or Ring or Gap gauges for checking the shaft
3. According to their design:
a) Single limit or double limit gauges
b) Single ended or double ended gauges
c) Fixed and adjustable gauges
Difference between work shop gauge and Inspection
gauge?
Work shop gauge:
1.Used by the operator during manufacture of a part in shop.
2.Usually have limits with in those of components being
inspected.
3.The tolerance is arranged to fall inside the work tolerance.
4.Some of the components which are in work tolerance limit
may be rejected under these gauges.
Inspection Gauge:
1.Used by inspector for the final inspection.
2.These gauges are made slightly larger tolerance than the
work shop gauges.
3.The tolerance on inspection gauges is arranged to fall
outside the work tolerance.
4.Some rejected parts may be accepted. IT of Inspection Gauge>Work tolerance>W/S Gauge Tolerance.
Gauge Tolerance/ Gauge Maker’s Tolerance/
Manufacturing Tolerance: In actual practice Gauges can’t be manufactured to the exact size (Due to
imperfection in the process). Some allowance must be provided to the
gauge maker known as gauge tolerance.
Gauge tolerance should be kept as small as possible but this will increase
the cost of manufacturing the gauges.
Gauge tolerance of limit gauges (GT)= 1/10th of Work Tolerance (WT)
Or Work shop Gauges (GO, NOGO Gauges) (10%)
Gauge tolerance for Inspection gauges (GT) = 5% of WT
(GO, NOGO Gauges)
Gauge tolerance for Master/Reference gauges (GT) = 10% of WT
Wear Allowance:
The measuring surfaces of GO gauges rub constantly against
the surfaces of work piece during checking. This results in
wearing of measuring surfaces of gauges.
The size of GO plug gauges is reduced and that of Ring or
Snap Gauges is increased.
Wear allowance is provided to the gauges in the direction
Opposite to that of the wear.
WA is not provided for NOGO gauges as they are not
Subjected to much wear compared to GO gauges.
GO plug gauges => WA is added.
GO snap or ring gauges => WA is subtracted.
WA = 5% of WT or 10% of GT.
WA may be avoided in clearance fit.
WA is applied to W/S GO gauges not to Inspection GO
gauges.
Providing WA, the GO gauge will reject more number of
acceptable parts as compared to gauge with only GT.
WA is provided when WT>0.09 mm.
Three basic size of Gauges:
1.Work shop gauge:
GT is with in WT, some accepted parts are rejected,
WA is given to W/S GO Gauge.
1.Inspection gauge:
GT is out side the WT, some rejected parts are Accepted,
As GO gauge for inspection is fairly slack, no WA is required.
1.General gauge:
To over come the draw back of w/s and inspection gauge,
general gauge has been recommended.
Tolerance zone of GO gauge placed inside Work tolerance.
Tolerance zone of NOGO gauge placed outside work
tolerance.
GO gauge of General gauge is taken same as W/S gauge.
NOGO gauge of General gauge is taken same as
Inspection gauge.
Taylor’s Principle of Gauge Design: It states that
1. “Go gauges should be designed to check the Maximum Metal Limit
(MML) while the NO GO gauge should be designed to check the Least
Metal Limit (LML).”
GO plug gauge should correspond to LL of Hole.
NOGO plug gauge should correspond to UL of Hole.
GO snap gauge should correspond to UL of shaft.
NOGO snap gauge should correspond to LL of shaft.
The difference between the GO and NOGO plug gauge as well as the
difference in size between GO and NOGO snap Gauge is approximately
equal to the work tolerance.
2. “GO gauges should check all the related dimensions (Roundness, size,
location, straightness etc).
NOGO gauges should check only one element of the dimension at a
time.”
GO plug gauge should have a full circular section and full
Length of the hole it has to check. It ensures that any lack
Of straightness or roundness of the hole will prevent the entry
Of full length GO gauge.
The length of GO plug gauge should not be less than 1.5
times the diameter of the hole to be checked.
Calculate the dimension of Plug and ring gauges to control
the production of a part 50H7d8. Given: 50 mm lies in the
step 30-50. For d shaft FD= - 16 D 0.44µ. IT6=10i and above it
tolerance magnitude is multiplied by 10 at each fifth step.
Determine actual dimension to be provided for shaft and
hole of 90 mm size for H8/e9 type of fit. Size 90 falls on
Diameter steps of 80 and 100. FD for “e” type shaft is
= - 11 D 0.41µ. Also design “GO” and “NOGO” gauges.
IT8=25i and IT9=40i.
Calculate the limits of size for inspection gauges conforming
to Taylor’s principle to check the rectangular hole . The limits
of size for a 50 mm H8 hole are low limit 50.000 mm and
high limit 50.039 mm. The limits of size for a 75 mm H8 hole
are low limit 75 mm and high limit 75.046 mm.
50 mm diameter step lies 30 – 50.
75 mm diameter step lies 50 – 80.
State Taylor’s Principle of Gauge Design of Limit gauges.
Design the “Work shop” “Inspection” and “General” type of
GO and NOGO gauges for checking the assembly
Φ 30 (mm) H7/f8. Fundamental deviation for “f” shaft is =
-5.5 D 0.41µ. Diameter step for Φ 30 is 18 – 30 mm.
Fundamental tolerance for IT7 and IT8 are 16i and 25i
respectively. Also determine
I. Type of fit
II. Allowance for the above fit
III. Other shafts giving the same type and same
degree of fit
IV. Equivalent fit in shaft based system
Part A Tolerance of
Part
B
Tolerance of B
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