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 eample, 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 fiing 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
