Unit I

Definition of Metrology:

Metrology is the name given to the science of pure measurements. For engineering purposes, it is

restricted to measurements of length and angle and quantities which are expressed in linear or angular terms.

Measurement:

Measurement is a process of comparing quantitatively an unknown magnitude with a predefined

standard.

Objectives of metrology

The basic objective of metrology is to determine whether a component has been manufactured to the

required specification. The mass production of modern ultra-precise apparatus was possible with the advances

in metrology.

The basic objectives of metrology are as follows:

To provide the required accuracy at minimum cost.

To reduce the cost of inspection by effective and efficient utilization of available facilities.

To reduce cost of rejection and rework by applying statistical quality control techniques.

To determine process capabilities.

To standardize measuring methods by proper inspection methods at he development stage itself.

To maintain the accuracies of measurement through periodical calibration of the measuring instrument.

To prepare designs for gauges and special inspection fixtures.

To asses the measuring instrument capabilities and ensure that they are adequate for their specific

measurement.

Definition of standards

A standard is defined as “something that is set up and established by an authority as a rule for the

measure of quality, weight, extent, valve or quality”.

For example:

A meter is a standard established by an international organization for the measure of length.

Role of standards

The role of standard is to achieve, uniform, consistent and repeatable measurements and to support the

systems which make such measurements possible throughout the world. 1

Standards of length

The accurate measurement must be made by comparison with a standard of known dimension and such

a standard is called “primary standard”.

Rapid advances were made in engineering due to the improved materials available and the development

of more accurate measuring techniques.

The first accurate standard was made in England and was known as imperial standard yard. This was

followed by international prototype meter made in France.

Since these two standards of length were made of metal alloys they are called material length standards.

International Prototype Meter

International prototype meter is defined as the straight line distance, at 0o

C between the engraved lines

of pure platinum-iridium alloy (90 % platinum and 10% iridium) of 1020 mm total length and having a

tresca cross section as shown in figure.

The graduations are on the upper surface of the web which coincides with the neutral axis of the

section.

The sectional shape gives greater rigidity for the amount of metal involved and is therefore economic in

the use of an expensive metal.

In this standard, platinum- iridium alloy was used because it is non-ox disable and retain good polished

surface required for engraving good quality lines.

Imperial Standard Yard

An imperial standard yard shown in figure is a bronze (82% cu, 13% tin, 5% zinc) bar of 1 inch square

section and 38 inches long.

A round recess, one inch away from the two ends is cut at both ends up to the Centre or neutral plane of

the bar. Further a small round recess of 1/10 inch in diameter is made below the Centre.

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Two gold plugs of 1/10 inch in diameter having engraving are inserted into these holes so that the lines

(engravings) are in neutral plane.

Yard is defined as the distance between the two central transverse lines of the gold plug at 62oF.

The purpose of keeping the gold plugs in line with the neutral axis is to ensure that the neutral axis

remains unaffected due to bending and to protect the gold plugs from accidental damage.

Disadvantages of Material Length Standards

Material length standards varies in length over the years owing to molecular changes in the

alloy from which they were made thus seriously affecting the fine measurements.

The exact replicas of material length standards were not available for use somewhere else.

If these standards are accidentally damaged or destroyed then exact copies could not be made.

Conversion factor has to be used for changing over to metric system.

Light Wave (Optical) Length Standard

Because of the problems of variation in length of material length standards, the possibility of using

light as a basic unit to define primary standard has been considered.

In order to define a standard length in this way it was necessary to find a suitable light source from

which a given radiation could be readily selected.

The wavelength of the selected radiation was measured and used as the basic unit of length.

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Since, wavelength standard is not a physical one, it need not be preserved. Further, it is easily

reproducible and the error of reproduction is in the order of one part in 100 million.

The eleventh general conference on weights and measures held in Paris in 1960 defined the meter as

equal to 1650763.73 wavelengths of the orange radiation of krypton-86 isotope in vacuum maintained

at 68 K.

Definition: The meter is defined as 1650763.73 wavelengths of the orange radiation in vacuum of the krypton-

86 isotope.

The yard is defined as 1509458.35 wavelength of the orange radiation in vacuum of the krypton-86

isotope. The yard is also defined as 0.9144 meter.

Note: the substance krypton-86 was used because it produces sharply defined interference lines and its

wavelength was the most uniform known at that time.

Advantages of Wave (Optical) Length Standard

Length does not change.

It can be reproduced easily if destroyed.

It can be used for making comparative measurements with much higher accuracy than with the

material standards.

Wavelength standards can be reproduced consistently at any time and at any place.

Subdivision of standards

The Imperial Standard Yard and International Prototype Meter, defined previously are master standards

and cannot be used for ordinary purposes. Thus, depending upon the importance of accuracy required,

the standards are sub-divided into four grades.

Primary standards:

For precise definition of a unit i.e., imperial standard yard international prototype meter, it is essential

that there should be one, and under most careful conditions. This has no direct application to a measuring

problem encountered in engineering. They are used only at rare intervals of 10 or 20 years solely for

comparison with secondary standards.

Secondary standards:

These are close copies of primary standards with respect to design, material and length. These are

made, as far as possible exactly comparison with primary standards after long intervals. They are kept at

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number of places under great supervision and are used for comparison with tertiary standards whenever

desired. This also acts as safeguard against the loss or destruction of primary standard.

Tertiary standards:

The primary or secondary standard exists as the ultimate controls for reference at rare intervals.

Tertiary standards are reference standards employed by National Physical Laboratory standards are the first

standards to be used for reference in laboratories and workshops. They are also made as true copy of secondary

standards and are kept as reference for comparison with working standards.

Working standards

These standards are similar in design to primary, secondary and tertiary standards, but being less in cost

and are made of low grade materials. They are used for general applications in metrology laboratories.

Sometimes standards are also classified as

Reference standards

Calibration standards

Inspection standards

Working standards.

Line standards

When the length being measured is expressed as the distance between two lines then it is called “line

standard”.

Line standards do not provide high accuracy as that of end standards.

Ex: measuring scales, Imperial standard yard, International prototype meter, etc.

Characteristics of line standards

Scales can be accurately engraved but it is difficult to take the full advantage of this accuracy.

A scale is quick and easy to use over a range of measurement.

The scale markings are not subjected to wear although significant wear on leading ends results

in “under sizing”.

A scale does not possess a “built in” datum which would allow easy scale alignment with the

axis of measurement; this again results in under sizing.

Scales are subjected to parallax effect, which is a source of both positive and negative reading

error.

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Scales are not convenient for close tolerance length measurements expect in conjunction with

microscopes.

End standards

When the length being measured is expressed as the distance between two parallel end faces then it is

called “End standards”.

End standards can be made to a very high degree of accuracy.

They consist of standard blocks or bars used to build up the required length.

Characteristics of End standards

End standards are highly accurate and are well suited to measurements of close tolerances.

They are time consuming in use and prove only one dimension at a time.

Dimensional tolerance as small as 0.0005 mm can be obtained.

End standards are subjected to wear on their measuring faces.

Transfer from Line Standard to End Standard.

It has already been clarified that the line standard of length is a highly inconvenient form for general

measurement applications.

In order to determine the position of the defining lines in the standard, a special microscope has to be

employed.

The line standard was defined first, and end standards being of real importance and more utility; the

end standards had to be produced of the highest accuracy in relation to the line standard.

In end standards, the distance is defined between the working faces which are flat and mutually

parallel.

In order to transfer the line standard correctly to the ends of a bar, the use of an instrument called Line-

standard comparator is used.

It consists of two microscopes mounted about a yard apart over a table. A gauge, about 35 and halft

inches in length is produced with end faces flat and mutually parallel.

Two 1/2 inch blocks are taken and wrung at the ends of this gauge.

The two 1/2| inch blocks are engraved with a fine line on one surface approximately in the centre of the

two end faces.

Thus the distance between the centre lines is approximately 36 inches after wringing these 1/2 inch

blocks to the main35 and half inches gauge.

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The standard and the above blocks are mounted on the table. The microscopes have accurate

micrometer screw controlled eyepieces.

In eyepiece, there are cross wires to focus on the lines of the standard.

The table is capable of being traversed across so that either block may be brought under the

microscope.

The apparatus compares the positions of lines on the standard with lines on the gauge, and with

micrometer eyepieces any small longitudinal variations between them can be determined.

Let the actual length 35 and half inches gauge is L. The distance between two lines on line standard is

36″.

Let us ignore the effect of the wringing film between the surfaces in contact as it is always present in

the use of end bars and gauges.

The other possible errors are the misplacing of the line at the mid-position of the end faces of 1/2 inch

blocks and possible error in the length of 35 and half inches gauge.

The two blocks at end are arranged in four ways. (Fig. 3.5 shows one position and other three positions

are self-explanatory) and difference of readings between lines on standard and the line on gauges are

noted every time.

Let the difference be d1, d2, d3 and d4 respectively. Then for the successive positions of the| inch

blocks, we have are calibrated in this way, these are used as master gauges from which further sub-

divisions are obtained e.g. two 18″ gauges may be prepared and when wrung in combination, their

length is compared with that of 36″ end standard master gauge.

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( ) ∑

Differentiate between Line and End Standards

Slip gauges:

Slip gauges are used as standards of measurement in practically every precision engineering works.

These are invented by C.E. Johansen of Swedan and are also called Johansen gauges.

These gauges are otherwise called as Gauge blocks or Block gauges and are universally accepted as end

standards of length in industry.

Slip gauges are rectangular blocks of high grade steel (or tungsten carbide) with less co-efficient of

thermal expansion.

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These blocks are highly hardened (more than 800 HV) through out to ensure maximum resistance to

wear and are then stabilized by heating and cooling successively in stages so that the hardening stresses

are removed.

After hardening, they are subjected to lapping to a high degree of finish, flatness and accuracy.

The cross sections of these gauges are 9 x 30 mm for sizes up to 10 mm and 9 x 35 mm for larger sizes.

The dimension (height) is marked on one of the measuring faces of gauge blocks.

Wringing of Slip gauges:

The slip gauges are wrung together by hand through a combined sliding and twisting motion.

The air gap between the gauge faces is expelled out and the adhesion is caused partly by molecular

attraction and partly by atmospheric pressure.

The gap between the two wrung slip gauges is only of the order of 0.00635 micro meters which is

negligible.

Selection of Slip gauges for required dimension: Always start with the last decimal place and deduct this from the required dimension.

Select the next smallest figure in the same way, find the remainder and continue this until the required

dimension is completed. Minimum number of slip gauges should be selected to build up the given

dimension.

Indian standard on slip gauges

Slip gauges are graded according to their accuracy as Grade 0, Grade I, And Grade II. Grade II is

intended for use in workshops during the actual production of components, tool and gauges.

Grade I is of higher accuracy and used in inspection departments. Grade 0 is used in laboratories and

standard room which serves as standard for periodically checking the accuracy of Grade I and Grade II

gauges.

Generally, two sets of slip gauges are used namely: (i) Normal set and (ii) special set are used. A set M

112 consists of the following gauges:

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Range (mm)

Steps(mm)

No.of pieces

1.001 to 1.009

1.01 to 1.49

0.5 to 24.5

25, 50, 75, 100

1.0005

0.001

0.01

0.50

25

-

9

49

49

4

1

Total

112

Interchangeability

An interchangeable part is one which can be substituted for similar part manufactured to the same

drawing.

In earlier times production used to be confined to small number of units and the same operator could

adjust the mating components to obtain desired fit.

With time the concept of manufacturing techniques kept on changing and today the same operator is no

more responsible for manufacture and assembly too.

With economic oriented approach, mass production techniques were inevitable, that led to breaking up

of a complete process into several smaller activities and this led to specialization.

As a result various mating components will come from several shops; even a small component would

undergo production on several machines.

Under such conditions it becomes absolutely essential to have strict control over the dimensions of

portions which have to match with other parts.

Any one component selected at random should assemble correctly with any other mating component

that too selected at random.

When a system of this kind is ensured it is known as interchangeable system. Interchange ability

ensures increased output with reduced production cost.

In interchangeable system, every operator being concerned only with a limited portion of overall work,

he can easily specialize himself in that work and give best results leading to superior quality.

He need not waste his skill in fitting the components by hit and trial and assembly time is reduced

considerably.

In the case of big assemblies, several units to manufacture individual parts can be located in different

parts of country depending on availability of specialized labour, raw material, power, water and other

facilities and final assembly of all individual components manufactured in several units can be done at

one place.

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The replacement of worn out or defective parts and repairs is rendered very easy and the cost of

maintenance is very much reduced and shut down time also reduced to minimum.

Interchange ability is possible only when certain standards are strictly followed.

Universal interchange ability (i.e. parts drawn from any two altogether different manufacturing sources

for mating purposes) is desirable and for this it is essential that common standards be followed by all,

and all standards used by various manufacturing units should be traceable to a single source, i.e.

international standards.

When all parts to be assembled are made in the same manufacturing unit, local standards may be

followed (condition being known as local interchange ability) but for reasons of obtaining spares from

any other source it is again desirable that these local standards be also traceable to international

standards.

The required fit in an assembly can be obtained in two ways, namely

(i) Universal or full interchange ability, and

(ii) Selective assembly.

Full interchange ability means that any component will mat with any other mating component without

classifying manufactured components in subgroup o without carrying out any minor alterations for

mating purposes.

This type of interchange ability is not must for interchangeable production and many times not feasible

also as it requires machines capable of maintaining high process capability and very high accuracy, and

very close supervision on production from time to time.

For full interchangeable assembly it is essential that only such machines be selected for manufacturing

whose process capability is equal to or less than the manufacturing tolerance allowed for that part.

Only then every component will be within desired tolerance and capable of mating with any other

mating component.

Definition of Limits

Maximum and minimum permissible sizes within which the actual size of component lies are called

limits.

Limits of size

In deciding the limit for particular dimension it is necessary to consider the following:

Functional requirement – the intended function that a component should perform

Interchangeability – replacement of the component in case of failure without difficulty.

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Economy in production time and cost.

Tolerance

It is impossible to make anything to an exact size; therefore, it is essential to allow a definite tolerance

or permissible variation on every specified dimension.

Consider the dimensioning shown in figure. When making the part we try to achieve a diameter of

40mm. This is called the basic or nominal diameter.

The shaft will be satisfactory if its diameter lies between 40.00 + 0.05 = 40.05 mm and 40.00 – 0.05 =

39.95 mm. The dimension 40.05 mm is called upper limit and the dimension 39.95 mm is called the lower

limit. The difference between the upper and lower limits is called tolerance.

Definition of Fit

Fit is the general term used to signify the range of tightness or looseness that may result from the

application of a specific combination of allowances and tolerances in mating parts. There are three types of fits

between parts:

Types of fit and their designation

Clearance Fit:

An internal member fits in an external member (as a shaft in a hole) and always leaves a space or

clearance between the parts.

Minimum air space is 0.002”. This is the allowance and is always positive in a clearance fit.

Interference Fit:

The internal member is larger than the external member such that there is always an actual interference

of material. The smallest shaft is 1.2513” and the largest hole is 1.2506”, so that there is an actual interference

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of metal amounting to at least 0.0007”. Under maximum material conditions the interference would be

0.0019”. This interference is the allowance, and in an interference fit it is always negative.

Transition Fit:

May result in either a clearance or interference condition. In the figure below, the smallest shaft

1.2503” will fit in the largest hole 1.2506”, with 0.003” to spare. But the largest shaft, 1.2509” will have to be

forced into the smallest hole, 1.2500” with an interference of metal of 0.009”.

The parts must always be forced together.

Tolerance

Tolerance is the total amount that a specific dimension is permitted to vary;

It is the difference between the maximum and the minimum limits for the dimension.

For Example a dimension given as 1.625 ± .002 means that the manufactured part may be

1.627” or 1.623”, or anywhere between these limit dimensions.

The Tolerance is 0.001” for the Hole as well as for the Shaft

Limit Gauges

A limit gauge is not a measuring gauge. Just they are used as inspecting gauges.

This gives the information about the products which may be either within the prescribed limit or not.

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Using limit gauges report, the control chart of P and C chart are drawn .

This procedure is mostly performed by QC department of each and every industry.

Limit gauges are mainly used for cylindrical holes of identical components with large number in mass

production.

Purpose of Using Limit Gauges

Components are manufactured as per the specified tolerance limit.

The dimension of each component should be within the upper and lower limit.

If the dimensions are not within the limit, the component rejected.

If we use any measuring instrument to check these dimensions, it will consume more time.

Still we are not interested in knowing the amount error in dimensions.

It is just whether the size of component is within the prescribed limits or not.

For this purpose we can make use of gauges known as limit gauges.

The common types are

Plug gauge

Ring gauge

Snap gauge

Plug Gauges

The end are hardened and accurately finished by grinding.

One end is GO end and other end is NOGO end.

In plug gauge, the GO end will be equal to the lower limit size of the hole and the NOGO end will be

equal to the upper limit size of the hole.

If the size of the hole is within the limits, the GO should go inside the hole and NOGO end should not

go.

If the GO end does not go, the hole is under size and also if NOGO end goes, the hole is over size.

Hence the components are rejected in both cases.

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Taper Plug Gauges

Taper plug gauges are used to check taper holes.

It has two check line.

One is a GO line and another is a NOGO line.

During checking of work, NOGO line remains outside the hole and GO line remains inside the hole.

They are various types taper plug gauges are available

1. Taper plug gauge - plain

2. Taper plug gauge-tanged

3. Taper ring gauge – plain

4. Taper ring gauge-tanged

RING GAUGES

Ring gauges are mainly used for checking the diameter of shafts having a central hole.

The hole is accurately finished by grinding and hardening process.

The periphery of the ring is knurled to give more grips while handling the gauges.

Two ring gauges are made separately to check the shaft such as

GO ring gauge and

NOGO ring gauge

GO ring Gauge NOGO Ring Gauge

The hole of GO ring gauge is made to upper limit size of the shaft.

NOGO for the lower limit size of the shaft.

To identify the NOGO ring gauges easily, a red mark or a small groove cut on its periphery.

Snap Gauges

Snap gauges are used for checking external dimensions.

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They are also called gap gauges

The GO anvil is made to lower limit and NOGO anvil made to upper limit of the shaft.

The different types of snap gauges are:

Double ended snap gauge

Progressive snap gauge

Adjustable snap gauge

SLIP GAUGES

Slip gauges are used as measuring blocks.

It is also called gauge blocks.

They are made of hardened alloy steel rectangular cross-section.

The surface of slip gauges are made to a high degree of accuracy.

The distance between the two opposite faces indicates the size of the gauges.

They are used in comparators and sine bars.

They are mainly used for testing and calibrating instruments in metrology.

Different sets of slip gauges are manufactured 32 pieces, 45 pieces and 88 pieces.

A normal set of slip gauges has 45 pieces.

Slip gauges are classified into various types according to their use as follows:

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1. Grade 1

2. Grade 2

3. Grade 0

4. Grade 00

5. Calibration Grade

Grade 1:

It is used for tool room work

Grade 2:

It is used workshop for setting tools and checking dimensions roughly.

Grade 0:

It is used in inspection department.

Grade 00:

It is used in error detection in instrument.

Calibration Grade:

It is used for calibrate the instrument

Taylor’s Principle in the Design of Limit Gauges

It states that GO gauge should check all related dimensions. Simultaneously, NOGO gauge

should check only one dimension at a time.

According to Taylor “GO” and “NOGO” limit gauges should be designed to determine the maximum

and minimum metal limits.

Go limit gauge: A GO gauge corresponds to maximum metal condition. For example upper limit of a shaft or

lower limit of a hole.

The “GO” snap gauge corresponds to upper limit of the shaft, while the “GO” plug gauge corresponds

to lower limit of the hole. The “GO” gauges should check all the possible elements of dimensions at a time

(roundness, location size etc). A GO plug gauge must be of corresponding mating section and preferably to the

full length of the hole so that straightness of the hole can be checked.

NO GO limit gauge: A NOGO gauge corresponds to minimum metal condition. For example, lower limit of a

shaft and the upper limit of a hole. It should check only one feature of the component at a time. The NOGO

snap gauge corresponds to lower limit while the NOGO plug gauge corresponds to upper limit.

Problem:

A dimension of 57.895 mm is required to be set with the help of slip gauges using M112 set.

57.895 = 1.005 + 1.39 + 5.5 + 50

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