1.Length Standards

7/25/2019 1.Length Standards

1/23

STANDARDS OF MEASUREMENTS

Definition of Standards:

A standard is defined as something that is set up and

established by an authority as rule of the measure of

uantity! "eight! e#tent! $alue or uality.

For example, a metre is a standard established by an

international organization for measurement of length.

Industry, commerce, international trade in modern civilization

would be impossible without a good system of standards.

Role of Standards: The role of standards is to a%hie$e

uniform! %onsistent and repeatable measurements

throughout the "orld&

Today our entire industrial e%onomy is based on the

inter%hangeabilityof parts the method of manufa%ture&

To a%hie$e this! a measuring system adeuate to define

the features to the a%%ura%y reuired ' the standards of

suffi%ient a%%ura%y to support the measuring system are

ne%essary&

!A"#A$# %F &'"(!)

In practice, the accurate measurement must be made by

comparison with a standard of *nown dimension and such astandard is called +rimary tandard

!he first accurate standard was made in 'ngland and was

*nown as Imperial tandard yard which was followed by

International +rototype metre made in France. ince these


2/23


3/23

Historical International Prototype Metre bar,made of an alloy of platinum and

iridium, that was the standard from 1889 to 1960.

!he tresca cross section gives greater rigidity for the amount of material

involved and is therefore e%onomi% in the use of an expensive metal. !he

platinum(iridium alloyis used because it is non o#idi)ableand retains good

polished surfa%e reuired for engra$ing good uality lines .

*mperial Standard yard:

An imperial standard yard, shown in fig, is a bronze 2;84 0u, 6inc7

bar of 6 inch s?uare section and


4/23

Bronze Yard was the official standard of lenth for the United States between

18!! and 189", when the #$ went to metric standards. 1 yard = 0.9144 meter.

%he yard is used as the standard unit of field&lenth measurement in 'merican,

(anadian and 'ssociation football, cric)et pitch dimensions, swimmin pools, and

in some countries, olf fairway measurements.

Disad$antages of Material length standards:

6. Daterial length standards vary in length over the years

owing to molecular changes in the alloy.

8. !he exact replicas of material length standards were not

available for use somewhere else.


5/23

' )rypton&filled dischare tube in the shape of the element*s atomic symbol. '

colorless, odorless, tasteless noble as, )rypton occurs in trace amounts in the

atmosphere, is isolated by fractionally distillin li+uefied air. %he hih power and

relatie ease of operation of )rypton dischare tubes caused(from 1960 to 1983)the

official meter to be defined in terms of one orane&red spectral line of )rypton&86.

etre as on !oday" #n 1983$ t%e 1&t% 'enera onferene on wei'%ts *

meas+res ,ro,osed t%e +se of s,eed of i'%t as a te%niay feasibe *

,ratiabe definition of meter.

eter is now defined as t%e en't% of ,at% of tra-eed by i'%t in -a++m in

(1 /99&9/48) seond. !%e i'%t +sed is iodine stabiized %ei+mneon aser.

Ad$antages of using "a$e length standards:

.& +ength does not %hange&

/& *t %an be easily reprodu%ed easily if destroyed&

0& This primary unit is easily a%%essible to any physi%al laboratories&

1& *t %an be used for ma2ing measurements "ith mu%h higher a%%ura%y than

material standards&

3& 4a$elength standard %an be reprodu%ed %onsistently at any time and at anypla%e&

Subdi$ision of standards:

!he imperial standard yard and the international prototype meter are masterstandards 5 cannot be used for ordinary purposes. !hus based upon the accuracy

re?uired, the standards are subdivided into four grades namely

.& 5rimary Standards

/& Se%ondary standards

0& Teritiary standards

1& 4or2ing standards

5rimary standards:

They are material standard preser$ed under most %areful %onditions&

These are not used for dire%tly for measurements but are used on%e in .6 or

/6 years for %alibrating se%ondary standards&

2" *nternational 5rototype metre! *mperial Standard yard&

Se%ondary standards:

These are %lose %opies of primary standards "&r&t design! material ' length&

Any error e#isting in these standards is re%orded by %omparison "ith


6/23

primary standards after long inter$als& They are 2ept at a number of pla%es

under great super$ision and ser$e as referen%e for tertiary standards& This

also a%ts as safeguard against the loss or destru%tion of primary standards&

Teritiary standards:

!he primary or secondary standards exist as the ultimate controls for reference at

rare intervals.

Tertiary standards are the referen%e standards employed by National

5hysi%al laboratory ,N&5&+- and are the first standards to be used for

referen%e in laboratories ' "or2shops& !hey are made as close copies of

secondary standards 5 are *ept as reference for comparison with wor*ing

standards.

4or2ing standards:

These standards are similar in design to primary! se%ondary ' tertiarystandards& 7ut being less in %ost and are made of lo" grade materials! they

are used for general appli%ations in metrology laboratories&

ometimes, standards are also classified as

8 Referen%e standards ,used as referen%e purposes-

8 9alibration standards ,used for %alibration of inspe%tion ' "or2ingstandards-

8 *nspe%tion standards ,used by inspe%tors-

G 4or2ing standards ,used by operators-

+*NE STANDARDS

4hen the length being measured is e#pressed as the distan%e bet"een t"o

lines! then it is %alled +ine Standard;&

E#amples: Measuring s%ales! *mperial standard yard! *nternational prototype meter!et%&

9hara%teristi%s of +ine Standards:

.& S%ales %an be a%%urately engra$ed but it is diffi%ult to ta2e the full ad$antageof this a%%ura%y& 2"A steel rule %an be read to about < 6&/ mm of truedimension&

/& A s%ale is ui%2 and easy to use o$er a "ide range of measurements&0& The "ear on the leading ends results in +nder sizin'5

1& A s%ale does not possess a =built in> datum "hi%h "ould allo" easy s%alealignment "ith the a#is of measurement! this again results in =undersi)ing>&

3& S%ales are sub?e%ted to paralla# effe%t! "hi%h is a sour%e of both positi$e

' negati$e reading errors>


7/23

@ S%ales are not %on$enient for %lose toleran%e length measurementse#%ept in %on?un%tion "ith mi%ros%opes&

END STANDARDS

4hen the length being measured is e#pressed as the distan%e bet"een t"o

parallel fa%es! then it is %alled 2nd standard5.End standards %an be made to a $ery high degree of a%%ura%y&

2" Slip gauges! ap gauges! Ends of mi%rometer an$ils! et%&

9hara%teristi%s of End Standards:

.& End standards are highly a%%urate and are "ell suited for

measurements of %lose toleran%es as small as 6&6663 mm&

/& They are time %onsuming in use and pro$e only one dimension at a time&

0& End standards are sub?e%ted to "ear on their measuring fa%es&

1& End standards ha$e a =built in> datum! be%ause their measuring fa%es are flat'

parallel and %an be positi$ely lo%ated on a datum surfa%e&

3& They are not sub?e%ted to the paralla# effe%t sin%e their use depends onfee7.

@& roups of blo%2s may be wr+n'7together to build up any length& 7ut

faulty "ringing leads to damage&

B& The a%%ura%y of both end ' line standards are affe%ted by temperature

%hange.

TRANSFER FROM +*NE STANDARD TO END STANDARD

2"+& method of deriving 'nd standard from line standard7

+ine Standard 9omparator:

Deasured differenced Hx61x8

x66

x8


8/23

6C8inch bloc*inch bloc*


9/23

A line standard %omparatoris used to transfer the line standard correctly to the

ends of a bar.

It consists of two microscopes mounted about a yard apart over a table. An end

standard about 03.C/ in%h in lengthis produced with flat 5 parallel faces. !wo 6@8

inch bloc*s with centrally engraved lines are wrung- to the ends of this end standard,

such that the distance between the center lines is approximately


10/23

!hen the other 6@8 inch bloc* is wrung with it 5 again is compared with the end

bar 2to be calibrated7 5 the deviation #8is noted. If & is the actual length of the


11/23


12/23

6;m

&A8


13/23

S+*5 AUES OR AUE 7+O9S

2J%)A"%" (AK('7

lip gauges are rectangular bloc*s of steel having cross section of


14/23

lip gauges are bloc*s of steel that have been hardened and stabilized by heat

treatment. !hey are ground and lapped to size to very high standards of accuracy and

surface finish. A gauge bloc* 2also *nown Johansson gauge, slip gauge, or Jo bloc*7 is a

precision length measuring standard consisting of a ground and lapped metal or ceramic

bloc*. lip gauges were invented in 6;39 by wedish machinist 0arl 'dward Johansson

The cross- sections of these gauges are 9mm 30mm for sizes up to 10mm and

9mm35mm for larger sizes. Any two slips when perfectly clean may be wrung together.

The dimensions are permanently marked on one of the measuring faces of gauge blocks.

Gauges blocks are used for:

(i) Direct precise measurement, where the accuracy ofthe work piece demands it.

(ii)For checking accuracy of venire calipers, micro

metes, and such other measuring instruments.

(iii)Setting up a comparator to specific dimension.

(iv)For measuring angle of work piece and also for

angular setting in conjunction with a sine bar.

(v) The distances of plugs, spigots, etc. on fixture areoften best measured with the slip gauges or end

bars for large dimensions.

(vi)To check gap between parallel locations such as in

gap gauges or between two mating parts.

There are many measurements which can be made with slip gauges either alone or in

conjunction with other simple apparatus such as straight edges, rollers, balls sine barsetc.


15/23

:hen correctly cleaned and wrung together, the individual slip gauges adhere to

each other by molecular attraction and, if left li*e this for too long, a partial cold

weld will ta*e place.

If this is allowed to occur, the gauging surface will be irreparable after use, hence the

gauges should be separated carefully by sliding them apart. !hey should then be

cleaned, smeared with petroleum Lelly 2Maseline7 and returned to their case.

5rote%tor Slips:

*n addition! some sets also %ontain prote%tor slips that are /&36mm thi%2 and are

made from a hard! "ear resistant material su%h as tungsten %arbide& These are

added to the ends of the slip gauge sta%2 to prote%t the other gauge blo%2s from

"ear& Allo"an%e must be made of the thi%2ness of the prote%tor slips "hen they

are used&

4ringing of Slip auges:

lip gauges are wrung together to give a stac* of the re?uired dimension. In

order to achieve the maximum accuracy the following precautions must be ta*en.

G Kse the minimum number of bloc*s.

G :ipe the measuring faces clean using soft clean chamois leather.

G :ring the individual bloc*s together by first pressing at right angles, sliding 5

then twisting.

5rinin of $lip aues


16/23

6 7ohansson aue bloc)s wrun toether easilysupport their own weiht

*ND*AN STANDARD ON S+*5 AUES ,*S/1(.@@-

lip gauges are graded according to their accuracy as

(rade /, (rade I 5 (rade II. (rade II is intended for use

in wor*shops during actual production of components,

tools 5 gauges.

(rade I is of higher accuracy for use in inspectiondepartments.

(rade / is used in laboratories and standard rooms for

periodic calibration of (rade I 5 (rade II gauges.


17/23

*mportant notes on building of Slip auges:

8 Al"ays start "ith the last de%imal pla%e&

8 Then ta2e the subseuent de%imal pla%es&

8 Minimum number of slip gauges should be used

by sele%ting the largest possible blo%2 in ea%h

step&

If in case protector slips are used, first deduct theirthic*ness from the re?uired dimension then proceed as


18/23

per above order.

Numeri%al problem(.

3uild the followin dimensions usin M&8 set. i: 29.8"!mm ii: 8."1! mm

Solution:

,i- To build 1&0/3 mm:

0ombination of slips E/96.


19/23

Numeri%al problem(/

3uild up a lenth of !.28! mm usin M11" set. #se two protector slips of ".! mm each.

Solution:

0ombination of slips 8.=8=86.E;6.//N6.///=8.= H


20/23

As regards grades or classes of slip gauges, these could

also be designed in five grades as under

rade /&!his is the wor*shop grade. !ypical uses includesetting up machine tools, positioning milling cutters and

chec*ing mechanical widths.

rade .&Ksed for more precise wor*, such as that carried

out in a good1class toolroom. !ypical uses include setting up

sine bars and sine tables, chec*ing gap gauges and setting

dial test indicators to zero.

rade 6&!his is more commonly *nown as the Inspectiongrade, and its use is confined to toolroom or machine shop

inspection. !his means that it is the Inspection #epartment

only who have access to this grade of slips. In this way it is

not possible for these slip gauges to be damaged or abused

by the rough usage to be expected on the shop floor.

rade 66&!his grade would be *ept in the tandard $oom

and would be *ept for wor* of the highest precision only. Atypical example would be the determination of any errors

present in the wor*shop or (rade 8 slips, occasioned by

rough or continual usage.

0alibration grade. !his is a special grade, with the actual

sizes of the slips stated or calibrated on a special chart

supplied with the set. !his chart must be consulted when

ma*ing up a dimension, and because these slips are not

made to specific or set tolerances, they are not as expensive

as the (rade //. It must be remembered that a slip gauge,

li*e any other engineering component, cannot be made to an


21/23

exact size. All slip gauges must have tolerances on the

length, flatness and parallelism of measuring faces.

'xcept for the calibration grade, all slip gauge sets aremanufactured to within specified limits the closer the limits

the more expensive the slip gauges, but in the case of the

calibration grade, greater tolerances on length are

permissible. Because the actual lengths are *nown or

recorded in the calibration chart, due allowance can be

made when the slips are used.

Tungsten Carbide Slip Gauges, Ceramic Slip Gauges

Tungsten Carbide Slip Gauges are manufactured from speciallyselected grade of Carbide with hardness of 1500 Vickers. igid

attention is paid throughout the manufacturing process forflatness! parallelism and e"ery slip gauge is rigorously checked

and calibrated in #$%& accredited laboratory.

Why Tungsten Carbide ?

The mere suspicion that carbide is a brittle material had heldback its use for 'uite some time! till a number of actual trialswere conducted. (t is some 50 years! since tests in the )S$

indicated that hard metal slips could outlast steel by at least tentimes and in certain conditions e"en 100 times.

Features

*imensional Stability

$bility to +ring

esistance to Corrosion

esistance to +ear

,conomy


22/23

Tungsten Slip Gauges Ceramic Slip Gauges

Salient Features

CORROSION Resistant

G)naffected by water! acids

and alkalis.G imple maintenance with no

re?uirements of any anti 1corrosion treatment

G "o adverse effects due tofingering while using.

Resistant to I!"CTG .-irconia ceramic blocks

are hard and highly tough

to withstand the knocks

and drops that occur during

use and will not chip orfracture easily

Resistant to W#"R

G $bout 10 times life or e"enmore compared to steel slip

gauges. The natural

stability and durability dueto its low friction co

efficient maintains theirgeometry longer.

SuperiorWRING"$I%IT& G The uniform and close

grain structure enables

the blocks rigidly wrung

together and easy tooperate

G

*ents and burns are not

easily produced on gaugesurface

T'#R"% e(pansion

G

The lowest thermal

conducti"ity makes thesegauge blocks easy to use

at different temperatures.


23/23

1.Length Standards

Documents

Transcript of 1.Length Standards