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Chapter 6: Standards of Length

Overview:

Introduction to the

basic principles andconcepts

Gauge blocks

Measurement Errors

Mitutoyo Steel Square Gage Block Set, ASME

Grade 0, Inch (Image source: Amazon)

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Introduction

For a workshop to create a product withsatisfactory dimensions, there has to be

measurement of its dimensions.

Thus, the operation of a workshop mustinclude some knowledge of metrology, which

is the science of measurement.

Specifically, a subset of metrology calledengineeringmetrology.

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Introduction

Engineering metrology concerns (but not just)measurement of dimensions. Dimensions include:

A length measuring tool can measure

thickness and diameters too.

(Image source: Craftman Space)

Length (scope

of chapter)

Thickness

Diameter

Taper

Angle

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Introduction

The practice of metrology/measurement involves

precise measurements through the use of

instruments and any necessary adjustments to

obtain the degree of accuracy required.

Layman explanation:

The act of measurement is the use of the mostappropriate instrument to make the measurementwith, e.g. tape rulers for straight lines.

The instrument must be applied correctly for areliable reading, e.g. the tape ruler has to be securedat the ends of the lines.

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Introduction

The practice of metrology/measurement alsoinvolves industrial inspections.

If the product is part of a bigger assembly, the

assembly is also inspected. To check whether all parts have come together

properly.

An inspection is the checking of a product at

various stages in its manufacturing, from when itis still raw material, up to when it is finished.

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Introduction

When this practice is applied on workshops,products made in a workshop must have their

dimensions checked regularly in order to discover

any problems with workshop equipment or

procedures.

For example, if something cut by a cutting tool

has dimensions which are off target when

measured, something may be wrong with the

cutting tool.

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Main Objectives of Measurement

The main purposes of measurement, with regard

to operation of a workshop, are to:

Maintain quality production

Maintain reliability of production.

The following slides break this purpose down

into practical objectives.

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Objectives of Measurement

Obtain measurements at required accuraciesand as efficiently as possible.

E.g. use stiff measurement tools to measure

stiff, long parts.

Easier alternative phrase: efficient and effective use ofmeasurement methods

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Objectives of Measurement

Evaluating newly developed products, inorder to ensure that their components are

within the process limits and measuring

instrument capabilities of the plant

In simpler words, the measurements are to

prove that the product is made correctly

within the workshop and with its resources.

If the measurements have unexpected

accuracies, the product may have been

tampered, or equipment might be faulty.9

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Objectives of Measurement

To ensure that the production process createcomponents and products within their relevanttolerances

In other words, ensure that the product/outputis within desired specifications.

Wheel alignment specs for old cars in ranges of degrees & lengths.

Source: Automotive Repair Guides, Blogspot10

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Objectives of Measurement

To ensure that instruments are adequate fortheir respective measurements.

Example: A workshop may use rulers extensively

to measure its products, but there may be a few

special cases where another instrument may be

more useful, e.g. a caliper is more appropriate to

measure the inside diameter of a tube from withinthe tube than at its ends.

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A ruler could be used to measure the internal diameter bypressingthe ruler against the end of this component, but a caliper gives a

more reliable reading.

(Image source: Wonkee Donkee Tools, UK)

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Objectives of Measurement

To reduce number of rejects and reworkthrough statistical quality control.

Measurements provide hard data on the outcome

of the production process.

This hard data can be analyzed statistically to

figure out the cause of problems.

E.g. occasional cases of dimensions out of

acceptable tolerances may suggest onset of

tool failure.

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Measurement is an important component of quality

control cycles, such as 6-Sigma.

(Image source: Revelle Training)

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(Other) Objectives of Measurement

These other objectives are not directly associatedwith the quality and reliability of production.

Instead, they contribute to the reliability of the

act of measurement itself. (More reliable measurement is always good.)

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(Other) Objectives of Measurement

This is usually done in the planning and

organizing phase of production. Incomplete and complete prototypes/replicas of

the product are fabricated so as to be subjected to

measurement with various methods and tools.

The most convenient and reliable method/tool of

measurement is selected as the as the standard for

the production process.

Standardization of measuring methods for aproduction process.

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(Other) Objectives of Measurement

Maintenance/Troubleshooting of measurementtools

Measurement tools are generally regularlycalibrated.

The act of calibration usually involves takingmeasurements of objects with standardizeddimensions.

When troubleshooting faulty measurement tools,

Measurements are taken to estimate progresstowards fixing the tools, e.g. improved accuracyafter a repair solution means that the solution has

been effective. 17

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To continuously achieve all of the objectives

above, there must be standards for measurement.

Achievement of these standards is defined by:

Precision

Accuracy

Of the measurements.

The practical nature of the precision andaccuracy of measurements depends on the

dimension which is being measured.

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Precision is the repeatability of ameasurement method/tool.

Concept of Precision

The precisionof a measurement method/tool is

synonymous with its reliability.

NOTE: Consistent readings do not

necessarily indicate good accuracy.

Signs of reliable measurements:

Consistent readings

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Concept of Precision

Examples of unreliable measurement: If a measurement tool gives slightly

inconsistent readings, either the tool is

damaged, or there is interference from theenvironment.

If a measurement method gives completelyrandom readings, the method has failed to

take into consideration external factors whichaffect the readings.

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Faded print on the scales of a ruler makes its precision

questionable.

(Image source: Walke Moore Tools)21

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Concept of Accuracy

Accuracy is the ability of a measurementmethod/tool to provide readings close to the true

value of a measured quantity.

Accuracy is assured when:

The measurement method/tool gives thecorrect reading when used to measure an

object with standardized dimensions.

E.g. a tape ruler is accurate if it cancorrectly measure a gauge block.

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Precision vs. Accuracy

A measurement method/tool can be precise, but not

necessarily accurate. Inaccuracy can be caused by:

Lack of calibration of tools.

Problems with the subject of measurement.

Environmental factors.

Example: The shaking of worktables in

workshops (usually due to bumping or vibrationfrom nearby machines) make lengthmeasurements of parts on the worktablesinaccurate.

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Precision vs. Accuracy

A measurement method/tool cannotbe: accuratebut not precise.

Precision is a requirement for accuracy.

A measurement method/tool can give readings

which change from attempt to attempt.

This is usually due to instability in the subjectof measurement.

In this case, it is best to consider another

method/tool of measurement. 24

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If this occurs, please notify the workshop staff or

any supervisor/instructor.

In a workshop environment, it is unlikely that a

subject of measurement would be unstable.

This is because most objects which aremeasured are made of tough solids, such as

metal or wood.

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If there are very different readings from the sameobject, this means that:

The measurement tool may be faulty, or

The tool or object has not been set upproperly.

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In a workshop environment, a relatively clear (and flat)

table is suitableand safefor precise and accurate

length measurements.

(Image source: Hisia Furniture) 26

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StandardsEarlier slides have mentioned objects such as gauge

blocks.

These objects are physical representations of the notion

of standards.

As a word, standard has a lot of meanings. Forthis subject, the scope of this word is restrictedto:

Scientific standards

Practiced standards based on scientificstandards.

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Scientific Standards

In metrology, a standard is anobject which define

a unit of measurement of a physical quantity.

A standard is generally agreed upon byestablished organizations with long histories

and vast knowledge of metrology, e.g.

International Bureau of Weights and Measures.(This link opens a webpage.)

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http://en.wikipedia.org/wiki/International_Bureau_of_Weights_and_Measureshttp://en.wikipedia.org/wiki/International_Bureau_of_Weights_and_Measures

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Scientific Standards

Standards are not unanimously agreed upon:For example, Commonwealth nations and other

former colonies generally use the S.I. system ofstandards,

However, some nations use the Imperial(English) system.

On the other hand, unanimous agreement is not

neededto validate any system of standards.

Instead, any system of standards must be

validated by scientific reasons.29

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Scientific Standards

Usually, these scientific reasons involvechemically and physically stable materials.

For example, the S.I. system defines the metre as

a portion of the length of distance travelled bylight in vacuum.

The metres physical representation is the

Historical International Prototype Metre Bar,which is made from a very wear-resistant alloy

of platinum and iridium.

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The prototype metre bar preceded the invention of gauge

blocks.

(Image source: National Institute of Standards and

Technology, USA) 31

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Practiced Standards

Practiced standards are custom-designedsystems of standards which are implemented in

industries and other private or public endeavours.

They are basedon from scientific standards.

These practiced standards are also calledstandards, for convenience.

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Need for Standards

In other words, standards are convenient

references for knowledge of units of measurementin cost-efficient production.

Standards promote economic and quality productionby providing technical criteria accepted by consensus.

Standards gives confidence to manufacturers and

consumers who contribute to this consensus.

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Aims of StandardsMake industrial, economic and scientific

communication easier. When manufacturers, regulators, engineers,

researchers and consumers use the same systemof standards, less time is spent on unit

conversions.

Contribute to economic expenditure of

resources.Making use of standards makes it easier to

coordinate the use of resources.

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Aims of Standards

Protect consumer interests.Consumers who are informed about

standards are more capable of estimating thequality and consistency of goods and services

than those who are not.

(Indirectly) promote safety, health and protection

of the environment.For example, keeping to standards prevents

wastage; wastage leads to safety, health and

environmental problems, so reducing it is good.35

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Aims of Standards

Encouraging trade and the removal of trade

barriers between trade parties, e.g. countries,

which practice the same standards.

If the previous aims of standards are

achieved, the achievement of this standard is

made easier.

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Gauge blocks have to be designed according to proven

standards, such as ISO 3650.

(Image source: ISO 3650 documentation, 1998)

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https://www.iso.org/obp/ui/https://www.iso.org/obp/ui/

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Gauge Blocks

Gauge blocks are physical representations of

practiced standards of length.

In a workshop environment, gauge blocks are

not versatile but are easy to use.

Gauge blocks are used for producing precise

lengths.

Therefore, gauge blocks often come in a set.

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Gauge Blocks

Individual gauge blocks are generally made of

tough hard-wearing metal or ceramic, or a

composite of both.

Gauge blocks of the sameset are usually made of the

same material.

Another set of Mitutuyobrand gauge

blocks.

The dimensions ofgauge blocks must be

maintained.

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Gauge Blocks

Gauge blocks are fabricated according to specified

lengths/dimensions, and must maintain these for as

long as possible for them to remain useful.

Gauge blocks are generally subjected to a lot

of handling, either by human hands, or by

machines.

Hence the use of hard-wearing materials.

Therefore, their materials have to be tough too.

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Looks of Gauge BlocksGauge blocks look like blocks of metal material.

In a workshop environment, they can bemistaken as other things such as raw work-pieces.

However, they have the following details:

Stamping of product codes.

Generally precise shapes.

Polished surfacesif the blocks are not too old.

Have oil on them.

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Gauge Blocks vs. Other Tools

There are considerable differences between

gauge blocks and other measurement tools.

The most significant are:

Gauge blocks cannot be recalibrated, because they do

not have devices which convert physical input into

visual output.

Gauge blocks are not multi-purpose, e.g. each gauge

block represents only one length of specificmagnitude and does nothing else.

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Gauge Blocks in the Workshop

In a workshop environment, gauge blocks areused to:

Calibrate measuring instruments (such as

micrometers). Set up precise and accurate angles (usually

with sine bars as well).

Position machine tool components.(continued in next page)

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Gauge Blocks in the Workshop

(continued from previous page)

Set comparators, dial indicators, and height

gauges to exact dimensions (e.g. helping some

other device measure something).

Inspect the accuracy of finished parts.

If the parts are designed with the gauge

blocks being used as templates, the gauge

blocks can be used again to check the parts.

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Materials for Gauge BlocksExamples of tough and hard-wearing materials

for gauge blocks include:

Cemented carbides: wear-resistant, but not as toughas other materials.

It is used for gauge blocks which are designedfor calibration. The wear resistance helpsmaintain precision.

Hardened steel alloy: not as wear-resistant as

carbides, but tougher. Gauge blocks of this material are used to

position machine components or used astemplates.

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Cemented carbide blocks do not last forever; notice the

chipped 1-inch block. Their stamps can also fade.

(Image Source: YourToolingStore.com, used products)

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Other Materials for Gauge Blocks

Examples of other materials include: Zirconium composites: desired for their low

thermal expansion coefficients.

Useful for high-temperature environments. High temperatures cause other gauge blocks to

expand and lose their accuracy.

Other materials are used when cost is not thebiggest factor.

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Surfaces of Gauge Blocks

In the workshop, gauge blocks come into contactwith many things, including hard, rough and/or

sharp surfaces.

Therefore, their surfaces are treated, groundand polished to exact dimensions, and for

additional wear resistance and hardness.

The polishing also makes it easier to know

how old or how well-kept a gauge block is.

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Safety of Gauge Blocks

On their own, gauge blocks are relatively safe touse. However:

Do not use gauge blocks for anything other

than their intended use. They are hard, so they can pose a hazard.

Gauge blocks are made of materials which

are not entirely safe to humans.Wash hands thoroughly after workshop

sessions.

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Gauge Block Sets

Gauge blocks in a set usually use the same set ofstandards for their lengths,

Either the S.I. system

Or the Imperial system.

As mentioned earlier, gauge blocks often come insets, usually in a case with slots which fit all

blocks.

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Gauge Block Sets

Gauge blocks of the same set are generallyarranged in size steps.

For example, gauge blocks in S.I. may have

step increases of 0.5 cm, 1 cm, 1.5 cm, etc.The step sizes are not uniform.

(The containers for gauge block sets have to be

designed together with the gauge blocks, so the

step sizes are selected such that the gauge blocks

can fit into the container.)51

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Gauge blocks of the same set can be stacked onto each

other through wringing in case a big block does not

suffice.

(Image Source: Clarence Leon Goodrich & Frank Arthur

Stanley (1907) Accurate Tool Work, McGraw-Hill, New

York, p.185, fig.196)53

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Maintaining Gauge BlocksThe accuracy and precision of gauge blocks can

be eroded by wear-and-tear from usage.

Therefore, the maintenance of gauge blocks

generally include the following precautions:

54

1. Prevent gauge blocks from damage.

Do not cause them to drop from any height.

Avoid having the gauge blocks from

coming into contact with sharp edges,

including the edges of other gauge blocks.

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Maintaining Gauge Blocks

2. Keep the gauge blocks in their cases whenthey are not being used.

Clean and oil the gauge blocks before

keeping them in their cases.

If the gauge blocks have been subjected to

hot or cold environments, let them return to

original temperature before keeping them.

Observe and follow the spacing for the

gauge blocks in the case.

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Maintaining Gauge Blocks

3. When they are not in use but are not to bereturned to the case yet, keep them away from

other hard objects.

Do not stack gauge blocks onto each otherunless they are to be used in this manner.

Have some space on the

workstation/worktable to place the gaugeblocks on.

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Errors

As a reminder, error, is not defined as a mistake

in metrology, but rather

Error is the difference between the measured

value of a quantity and the true value of the

quantity.

Errors are not always undesirable.

Investigative research on the causes of errors can

yield useful knowledge, e.g. for improvement of

the measurement method.

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Categories of Errors

There are generally two categories of errors,

according to their causes:

Human errors: causes for these errors include

but are not limited to lack of skill in use and

handling,poor work attitude/lapse ofconcentration and bias caused by personal

accuracy achievementson the part of the person

making the measurement. These can be mitigated withproper training,

practice of disciplineandstrong grasp of

professional ethics, respectively.58

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Categories of Errors

Environmental errors: causes for these errorsare due to factors in the surroundings, e.g. theworkplace, such as

Changes in temperature due to heating from

harsh lighting, sunlight, and proximity to hotmachines

Vibrations from nearby heavy machinery (e.g.other workshop machines)

If environmental errors are significant, theworkplace may have to be rearranged toremove the causes.

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Common Human Errors

Misreading:

This is usually due to lack of attention.

The user may also be tired.

Rarely, onset of health issues.

Poor preparation:

For example, the wrong settings had beenselected when preparing a measurement tool.

Generally due to lack of attention.

These can be overcome with implementation of

checking regimes.60

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Common Human ErrorsMiscalculation

This can happen if the desired measurement is

manually derived from other measurements.

For example, an angle is to be obtained by

measuring lengths and then usingtrigonometry to calculate the angle.

This is generally due to lack of attention or lack

of competence of the person.

These can be prevented by having a measurement

tool or method make measurements directly.

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Common Human Errors

(Image Source: ChemWiki)

Parallax:

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Parallax in the Workshop

Cause of parallax is usually:

Misalignment of subject of measurement and

worktables, e.g. the subject is placed on a surface

such that the subject is not perpendicular to the

operators eyes.

The subject of measurement may have to be

placed elsewhere, if space permits.

Otherwise, corrections have to be applied to themeasurement after finding errors from

measuring gauge blocks.

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Environmental Errors

Thermal expansion due to temperature change

Usually, the workshop environment is not

expected to have considerable temperature

changes.

However, harsh sunlight or lamps can heat a

subject of measurement or a measurement tool.

The heating causes expansion.

E.g. gauge blocks may expand by micrometerswhen they heat up by a few degrees Celsius.

This is an issue ifvery high accuracy is desired.

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Other Causes of Errors

The subject of measurement or measuring tool

may not have been prepared for measurement.

For example, to measure the length of a part with

a ruler, the part should be clean before coming

into contact with the ruler.

Otherwise, substances which dirty the part

such as oil and grime will dirty the ruler as

well.

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This is a ruler

which has been

designed to be

deliberatelywarped, so that it

can be picked up

by users with

arthritis problems

in their hands. It

can be flattened

when it is used to

make

measurements.(Image Source:

DeZeen

Magazine)

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END OF CHAPTER