linear measurement

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HELPFUL FOR UNDERSTAND CHAPTER LINEAR MEASUREMENT MECH. ENGG

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MECHANICAL MEASUREMENT & METROLOGY LINEAR MEASUREMENT:

MECHANICAL MEASUREMENT & METROLOGY LINEAR MEASUREMENT By: Samir Jariwala

Definition:- It is the measurement of perpendicular distance between two point or surfaces.:

Definition:- It is the measurement of perpendicular distance between two point or surfaces. Instrument Use for Linear Measurement:- Vernier calliper Vernier height gauge Vernier depth gauge Micrometer Engineer’s steel rule or scale Callipers Bore gauge Telescopic gauge Slip gauge Dial indicator Comparators

Vernier calliper:- :

Vernier calliper :- Construction:- -It consist of two scale one is fixed called main scale & other is moveable called vernier scale. - For precise setting of the moveable jaw an adjustment screw is provided with movable jaw. - The vernier is also facilitated to lock the sliding scale with the fixed main scale.

Working:-:

Working:- - The work piece to be measured is kept between the jaws for measuring the outside dimension. The fine adjustment can be done by adjustment screw. Movable jaw is made fixed on main scale frame by clamping screw. The vernier calliper is taken from work piece for taking the reading from the main scale as well as the vernier scale. first the main scale reading is taken then, the vernier scale is examined to determine which of its division coincides with a division on the main scale. Least count =1/10=0.1 mm Diameter of work piece = Main scale reading +(No. of vernier division coinciding with main scale*least count)

Errors in Vernier calliper:-:

Errors in Vernier calliper:- Errors due to play between the sliding jaw & fixed scale bar. Zero error due to wear & wrapping of jaws. Errors due to incorrect observation of scale reading. Errors due to excessive force on moving jaw. Errors is also introduced if the line of measurement does no coincide with the line of the scale.

Vernier height gauge:- :

Vernier height gauge :- Construction:- - It is similar to the vernier calliper except that the fixed jaw in this case is replaced by fixed base which rests on the surface plate or table The upper & lower surfaces of the measuring jaw parallel to base so it can be used for measurement over or under a surface. The scriber when attached vernier is used for inspection of parts & for layout work. In some cases scriber is replaced by dial indicator .

Vernier depth gauge:- :

Vernier depth gauge :- Construction:- - It is used for measuring depths of holes, slots, recesses and distance from a plane surface to projection. - Before using the gauge make sure that the gauge base is satisfactorily true flat & square. - Then lower the blade into the hole until it contacts the bottom surface of hole. - The final adjustment is depends upon a sense of contact & is made by the final adjustment screw. - The instrument is then removed from the hole & reading is taken same as vernier calliper.

Micrometer:-:

Micrometer :- Principle:- -Micrometer works on the principle of screw & nut. -The screw is attached to a concentric cylinder which is divided into number of equal parts. -When the screw is turned through nut by one revolution, its axial movement is same as pitch of the thread of screw.

Least count:- -In a millimeter micrometer instrument the screw has a pitch of 0.5mm and thimble has 50 division so, The least count(L.C) = Value of smallest division on main scale or pitch No. of division on thimble =0.5/50 =0.01mm:

Least count:- -In a millimeter micrometer instrument the screw has a pitch of 0.5mm and thimble has 50 division so, The least count(L.C) = Value of smallest division on main scale or pitch No. of division on thimble =0.5/50 =0.01mm Types of micrometers:- 1. Outside micrometer 2. Inside micrometer 3. Vernier micrometer 4. Depth micrometer 5. Bench micrometer 6. Digital micrometer 7. Differential screw micrometer 8. Micrometer with dial gauge 9. Screw thread micrometer

Construction:- -It consists of 1. U-shaped frame 2. Anvil 3. Spindle 4. Thimble 5. Barrel or sleeve 6. Rachet 7. Locknut. -It is made of steel, cast iron or light alloy. -The gap between two ends of U-shape frame permits maximum diameter or length of job to be measured. :

Construction:- -It consists of 1. U-shaped frame 2. Anvil 3. Spindle 4. Thimble 5. Barrel or sleeve 6. Rachet 7. Locknut. -It is made of steel, cast iron or light alloy. -The gap between two ends of U-shape frame permits maximum diameter or length of job to be measured. -The one end of spindle is attached to the thimble and other is to movable anvil. -The screw thread is made on the spindle with specific pitch. -The barrel is accurately divided & clearly marked in 0.5mm along its length on main scale. -The thimble has 50equal division around its circumference & each division has value of 0.01mm.

- When anvil & spindle is bought together the instrument reads zero. - The ratchet is provided at end of thimble. It is used to assure accurate measurement & to prevent too much pressure applied on micrometer. - When the spindle reaches near the work surface the operator use the ratchet screw to tighten the thimble. - The ratchet is automatically slips when correct pressure is applied & prevent too much pressure. - The locknut is provided on spindle to lock & when the micrometer is at correct reading. :

- When anvil & spindle is bought together the instrument reads zero. - The ratchet is provided at end of thimble. It is used to assure accurate measurement & to prevent too much pressure applied on micrometer. - When the spindle reaches near the work surface the operator use the ratchet screw to tighten the thimble. - The ratchet is automatically slips when correct pressure is applied & prevent too much pressure. - The locknut is provided on spindle to lock & when the micrometer is at correct reading. Errors in micrometers:- The face of anvil & spindle may not be truly flat. Lack of parallelism and squareness of anvil or spindle at some or all part of scale. Setting of zero reading may be inaccurate. Inaccurate reading shown by fractional division on the thimble. Wear on the face of anvil, spindle the threads of spindle. Error due to too much on the thimble.

Engineer’s steel rule or scale:-:

Engineer’s steel rule or scale :- -It is the simplest line measuring instrument. It is made from hardened steel or stainless steel. Its accuracy graduated in mm or inch division. It works on basic measuring technique of comparing unknown length to the one previously calibrated. It manufactured in different size & style of 150mm,300mm,600mm,1000mm length.

Callipers:-:

Callipers :- -In some cases the dimensions of component can not be measured directly by scale. In this case callipers uses with scale for transferring length to the scale. It consist of two legs which hinged at top and end of legs made from carbon & alloy steel. Where high accuracy is necessary callipers not used. Classification:- 1.Based on design aspect (a)Firm joint callipers (b)Spring type callipers 2. Base on their use (a)Outside callipers (b)Inside callipers (c)Transfer callipers (d)Odd leg callipers

Bore gauge:-:

Bore gauge :- -It is used for measure the small bores. -There are different types of bore gauge but the mostly used are 1.Dial bore gauge & 2.Hemispherical bore gauge Dial bore gauge:- -It measure the bore directly. -It work on proven principle of two diametrically opposed measuring points, one fixed & one moving. -The measuring body is connected with the dial indicator by tube. -The push rod is provided inside the tube to transmit the movement of measuring head to dial indicator. -So, the horizontal movement is convert into vertical movement & dial indicator gives indication of variation of size. -The accuracy of bore gauge is 0.001mm -It is used in injection molding or in barrel. -It is rapid & accurate checking of bore for size, ovality, tapper & wear within a deep bore.

Hemispherical bore gauge:-:

Hemispherical bore gauge:- -This gauge consist of spring steel arms having Hemispherical ends. Inside the arm wedge shaped member attached to spring which moved endwise by rotating screw head. Due to up & down of wedge member with spindle, the hemispherical ends expand or contracts as per size. Then the bore is taken out & the reading is noted down by measuring over the hemispheres with an micrometer. This bore gauge is available in 4sizes to measure 3mm to 12mm diameter of bore.

Telescopic gauge:-:

Telescopic gauge:- It is an indirect measuring device & used for measuring internal diameter of holes, slots & grooves. It consists of handle, two telescopic rods & locking screw. One end of handle is connected with tube in which telescopic rods slides which has spherical contacts & it is forced by internal spring. For taking measurement, telescopic rods are compressed against spring & inserted into the hole whose diameter to be measured. Then extended up to the walls of hole. Then they locked by locking screw & rods can be measured by micrometer or vernier caliper. This is use in measure the diameter of IC Engines.

Slip gauge:-:

Slip gauge :- -It is also known as gauge blocks. -They are universally accepted as end standard of length in industry. -It is a rectangular block made up with high grade hardened steel. -It is able to resist wear. -They are independent of any variation of size & shape. They are stabilized by heating & cooling successively in stages so it removed its hardening stress. -After being hardened they are carefully finished by high grade lapping to the high degree of finish, flatness, and accuracy. -They are generally available in sets. -The slip gauge is inspected time to time by standard gauge blocks. -Tool steel, chrome plated steel, stainless steel, tungsten carbide and chrome carbide are used for making gauge.

Wringing of slip gauge:-:

Wringing of slip gauge:- -If two slip gauge are wrung together to each other and considerable pull is required to break the wring. -This phenomenon of wringing occurs due to molecular adhesion between a liquid film and mating surface. -This wringing process is used to build up desired dimension over a size of specific increment. -It depends on the surface finish and flatness of the blocks used and absence of dirt, grease, burrs and scratches. -When it correctly wrung together, the error in total length is negligible.

Application:-:

Application:- They are used to check the accuracy of vernier, micrometer, and other devices. They are used to set the comparator to a specific dimension. They are used for direct measurement where the accuracy of work piece is important. They are frequently used with sine bar to measure angle of work piece. They can be used for check gap between parallel location.

Dial indicators:-:

Dial indicators :- It is used for measuring and checking linear dimension. It is simpler to use than other instrument such as vernier and micrometer. It consist of one round dial which has two pointer one is main & other is smaller. It acts with the help of rack & pinion. The movement of spindle is transferred by rack to small pinion from where it transferred to the pointer through a gear train. The main dial is divided into the 100 equal division, each division shows the 0.01mm means for 1mm spindle moves 1 revolution. The small pointer records the number full turns of main pointer. It is very easy to use & less expensive than other instrument. Once the indicator is installed unskilled person easily operate it.

comparators:

comparators Comparators is a precision instrument used for comparing the dimensions of a work piece with a working standards or master setting which represents the basic size.

comparators:

comparators Classification of comparators: Mechanical comparators Reed type comparator Sigma comparators Johansson mikrokrator Mechanical optical comparators Electrical and Electronics comparators Pneumatic comparators Fluid displacement comparators Projection comparators Multi-check comparators Automatic gauging machines Electro-mechanical comparators High sensitive calibration comparators

MECHANICAL COMPARATORS:

MECHANICAL COMPARATORS Johansson mikrokator Working: This comparator works on principle that when a tension is applied to the twisted strip,this causes the strip to untwist resulting in rotation of pointer which attached at centre of twisted strip. Construction: The comparators consists of a twisted metal strip as shown in fig.and the thin glass pointer is attached at the centre of strip. The two halves of the strip from the centre are twisted in opposite directions so that any tension or pull on the strip will cause the centre to rotate. A measuring plunger moves upward due to contact with the work piece, the bell crank lever pulls the twisted strip. Hence the point which is attached at centre of twisted strip rotates. In order to prevent excessive stress on the central portion, the strip is perforated along the centre line.

PowerPoint Presentation:

Magnification: The magnification of this instrument is equal to the ratio of rate change of pointer rotation, to rate change in length of strip( dL ). The magnification of the instrument is given by, where, ѳ =twist of spring at centre with respect to the end L=length of twisted strip measured along its neutral axis w=width of twisted strip n=number of turns From above equation following points are noted: If length of strip increases, the magnification of instrument is increased. If width of strip decreases , the magnification is increases. The magnification is inversely proportional to the number of turns.

Reed type mechanical comparators:

Reed type mechanical comparators A reed type comparator uses the frictionless reed mechanism for magnifying small motions of the spindle. Working and construction: The reed type mechanical comparators as shown in fig., consists of a fixed block a which is rigidly fastened to the gauge head case and floating block B which carries the measuring spindle and connected horizantally to block A by reads C. A vertical reed c is attached to each block with upper ends joined together. Beyond this joint extend a pointer. Since vertical reeds are joined at their upper end instead of slipping the movement causes both reeds swing through an arc. The amount of reeds swing or pointer rotation is proportional to the distance the floating body B moves with the spindle . The mechanical amplification is possible by 100.

Sigma comparator:

Sigma comparator The sigma comparator works on leverage principle in which high magnification is obtained entirely by mechanical means. The magnification ranges from 300 to 5000 obtained by sigma comparators. Construction & working: The sigma comparator consists of plunger as shown in Fig , is mounted on two flat steel strings, this provides a frictionless linear movement for the plunger. The plunger carries a knife edge, which bears upon the face of the mounting block of a cross strip hinge. a thin phosphorus bronze ribbon is fastened to the end of forks of Y arm and wrapped around a small drum, mounted on a spindle carrying the pointer. A measuring plunger with knife edge moves upward due to contact with work piece get angular rotation about pivot. This causes the rotation of the Y arms. The metallic band attached to the arms makes the driving drum and hence pointer to rotate. If the length of the Y-arm is l and the distance from the hinge pivot to the knife edge is a, then the first stage of magnification is l/a If the radius of drum attached to the spindle is r and pointer radius is R, then second stage of magnification is R/ r.hence total magnification is given by M=l/a*R/r

PowerPoint Presentation:

Advantage of mechanical comparators Easy to handle. They do not required any external electrical energy. They are cheaper compare to other. They are portable. Disadvantage of mechanical comparators less accuracy. Less sensitive. Friction is more. Less magnification.

PowerPoint Presentation:

. Mechanical-optical comparators In this type of comparator, the small displacement of measuring plunger are amplified first by mechanical system and further amplification takes place by optical system. Working principle of optical comparator : The optical comparators works based on fundamental optical law as the edge of the shadow is projected on a curved graduated scale to indicate the comparison measurement. This is explained with the help of optical lever. In optical comparator, the mirror is deflected or titled by plunger movement. The movement of the reflected light is recorded as an image on screen. The main advantage using optical lever in comparator is that the reflected light beam deflects twice the angle of rotation of the mirror, this gives large magnification. Mechanical optical comparators consists of plunger which is attached with lever and a mirror on which the light strikes from the source. Let, plunger moves upward through distance dy , this causes the movement of mirror at lever end by dz. :. dz = dy *L2/L1 Hence mechanical amplification=L2/L1 Hence optical amplification=2*L4/L3 Therefore, overall magnification of the comparator = 2*L2/L1*L4/L3

PowerPoint Presentation:

Working principle of optical comparator : The optical comparators works based on fundamental optical law as the edge of the shadow is projected on a curved graduated scale to indicate the comparison measurement. This is explained with the help of optical lever. In optical comparator, the mirror is deflected or titled by plunger movement. The movement of the reflected light is recorded as an image on screen. The main advantage using optical lever in comparator is that the reflected light beam deflects twice the angle of rotation of the mirror, this gives large magnification.

PowerPoint Presentation:

Advantage of optical comparator: They have a less number of mechanical moving parts. gives higher accuracy. They have high magnification. Disadvantage of optical comparators: They requires the light source. The comparators is usually large and expensive. When the scale is projected on a screen, then it is necessary to use these instrument in a dark in order to take reading usually.

Pneumatic comparators:

Pneumatic comparators Working principle: When air passes through the narrow constant space between nozzle and the work piece the pressure of air is remain constant. This change in pressure is utilized to measure the deviation of the dimensions being measured from the standard dimensions. The magnification pneumatic comparator may be 30,000:1. The pneumatic comparator is normally used to determine the roundness of the job. Pneumatic comparator consists of vertical metal cylinder contains water to a predefined level and a dip tube immersed into it upto a depth corresponding to air pressure required. The system designed in order to supply at constant pressure to measuring jets. If the pressure of the air supplied higher than the predefined pressure, some air will be bubble out from the bottom of the dip tube and air moving to control orifice will be predefined constant pressure. But, when restriction is provided at the measuring jets, resulting increases the back pressure and level of water in manometer fall down which vary the height of water in manometer tube, a calibrated scale being set beside this tube. In other words the restriction to the escape of air depends upon the variations in the dimensions to be measured.

PowerPoint Presentation:

Advantage of pneumatic comparator There is no direct contact between the comparator measuring head and work piece, hence no wear takes place on the measuring head. They less number of moving parts hence less friction. Very high magnification can be achieved. Disadvantage of pneumatic comparator Scale is generally not uniform This comparator is not portable and hence may not useful for the industrial applications. It has less sensitivity compared to electrical and electronic comparator.

Electronic comparator:

Electronic comparator The electronic comparator works based on the principle of application of frequency modulation or radio oscillation. The changed frequency is annulled by suitable controls to restore it back to the basic value. The measuring instrument dial is graduated directly in terms of the physical standards of length which compare with the electronic standards of wavelength. This type of comparator is used for internal/external gauging, thickness gauging, flatness testing, gauging on curved or flat surfaces and measurement of electro-plate coating.

PowerPoint Presentation:

Advantage of electrical & electronic comparator They have small number of moving parts hence less friction, wear and accuracy is more. This comparator has a high magnification. Measuring unit can be remote the indicating instrument. The mechanism carrying the pointer is light and not sensitive to vibration. The operation of the comparator on A.C. supply reduces sliding friction errors. The measuring unit can be made very small and indicating unit need not be placed closed to the measuring unit. Small and compact.

PowerPoint Presentation:

Disadvantage of electrical & electronics comparators: The comparator requires electrical energy to operate. Heating of coils in the measuring unit may cause zero drift and alter the calibration. Fluctuation in the voltage or frequency of the electric supply may affect the results. When measuring unit is remote from the indicating unit, reliability is lower. Cost of comparator is more than other comparator. If only a fixed scale is used with a moving pointer with high magnification, a very small range is obtained.

Electrical comparator:

Electrical comparator Electrical comparators are also known as electro-mechanical measuring system as these employ an electromechanical device which converts a mechanical displacement into in electrical signal. Linear variable differential transformer is the most common electro-mechanical device used to convert mechanical displacement into electrical signal. displacement L.V.D.T voltage

PowerPoint Presentation:

Construction & working LVDT consists of three coils wound on a single non-magnetic tube. There is primary winding fed with a.c . supply and two secondary winding. The two secondary windings are symmetrically placed, are identical and are connected in series opposition. A perromagnetic core attached to the moving part which freely move inside the non-magnetic tube. When a.c . current is supplied to primary winding, the magnetic flux generated by the coil is distributed by the armature so that voltages are induced in the secondary winding. The position of the magnetic core determinates the flux linkages with each winding. When the core is placed centrally, at reference position, equal and opposite voltage induced in secondary windings. Hence net output from transformer equal to zero, since E2=E1 .∙.E2-E1=0. This is called the null position of armature. A variation in the position if the armature from its null position produces an output in terms of voltage. As the armature moves towards left,E2 increases & E1decreases. The net available is Vo=E2-E1 and in phase with E2. Similarly, as armature moves towards right E1 increases & E2 decreases. The net voltage available is Vo=E1-E2 and in phase with E1. It is observed that the magnitude of Vo is a function of the distance moved by the armature and its polarity or phase indicates as to in which direction it has moved. Hence, pressure acting against the elastic element like bourdon tube, bellows, or diaphragm can be measured in terms of unbalance voltage Vo between two secondary windings.

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