Mechanical Measurement

and Metrology

RESOURCE PERSON: KHURSHEED

AHMAD
 Interchangeability
2

Concept/Defn:
Interchangeability refers to assembling a number of
unit components taken at random from stock so as to
buildup a complete assembly without fitting or
adjustment.
When one component assembles properly (and which
satisfies the functionality aspect of the assembly) with
any mating component, both chosen at random, then
it is known as interchangeability.
The parts manufactured under similar conditions by
any company or industry at any corner of the world,
can be interchangeable.
 Interchangeability
3

Interchangeable Part:

Interchangeable part is one which can be

substituted for a similar part manufactured
from the same drawing.
 Advantages
4

1. It makes possible the standardization of products

and methods of manufacturing.
2. The assembly of mating parts is easier. Since any
component picked up from its lot will assemble with
any other mating part from another lot without
additional fitting and machining.
3. It saves time.
4. It enhances the production rate.
 Advantages
5

5. It brings down the assembling cost drastically.

6. Repairing of existing machines or products is
simplified because component parts can be easily
replaced.
7. Replacement of worn out parts is easy.
8. Without interchangeability mass production is not
possible.
Examples
6
Examples
7
 Types
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1. Universal Interchangeability: It means that similar

parts, derived from any source whatsoever, are
interchangeable.
2. Local interchangeability: In it, parts made at the specific
source (in particular factory, for instance) are
interchangeable, but such parts are not necessarily
interchangeable with similar parts manufactured else where.

Interchangeability is possible only when certain standards

are strictly followed. And all standards used by various
manufacturing units
should be traceable to a single source, i.e. international
standards.
 Interchangeability (Contd.)
9

Interchangeability of component parts depend on

two factors:
1. It is necessary for the relevant mating parts to be
designed incorporating specified limits of size.

2. Parts must be manufactured within the specified

limits, i.e. allowance; which must be controlled
rigidly.
 Interchangeability (Contd.)
10

Example:
A modern car, consists of many hundreds of
separate components each of which is manufactured
in large numbers. For complete interchangeability it
should be possible simple to collect at random the
constituent parts, then to assemble the whole
without the use of any cutting tools and for the
assembly to function satisfactorily.
 Interchangeability (Contd.)
11

Essential characteristics when a Hole mates

with a Shaft with a clearance fit:
1. Allowance (minimum clearance) is the
functional requirement.
2. Tolerance on (a) the Hole and (b) the Shaft is
essential for manufacturing the feature
concerned.
 Interchangeability (Contd.)
12

3. The fit between the mating features is

determined by the limits of (a) the two
maximum metal limits and (b) the two minimum
metal limits, which are known as the GO and
NOT GO limits respectively. A correct fit is
obtained only if the mating features are made
correctly within the prescribed extreme limit of
size.
4. The required fit is not obtained if the mating
features are made outside the prescribed
extreme limits of size.
 Interchangeability (Contd.)
13

5. Further, in practice, any feature made outside

the prescribed limits may be unable to
function with the mating feature, even if the
later is within the correct limits.
6. The conditions giving maximum metal limits to
both features result in the minimum clearance.
7. The conditions giving minimum metal limits to
both features result in the maximum clearance.
 Method for achieving precision and
accuracy
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The terms precision and accuracy are associated with measurement.

Precision: is defined as
 The repeatability of measuring process or

 The quality or state of being particular or

 Degree to which an instrument gives repeated measurement of the

same standard.
Accuracy:
 Is the agreement of the result of a measurement with the true value of
the measured quantity
 The closeness with which the reading approaches an acceptable
standard value or the true value.
 It is numerically equal to the degree of error in the final result.
 In any experiment, accuracy of the measured quantity is influenced
by the limits of the intrinsic error, limits of variation in the indication,
accuracy of the observer and the environment .
 Method for achieving precision and
accuracy
15

It is the precision which is of immense importance

in most measurements. The chief concern is with
comparing the dimension of measurement relative
to each other, being assumed that the scale used
for measurement is standard and accepted one.
Method for achieving precision and
accuracy
16

Higher accuracy can be achieved by incorporating

the magnifying devices in the instrument and
these magnifying devices carry with them their
own inaccuracies. By taking many precautionary
measures one can make these errors extremely
small.
 Method for achieving precision and
accuracy
17

An accurate measurement instrument should fulfill the

following requirements:
1. It should possess the requisite and constant accuracy.
2. As far as possible, the errors should be capable of
elimination by adjustment contained within the
instrument itself.
3. Every important source of inaccuracy should be
known.
4. When an error can not be eliminated, it should be
made as small as possible.
5. When an error can not be eliminated it should be
capable of measurement by the instrument itself and
the instrument calibrated.
 Error
18

Error:
 Difference between the measured value and the
true value of a quantity is called static (or absolute)
error or simply error of measurement.
 The error may be positive or negative.
 If error is on negative side then the corresponding
reading is on lower side.
 Where as if the error is on positive side, the instrument
reading is on higher side.
 Sources of error
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Error falls into two categories:

1. Controllable Errors 2. Random Errors

1. Controllable Errors: Such errors are called as

system errors and are controllable both in their
magnitude and stress. These can be determined
and reduced if attempts are made to analyze
them.
 Sources of error (Contd.)
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Controllable Errors can be due to:

1.1. Calibration errors: Actual length of
standard such as slip gauges and engraved scales
will vary from the nominal value by small amount.
Some times the instrument inertia and hysteresis
effects do not let the instrument translate with
complete fidelity.
 Sources of error (Contd.)
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1.2. Ambient Conditions: The variations in the

ambient conditions from internationally agreed
standard value of 20oC, barometric pressure
760mm of mercury, or 10mm of mercury vapor
pressure, can give rise to errors in the measured
size of component. Temperature is by far the
most significant of these ambient conditions and
due correction is needed to obtain results free
from error.
 Sources of error (Contd.)
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1.3. Stylus Pressure: Errors induced due to stylus

pressure are also appreciable. When ever any
component is measured under definite stylus
pressure the deformation of the work piece surface
and deflection of the work piece shape will occur.
1.4. Avoidable Errors: These errors include the
errors due to parallax and the effect of misalignment
of the work piece centres. Instrument location errors
such as placing a thermometer in sunlight when
attempting to measure air temperature also belong
to this category of errors.
 Sources of error (Contd.)
23

2. Random Errors:
The random errors occur randomly, the specific
causes of such error can not be determined.
The likely sources of this type of error are:
i. Small variation in the position of setting
standards and work piece.
ii. Slight displacement of lever joints(or any other
joint) in the measuring instrument.
 Sources of error (Contd.)
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2. Random Errors: (Cont..)

iii. Transition fluctuation in the friction in
measuring instrument.
iv. Operator errors in the reading scale and pointer
type displays or in reading engraved scales
positions.
 Sources of error (Contd.)
25

Conclusion:
Controllable errors are those which are repeated
consistently with the repetition of the experiment,
where as;
Random errors are those which are accidental and
whose magnitude and stress cannot be predicted
from the knowledge of measuring system and
conditions of measurement.
Classification of measuring instruments
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A measuring instrument is any device that may be

used to obtain dimensional or angular
measurement.
The function of measuring instrument is to sense or
detect a parameter encountered in a scientific
process or research, such as temperature, pressure,
resistance, current, voltage, flow and motion etc.
The measuring instrument should have the
capability of detecting any changes that occur in the
measured parameter.
 Selection of instruments
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The selection of measuring instrument depend

upon its technical specifications e.g.:
 Accuracy, Precison,
 Scale value, scale division, graduation range
 Linearity, threshold value, Sensitivity
 Measuring force
 Working capacity
 Error of measuring surface
 Error of the straightness of the guide ways, etc