Experiment No 3

Experiment No:3 Block Gauges :Objectives .To familiarize the student with the types, applications of block gauges .To learn how to use block gauges in calibration -

: Apparatus .Set of block gauges -1 .Surface plate-2 .utside micrometer -3

:Theory Gauge blocks !ccurate measurements of length can be accomplished only through comparison with . a standard, preferably one that is traceable to a primary standard "orking standards for the measurement of length, such as line standards and end standards are essential to manufacturing. Two marks engra#ed on a dimensionally stable material define length on a line standard. The length of an end standard is the .distance between its flat, parallel end faces

,$auge blocks are the most often used length standards. These are small, steel blocks usually rectangular in cross section, ha#ing two #ery flat and parallel surfaces that are a certain specified distance apart. $auge blocks %figure 3.1& are usually made of alloy steel or cemented carbide, hardened % to increase wear resistance & and carefully heat .treated to relie#e internal stresses and minimize subse'uent dimensional change The measuring surfaces of each bock are ground and then lapped to produce #ery flat and smooth surfaces that are parallel within a few tenths of a micrometer. "hen such surfaces are slid together with a slight contact pressure, the adhere with a significant force % adhesion is due to molecular attraction and atmospheric pressure &( this  procedure . of combining gauge blocks blocks is called wringing $auge blocks usually come in sets of #arious sizes and by wringing you can obtain .#arious combinations

!igure 3!" gauge blocks set

)$auge blocks also called slip gauges-are made in fi#e grades or classes or accuracy .$rade **, +alibration $rade, $rade *, $rade   $rade  +alibration grade has the highest le#el of accuracy in normal engineering practice and it is intended for calibrating other blocks. They are used where tolerance are < .2µm and are not intended for general gauge inspection $rade *, , are intended for general use. $rade * is intended for high precision uses, grade  is for general purpose manufacturing gauge %they are used with sine  bars&, . and grade  is intended for rough setting purposes and checking components )nstruction for wringing together two slip gauges ,Surfaces must be clean and free from burrs. They should be washed in petrol .1  benzene, carbon tetrachloride or other dielectric-greasing agents and dry on a .clean cloth. Then be wiped with clean soft chamois leather  Slip gauges then should be held across one another at right angles and wring them .2 .with a rotary motion( this reduces the amount of surface rubbing ! minute amount of grease or moisture must be present between the surfaces for  .3 them to wring satisfactorily. nless a #ery firm is obtained, there is always a  possibility . that the wringing film may be a micrometer thick  t is ad#isable to take the following points into consideration) n order to pre#ent )damage to gauge blocks ./rotect from dust, dirt and moisture .1 .!#oid magnetization .2 0andle lapped faces as little as possible to pre#ent etching from finger acid. "ipe .3 .all finger marks with clean soft chamois leather  .!lways wipe faces immediately before use e#en when use is continuous . !lways replace clean gauges in their bo and close it after use. f gauges are not . .in fre'uent use they should be coated to pre#ent corrosion 5o not handle gauges abo#e the open bo( they may cause damage to other  .4 .gauges if dropped

igure 3!# metho$ o% &ringing slip gauges

) 6ot .8or longer dimensions, length bars are used 7 To combine gauge blocks, one should use the minimum number of blocks in 7 .order to reduce wear and the accumulated error on the whole combined length

'alibration +alibration is the act or result of 'uantitati#e comparison between a known standard and the output of the measuring system measuring the same 'uantity. n a way, the  process of calibration is in effect the procedure for determining the scale of the . measuring system 8or eample, by application of a range of known #alues for the input and obser#ation of the system output, a direct calibration cur#e can be de#elop for the measurement system. n such a cur#e the input,  x, is plotted on the abscissa against the measured output, y, on the ordinate. ! calibration cur#e forms the logic by which a measurement system9s indicated output can be interpreted during an actual measurement . The calibration cur#e can be the basis for fiing the output display .scale on a measurement system

igure 3!3 'alibration o% a micrometer using block gauges

(roce$ure se the pro#ided metric unit slip % block & gauge set, list the slips to be wrung to .1  produce the following o#erall dimensions using the minimum number of block  .gauges se the block gauges set to calibrate the pro#ided micrometer, calculate the error .2 .in each reading, draw the calibration cur#e

:)esults .>eading 6o

Size or gauge block  ;icrometer reading %= %mm %< %mm 1 2 3   4

*iscussion an$ 'onclusions

%:rror %mm