Meterology and Techniques

8/6/2019 Meterology and Techniques

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EXPERIMENT NO-1

Date :

AIM : Study about Metrology & Techniques.

Objectives : At the end of the practical,the students shall be able to understand :

Metrology & Importance of Measurement,

Standards in metrology

Traceability

Geometric dimensions & tolerances

Tolerance analysis

Metrology :

Metrology is the science of measurement

Dimensional metrology is that branch of Metrology which deals with

measurement of dimensions of a part or workpiece (lengths, angles,

etc.)

Dimensional measurements at the required level of accuracy are theessential link between the designers intent and a delivered product.

Importance of measurement :

A sound measurement system is fundamental to the production and delivery of highquality goods and services in society. It provides the basis for:

industrial development, e.g., manufacturing processes, product testing,

food processing,

fair trading and consumer protection in the domestic and internationalarenas,

consumer confidence,

health and safety,

environmental monitoring,

adoption of new technology and

scientific advances

It provides the framework in which manufacturers can demonstrate compliance withspecifications within an internationally harmonised system.

A national measurement system comprises five components:

physical metrology: A National Measurement Institute (NMI) that

develops, maintains and disseminates national measurement standards

at the highest level appropriate to national needs, and develops and


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transfers new measurement technology to domestic users(corresponding international body: BIPM/CIPM)

legal metrology: A nominated national authority with responsibility

for overseeing legislation relating to measurement and its application

in everyday commerce (OIML)

laboratory accreditation: a National Accreditation Body(ies) thatensures appropriate linkages (traceability) from measurements

undertaken by NMIs to those undertaken in the workplace through an

accredited network of technically competent calibration laboratories(ILAC)

documentary standards: a National Body that oversees the

harmonisation of international and domestic written standards and theadoption of these at the national level (ISO, IEC, ITU)

accreditation of certification bodies: a National Body that is

concerned with national requirements for certification and quality

arrangements (IAF)

Standards (metrology)

In the science of measurement, a standard is an object, system, or experiment that bears

a defined relationship to a unit of measurement of a physical quantity. Standards are the

fundamental reference for a system of weights and measures, against which all other

measuring devices are compared. Historical standards for length, volume, and mass were

defined by many different authorities, which resulted in confusion and inaccuracy of

measurements. Modern measurements are defined in relationship to internationally-

standardized reference objects, which are used under carefully controlled laboratory

conditions to define the units of length, mass, electrical potential, and other physical

quantities.

The International Prototype kilogram is an artifact standard that is defined to be exactly

one kilogram mass.
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Primary measurement standards may be used strictly in measurement laboratories. Less

precisely controlled working standards are used forcalibration of industrial measurement

equipment. Primary standards that define units may be inconvenient for everyday use, so

working standards represent the primary definition in a form that is easier to use. For

example, the definition of the "metre" is based a laboratory experiment combining the

speed of light and the duration of a second, but a machine shop will have working

standard gauge blocks that are used for checking its measuring instruments.

Standard units for length would be embedded in the cornerstones of churches or

important public buildings, so that all people trading in an area could agree on the

units.

Initially many units of measure were defined in terms of unique artifacts which were the

legal basis of units of measure. A continuing trend in metrology is to eliminate as many

as possible of the artifact standards and instead define practical units of measure in terms

of fundamental physical constants, as demonstrated by standardized technique. One

example is the unit of electrical potential, the volt. Formerly it was defined in terms

ofstandard cell electrochemical batteries, which limited the stability and precision of the

definition. Currently the volt is defined in terms of the output of a Josephson junction,

which bears a direct relationship to fundamental physical constants. One advantage of

elimination of artifact standards is that inter-comparison of artifacts is no longer required.

Another advantage would be that the loss or damage of the artifact standards would not

disrupt the system of measures.

Examples of primary reference standards

The reference standard for a kilogram is the one kilogram mass of aplatinum-

iridium kilogram maintained by the Bureau International des Poids et Mesures in Svres,

France.

In contrast, the reference standard for the meteris no longer defined by a physical object.

In 1983, the standard meter was redefined as the distance light travels in a vacuum during

1/299,792,458 of a second.

Secondary reference standards
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Secondary reference standards are very close approximations of primary reference

standards. For example, national measuring laboratories such as the US'sNational

Institute of Standards and Technology (NIST) may house stainless steel balls of one

kilogram which are used to set standards for manufacturing supermarket

measuring scales.

Working standards used in industry have a traceable relationship to the secondary and

primary standards.

A set ofgauge blocks are used as a working standard to check the calibration of

measurement tools such as micrometers .

Traceability

A core concept in metrology is (metrological) traceability, defined as "the property of the result of

a measurement or the value of a standard whereby it can be related to stated references, usually

national or international standards, through an unbroken chain of comparisons, all having stated

uncertainties." The level of traceability establishes the level of comparability of the measurement:

whether the result of a measurement can be compared to the previous one, a measurement

result a year ago, or to the result of a measurement performed anywhere else in the world.

Traceability is most often obtained by calibration, establishing the relation between the indication

of a measuring instrument and the value of a measurement standard. These standards are

usually coordinated by national metrological institutes: National Institute of Standards and

Technology, National Physical Laboratory, UK, Physikalisch-Technische Bundesanstalt, etc.

The term measurement traceability is used to refer to an unbroken chain of comparisons relating

an instrument's measurements to a knownstandard.Calibration to a traceable standard can be

used to determine an instrument's bias, precision, and accuracy.

In many countries, national standards for weights and measures are maintained by a National

Measurement Institute (NMI) which provides the highest level of standards for the calibration /
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measurement traceability infrastructure in that country. Examples of government agencies include

the National Physical Laboratory, UK (NPL) the National Institute of Standards and Technology in

the USA, and Physikalisch-Technische Bundesanstalt (PTB) in Germany. As defined by NIST,

"Traceability of measurement requires the establishment of an unbroken chain of comparisons to

stated references each with a stated uncertainty."

Geometric dimensions & tolerances

GD&T (per ASME Y14.5M-1994) is an international language that is used on

engineering drawings to accurately describe the size, form, orientation, and location of

part features. It is also a design-dimensioning philosophy that encourages designers todefine a part based on how it functions in the final product or assembly.

GD&T is an exact language that enables design engineers to "say what they mean" on a

drawing, thus improving product designs and lowering cost. Process engineers and

manufacturing use the language to interpret the design intent and to determine the bestmanufacturing approach. Quality control and inspection use the GD&T language to

determine proper set-up and part verification.

By providing company-wide uniformity in the drawing specifications and interpretation,

GD&T reduces controversy, guesswork, and assumptions throughout the design,manufacturing and inspection process.

Understanding how to apply and interpret GD&T correctly will help you:

Create clear, concise drawings

Improve product design

Create drawings that reduce controversy, guesswork, and assumptions throughout

the manufacturing process

Effectively communicate or interpret design requirements for suppliers andmanufacturing

Geometric Characteristic Symbols

There are 14 geometric characteristic symbols used in GD&T. They are used to describe size,location, orientation, and form.
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Geometric Tolerance Classes and Types

Class Type Symbol Reference Entity

FormStraightness

Flatness

Circularity

Cylindricity

- The condition where an axis is a straight line(or, in the case of a centerplane, each lineelement is a straight line).

The condition of a surface having all of itselements in one plane

A condition where all points of a surface ofrevolution, at any section perpendicular to acommon axis, are equidistant from that axis.

A condition of a surface of revolution in which allpoints of the surface are equidistant from acommon axis.

Profile Line

Surface

A geometric tolerance that limits the amount oferror for line elements relative to their true profile.

-

A geometric tolerance that limits the amount oferror a surface can have relative to its trueprofile.

Orienta-

tion

Angularity

Parallelism

Perpendicularity

- The condition of a surface, centerplane or axisbeing exactly at a specified angle

The condition that results when a surface, axis orcenterplane is exactly parallel to a datum

The condition that results when a surface, axis,or centerplane is exactly 90 to a datum.


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Location Position

Concentricity

Symmetry

The zone within which the axis or center plane ofa feature is permitted to vary from true(theoretically exact) position.

The condition where the median points of alldiametrically opposed elements of a cylinder (ora surface of revolution) are congruent with theaxis of a datum feature.

The condition where the median points of allopposed elements of two or more featuresurfaces are congruent with the axis orcenterplane of a datum feature.

Runout Circular

Total

- A composite control that affects the form,orientation, and location of circular elements of apart feature relative to a datum axis.

A composite control affecting the form,orientation, and location of all surface elementsof a diameter (or surface) relative to a datumaxis.

GD&T Modifiers and Symbols

There are many other modifiers and symbols used in geometric tolerancing. Here are a few:

All-Around Symbol - A circle placed on the bend of the leader line of a profilecontrol.

Basic Dimension - A numerical value used to describe the theoretically exact size,true profile, orientation, or location of a feature or datum target.

Between Symbol - A double ended arrow that indicates the tolerance zone extendsto include multiple surfaces.


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Controlled Radius - A radius with no flats or reversals allowed. The symbol for acontrolled radius is "CR."

Datum Feature - A part feature that contacts a datum.

Datum Target - A symbol that describes the shape, size, and location ofgage elements that are used to establish datum planes or axes.

Feature Control Frame - A rectangular box that is dividedinto compartments within which the geometric characteristicsymbol, tolerance value, modifiers, and datum references areplaced.

Least Material Condition - The condition in which a feature of size contains theleast amount of material everywhere within the stated limits of size.

Maximum Material Condition - The condition in which a feature of size contains the

maximum amount of material everywhere within the stated limits of size.

Projected Tolerance Zone - A tolerance zone that is projected above the partsurface.

Radius - A straight line extending from the center of an arc or circle to its surface.

Tolerances types examples :

Size tolerances :


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Form tolerances :

Location tolerances :

Orientation tolerances :

Tolerance analysis

Tolerance analysis is the general term for activities related to the study of accumulated variation in

mechanical parts and assemblies. Its methods may be used on other types of systems subject to

accumulated variation, such as mechanical and electrical systems. Engineers analyze tolerances for


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the purpose of evaluating geometric dimensioning and tolerancing (GD&T). Methods include 2D

tolerance stacks, 3DMonte Carlo simulations, and datum conversions.

Tolerance stackups ortolerance stacks are terms used to describe the problem-solving process

in mechanical engineering of calculating the effects of the accumulated variation that is allowed by

specified dimensions and tolerances. Typically these dimensions and tolerances are specified on an

engineering drawing. Arithmetic tolerance stackups use the worst-case maximum or minimum values

of dimensions and tolerances to calculate the maximum and minimum distance (clearance or

interference) between two features or parts. Statistical tolerance stackups evaluate the maximum and

minimum values based on the absolute arithmetic calculation combined with some method for

establishing likelihood of obtaining the maximum and minimum values, such as Root Sum Square

(RSS) or Monte-Carlo methods.

While no official engineering standard covers the process or format of tolerance analysis and

stackups, these are essential components of good product design. Tolerance stackups should be used

as part of the mechanical design process, both as a predictive and a problem-solving tool. The

methods used to conduct a tolerance stackup depend somewhat upon the engineering dimensioning

and tolerancing standards that are referenced in the engineering documentation, such as American

Society of Mechanical Engineers (ASME) Y14.5, ASME Y14.41, or the relevant ISO dimensioning and

tolerancing standards. Understanding the tolerances, concepts and boundaries created by these

standards is vital to performing accurate calculations.

Tolerance stackups serve engineers by:

helping them study dimensional relationships within an assembly.

giving designers a means of calculating part tolerances.

helping engineers compare design proposals.

helping designers produce complete drawings.

]Concerns with tolerance stackups

A safety factor is often included in designs because of concerns about:

Operational temperature and pressure of the parts or assembly.

Wear.

Deflection of components after assembly.
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The possibility or probability that the parts are slightly out of specification (but passed

inspection).

The sensitivity or importance of the stack (what happens if the design conditions are not met).

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