MEASUREMENTS AND METROLOGY

MAYANK BAHETI (1RV09IM024)

MAHESH CHANDRA (1RV09IM021)

DEEPAK RATHOD (1RV09IM009)

BY:-

COMPARATORS

IntroductionA comparator works on relative measurements, i.e. to say, it gives only dimensional

differences in relation to a basic dimension. So a comparator compares the unknown

dimensions of a part with some standard or master setting which represents the basic size

anddimensional variations from the master setting are amplified and measured. The

advantages of comparators are that not much skill is required on the part of operator in its

use. Furtherthe calibration of instrument over full range is of no importance as comparison

is done with astandard end length. Zero error of instrument also does not lead to any

problem. Since rangeof indication is very small, being the deviation from set value, a high

magnification resultinginto great accuracy is possible. The comparators are generally used

for linear measurements,and various comparators available differ principally in the method

used for amplifying and recording the variations measured. According to the principles used

for obtaining suitabledegrees of magnification of the indicating device relative to the change

in the dimension beingmeasured, the various comparators may be classified as follows :

(1) Mechanical comparators (2) Mechanical-optical comparators

(3) Electrical and Electronic comparators (4) Pneumatic comparators

(5) Fluid displacement comparators  (6) Projection comparators

(7) Multi-check comparators  (8) Automatic gauging machines.

Uses of Comparators

The various ways in which the comparators can be used are as follows :(i) In mass production, where components are to be checked at a very fast rate.(ii) As laboratory standards from which working or inspection gauges are set and correlated.(iii) For inspecting newly purchased gauges.(iv) Attached with some machines, comparators can be used as working gauges toprevent work spoilage and to maintain required tolerances at all stages of manufacturing.(v) In selective assembly of parts, where parts are graded in three or more groupsdepending upon their tolerances.

Mechanical ComparatorsIn these comparators, magnification is obtained by mechanical linkages and othermechanical devices.

Systems of Displacement Amplification used in Mechanical Comparators

(i) Rack and Pinion. In it the measuring spindle integral with a rack, engages a pinionwhich amplifies the movement of plunger through a gear train. (Refer Fig. 5.1)

Fig. 5.1. Rack and pinion.

(ii)Cam and gear train: In this case the measuring spindle acts on a cam whichtransmits the motion to the amplifying gear train. (Refer Fig. 5.2)

Fig. 5.2. Cam and gear train

(iii)Lever with toothed sector. In this case a lever with a toothed sector at its endengages a pinion in the hub of a crown gear sector which further meshes with a final pinion to produce indication with band wound around drum.

 Fig. 5.3. Lever with toothed gear.

(iv) Compound Levers. Here levers forming a couple with compound action areconnected through segments and pinion to produce final pointer movement.

 Fig. 5.4. Compound levers.

(v) Twisted Taut Strip. The movement of measuring spindle tilts the knee causingstraining which further causes the twisted taut band to rotate proportionally. The motion of strip is displayed by the attached pointer.

 Fig. 5.5. Twisted taut strip.

(vi) Lever combined with band wound around drum. In this case, the movementof the measuring spindle tilts the hinged block, causing swing of the fork which induces rotation

Fig. 5.6. Lever combined

The Johansson 'Mikrokator'.This comparator was made by C.F. Johansson Ltd. and therefore named so. It is shown

diagrammatically in Fig. 5.7. This instrument uses the simplest and most ingenious method

for obtaining the mechanical magnification designed by H. Abramson which is called

Abramson Movement. It works on the principle of a button spinning on a loop of string. A

twisted thin metal strip carries at the centre of its length a very light pointer made of thin

glass. The two halves of the strip from the centre are twisted in opposite directions so that

any pull on the strip will cause the centre to rotate. One end of the strip is fixed to the

adjustable cantilever strip and the other end is anchored to the spring elbow, one arm of

which is carried on the measuring plunger. As the measuring plunger moves either upwards

or downwards, the elbow acts as a bell crank lever and causes twisted strip to change its

length thus making it further twist or untwist. Thus the pointer at the centre of the twisted

strip rotates by an amount proportional to the change in length of strip and hence

proportional to the plunger movement. The bell crank lever is formed of flexible strips with a

diagonal which is relatively stiff.The length of cantilever can be varied to adjust the

magnification of the instrument. Since the centre line of the strip is straight even when

twisted, therefore, it is directly stretched by the tension applied to the strip. Thus in order to

prevent excessive stress on the central portion,the strip is perforated along the centre line

by perforations as shown in Fig. 5.7.

 

Fig. 5.7. Johansson Mikrocator. 

mid-point of strip with respect to the end, I is the length of twisted strip measured along its

neutral axis and w is the width of twisted strip and n is the number of turns.

It is thus obvious that in order to increase the amplification of the instrument a very

thin rectangular strip must be used. Further amplification can be adjusted by the cantilever

strip- which also provides anchorage. It increases or decreases effective length of strip.

Final

setting of the instrument amplification is made by a simple adjustment of the free length of

cantilever strip.

A slit C washer as shown in Fig. 5.7 is used for the lower mounting of plunger. Thus

this instrument has no mechanical points or slides at which wear can take place. The

magnification of the instrument is of the order of x 5000.

Dial Indicator.One of the most commonly used mechanical comparators isessentially of the same type as a dial indicator. It consists of a robust base whose surface isperfectly flat and a pillar carrying a bracket in which is incorporated a spindle and indicator.The linear movement of the spindle is magnified by means of a gear and pinion train intosizable rotation of the pointer on the dial scale. The indicator is set to zero by the use of slipgauges representing the basic size of the part. This is generally used for inspection of smallprecision-machined parts. This type of comparator can be used with various attachments so

that it may be suitable for large number of works. With a V-block attachment it can be usedfor checking out-of-roundness of a cylindrical component.

Reed Type Mechanical Comparator.In mechanical comparator, the gauging head is usually a sensitive, high quality, dial

indicator mounted on a base supported by a sturdy column. Fig. 5.8 shows the reed type

mechanical comparator.The reed mechanism is frictionless device for magnifying small

motions of spindle. It consists of a fixed block A which is rigidly fastened to the gauge head

case, and floating block B, which carries the gauging spindle and is connected horizontally

to the fixed block by reeds C. A vertical reed is attached to each block with upper ends

joined together. These vertical reeds are shown in the figure by letter D. Beyond this joint

extends a pointer or target. A linear motion of the spindle moves the free block vertically

causing the vertical reed on the floating block to slide past the vertical reed on the fixed

block. However, as these vertical reeds are joined at the upper end, instead of slipping, the

movement causes both reeds swing through an arc and as the target is merely an extension

of the vertical reeds, it swings through a much wider arc. The amount of target

swing is proportional to the distance the floating block has moved but of course very much

magnified.The scale may be calibrated by means of gauge block (slip gauges) to indicate

any deviation from an initial setting.Comparators using this type of linkage have

sensitivities of the order of 0.25 micron per scale division.The mechanical amplification is

usually less than 100, but it is multiplied by the optical lens system. It is available in

amplifications ranging from x 500 to x 1000.

Fig. 5.8. Reed type mechanical comparator.

The Sigma ComparatorFig. 5.9 shows the constructional details of the Sigma Mechanical Comparator. The vertical beam is mounted on flat steel springs A connected to fixed members, which in turnare screwed to a backplate. The assembly provides a frictionless movement with a restraint from the springs.The shank B at the base of the vertical beam is arranged to take a measuring contact, selecting from the available range. The stop C is provided to restrict

movement at the lower extremity of the scale.Mounted on the fixed members, is the hingedassembly D carrying the forked arms E. This assembly incorporates a hardened fulcrum (provided with means for adjustment of controlling the ratio of transmitted motion) operative on the face of a jewelled insert on the flexible portion of the assembly.The metal ribboni^, attached to the forked arms,passes around the spindle G causing it to rotate in specially designed miniature ball bearings. Damping action to the movement is affected by a metal disc,mounted on the spindle, rotating in a magnetic field between a permanent magnet and a steel plate. The indicating pointer H is secured to a boss on the disc.

Fig. 5.9. Sigma mechanical comparator.

The trigger J (opposite K) is used to protect the measuring contact. At the upper end ofthe vertical beam, an adjusting screw is provided for final zero setting of the scale.A new patented feature is shown at K. This is a magnetic counter-balance which servesto neutralise the positive "rate" of springs reaching on the measuring tip. In this way a constant pressure over the whole scale range is achieved.The instrument is available with vertical capacities of 150 mm, 300 mm and 600 mm and magnifications of 500,1000,1500,3000 and 5000. The scales are graduated in both English and Metric systems.The least count which one division represents is of the order of 0.25 microns.Advantage : It has got a bold scale and larger indicating pointer.Disadvantages : (i) Due to motion of the parts, there is wear in the moving parts.(ii) It is not as sensitive as optical or other types of comparators due to friction beingpresent in the moving parts.

Diagrammatic sketch showing the movement of sigma comparator.The various movements in the 'sigma' comparator discussed in Art. 5.5.6, will be very clear

with the help of Fig. 5.10 which shows the various movements in diagrammatic form.

 

Fig. 5.10. Movement of sigma comparator.

The plunger in Fig. 5.10 is mounted on a pair of slit diaphragms in order to have

frictionless linear movement. A knife edge is mounted on it and bears upon the face of the

moving member of a cross strip hinge. For details of cross strip hinge refer Fig. 5.11. The

cross strip hinge consists of the moving component and a fixed member which are

connected by thin flexible strips alternately at right angle to each other. Thus if an external

force is applied to the moving member ; it will pivot, as would a hinge, about the line of

intersection of the strips.To the moving member an arm of Y shape and having effective

length I is attached. If the distance of the hinge from the knife edge be a then the

magnification of the first stage

 

Fig. 5.11. The cross strip hinge used in sigma comparator.

 

(a) Comparator (b) Movement used in comparator.

Fig. 5.12. Mechanical comparator using rocking prism.

In order to adjust the magnification, distance a must be changed by slackening and

tightening the two screws attaching the knife edge to the plunger.

Some of the interesting features of the instrument are :

(1) As the knife edge moves away from the moving member of the hinge and is followed

by it, therefore, if too robust movement of the plunger is made due to shock load, that will

not be transmitted through the movement.

(2) By mounting a non-ferrous disc on the pointer spindle and making it move in field

of a permanent magnet, dead beat readings can be obtained.

(3) The error due to parallax is avoided by having a reflective strip on the scale.

(4) The constant measuring pressure over the range of the instrument is obtained by

the use of a magnet plunger on the frame and a keeper bar on the top of the plunger. As the

plunger is raised the force required increases but the keeper bar approaches the magnet

and the magnetic attraction between the two increases. Thus as the deflecting force

increases,the assistance by the magnet increases and total force remains constant.

Mechanical ComparatorThe diagram of a type which is used for comparative measurement of external sur-faces is shown in Fig. 5.12 (a). In this instrument the movement of the measuring tip attached at the end of the spindle is transmitted to the pointer through a mechanism shown in Fig.5.12 (b). The upper end of the spindle bears against a rocking prism (knife-edge). There is a frame member having two V-slots offset to each other by distance a. The end of the spindle rests against the first V-slot and its movement is transmitted to this frame through a prism. A knife edge which is stationary relative to the body of the instrument enters the upper V-slot.The apex of the upper knife-edge is the centre for all the movingparts of the comparator. The distance a between; the V-slots forms the shorter lever arm of the system, whereas, the longer lever arm is the distance L from the centre of rotation of the system to the other

end of hand,

Fig. 5.13. Mechanical comparatorusing sector and pinion.which moves along the comparator scale. Thus the magnification of the instrument is L/a and is of the order of 1000. The contact pressure is of the order of 300 to 400 grammes and is provided by a spring. The use of knife-edge pivots in the comparator movement excludes the influence of possible clearance in the pivots on the accuracy of this instrument. Yet another type of mechanical comparator is shown in Fig. 5.13 in which the movement of the plunger (contact member) is transmitted to the pointer through an angular lever which ends in the form of a sector and pinion. It may be noted that a spring attached with the angular lever avoids the play in comparator.

Mechanical Optical Comparators In mechanical optical comparators small displacements of the measuring plunger areamplified first by a mechanical system consisting of pivoted levers. The amplified mechanicalmovement is further amplified by a simple optical system involving the projection of an image.The usual arrangement employed is such that the mechanical system causes a plane reflectorto tilt about an axis and the image of an index is projected on a scale on the inner surfac^fftaground-glass screen. Optical magnification provides high degree of measuring precision |Mpto reduction of moving members and better wear resistance qualities. Optical magnificatioijis__also free from friction, bending, wear etc.

 Fig. 5.14. Principle of Optical Comparator.The whole system could be explained diagrammatically by Fig. 5.14, which gives verysimple arrangement and explains the principle of above comparator. In this system, Mechanical amplification = tyli and Optical amplification = x 2. It is multiplied by 2, because if mirror is tilted by an angle 60, then image will be tilted by 2 x 66. Thus overall magnification of this system = 2 x (Z^i) (J4//3).Thus it is obvious that optical comparators are capable of giving a high degree of measuring precision owing to high magnification and the reduction of moving members to minimum. Further these possess better wear resistance qualities as the only wearing members are the plunger and its guide and the mirror pivot bearing. Another advantage of the optical comparators is that provision of an illuminated scale enables readings to be taken without regard to the room lighting conditions. The point of importance in optical comparator is that mirror used must be of front reflection type and not of normal back reflection type. In normal back reflection type there are two reflected images, one each from front and back. Thus the reflected image is not well defined one, as one bright and other blurred image are observed. If front reflection type of mirror is used, then it requires considerable care in its use to avoiddamage to the reflecting surfaces.

Zeiss Ultra-optimeter.

The optical system of this instrument involves double reflection of light and thus gives higher degree of magnification. (Refer Fig. 5.16) A lamp sends light rays to green filter, which filters all but green light, which is less fatiguing to the eye. The green light then passes to a condenser which via an index mark projects it on to a movable mirror Mi, whence it is reflected to another fixed mirror M2, and then back again to the first moveable mirror. The second objective lens brings the reflected beam from the first mirror to a focus at a transparent graticule containing a precise scale which is viewed by the eye-piece. The projected image of index line on the graticule can be adjusted by means of screw in order to set the zero. When correctly adjusted, the image of the index line is seen against that of the graticule scale. The special end of the contact plunger rests against the outer end of the first movable mirror so that any vertical movement of the plunger will tilt the mirror. The extreme sensitivity of this instrument necessitates special precautions in its operation to avoid temperature effects.

 

Fig. 5.16. The Optical arragnement of the Zeiss Ultra-optimeter.

Electrical and Electronic Comparators These comparators depend for their operation on Wheatstone bridge circuit. In d.c.circuit, a change of balance of the electrical resistance in each arm of the bridge is caused bythe displacement of an armature relative to the arm under the action of the measuring plunger.Once out of balance is caused in the bridge, it is measured by a galvanometer graduated toread in units of linear movement of plunger. This circuit is operated by battery. For the bridgeto balance, the ratios of the resistances in two arms must be equal. If alternating current is applied to the bridge, the inductance and capacitance of the arms must also be accounted for along with resistance. In actual measuring instruments, one pair of inductances is formed by a pair of coils in the measuring head of the instrument. The movement of the plunger displaces an armature, thus causing a variation in the inductance of a pair of coils forming one arm of a.c. bridge. The arm carriesthe armature (Fig. 5.20) and the inductance in the coils is dependent upon the displacement of the armature relative to the coils. There are other refinements in actual instrument such as an electrical method of zero adjusting and a switch to change the magnification. The amount of unbalance caused by movement of measuring plunger is amplified and shown on a linear scale. Magnifications of the order of x 30,000 are possible with this system. Commonly used instruments are Electrichek, Electricator,

 Fig. 5.20. Electrical comparator based on a.c. bridge.Electrigage, Electrolimit and Electronic Measuring Equipment. A brief description of a few ofthese is given from 5.7.2 to 5.7.7.5.7.1. 

Electrical Comparators.Electrical comparators are also known as electro-mechanical measuring systems as these

employ an electro-mechanical device which converts a mechanical displacement into

electrical signal.Linear variable differential transformer (LVDT) is the most popular electro-

mechanical device used to convert mechanical displacement into electrical signal. It, in

effect, is a transformer consisting of three symmetrically spaced coils carefully wound on an

insulated bobbin. It works on mutual inductance principle and consists of a primary coil

wound on an insulating form (bobbin) and two identical secondaries symmetrically spaced

from the primary. AC carried excitation is applied to the primary and two secondaries are

connected externally in a series opposition circuit. The lead wires exit through an opening in

the outer shield, usually in the end-cover washers. A cylindrical shield of ferromagnetic

material is spun over the metallic end-washer after the windings have been vacuum

impregnated with a potting compound suitable for the application environment. The

finished transformer there after becomes quite impervious to humidity or ordinary

magnetic influences. The device thus also becomes extremely rugged and reliable. There is

a non-contacting magnetic core, made from a uniformly dense cylinder of nickel-iron alloy,

carefully annealed to improve and homogenise its magnetic permeability, which moves in

the centre of these coils wound on the insulating form and the motion of this core varies the

mutual inductance of each secondary to the primary, which determines the voltage induced

from the primary to each secondary. If the core is centered in the middle of the two

secondary windings, then voltage induced in each secondary winding will be identical and

180° out-of-phase, and the net output will be zero. If the core is moved off middle position,

then the mutual inductance of the primary with secondary will be greater than the other, and

a differential voltage will appear across the secondaries in series. For off centre

displacements within linear range of operation, the output is essentially a linear

 

Fig. 5.22. Schematic Arrangement of LVDT

(core-shown in the middle position).

 

Fig. 5.23. Phase referenced output voltage of LVDT.

 

Fig. 5.24. Absolute value of output voltage of LVDT.

function of core displacement. The various unique features of LVDT are : Due to no physical

contact between the core and coil its mechanical components do not wear out; as a result

the friction is absent and true infinite resolution with no hysteresis is obtained. The small

core mass and the lack of friction enhance response capabilities for dynamic measurements

and thus it becomes very suitable for taking measurements for on-line machining. Further

as it is not affected by overload, its reliability is high.

5.7.2. Electronic Gauging. The electronic Gauging System is designed to fulfill the

increasing demand for equipment particularly suited to the accuracy and versatility required

by up-to-date engineering practice.

5.7.2.1. Principle of Operation. The principle of operation is shown in Fig. 5.25. The

movement at the probe tip actuates inductance transducer which is supplied with an

alternating current from the oscillator.

 

Fig. 5.25. Principle of operation in electronic gauging.

The transducer converts this movement into an electrical signal which is then amplified

and fed via an oscillator to the demodulator. The current in D.C. form, then passes to the

meter and the probe tip movement is displayed as a linear measurement. Various

measuring and control units can be incorporated which provide for an extremely wide range

of single or multiple measurements to be made simultaneously.It consists of a gauging head

and an amplifier. The mechanical movement of the contact probe alters the output or

voltage of the gauge head proportional to displacement. The small alteration in voltage is

amplified by means of a sensitive meter, calibrated in terms of units of length. Switching

selection enables a convenient range of magnification. Such an instrument has many

functional advantages like high magnification, range of magnifications, light gauging force

(40 grams or less), rugged construction, immediate response, unlimited applicability to

inspection procedures.

Pneumatic Comparators

Air gauging has rapidly increased during some past time due to the following importantcharacteristics:(a) Very high amplifications are possible. It can be used to measure diameters, length,squareness, parallelism, concentricity, taper, centre distance between holes and othergeometric conditions.(b) As no physical contact is made either with the setting gauge or the part beingmeasured, there is no loss of accuracy because of gauge wear. For this reason, air spindle and air snap gauges last very long. Also very soft parts which are easily scratched, can be gauged.(c) Internal dimensions can be readily measured not only with respect to toleranceboundaries but also geometric form. In other words, while measuring a bore it can revealcomplete story of size, taper, straightness, camber and bell mouth etc.(d) It is independent of operator skill.(e) High pressure air gauging can be done with cleansing of the parts which helps toeliminate errors due to dirt and foreign matter.(f) Gauging pressures can be kept sufficiently low to prevent part deflection.(In general, high pressure gauges are suitable for those parts in which tolerances arerelatively large and low pressure air gauges are preferable for highly precise work.)(g) Dimensional variations throughout the length of shaft or cylinder bore can beexplored for out of roundness, taperness, concertricity, regularity and similar conditions.(h) Not only it measures the actual size, but it can also be used to salvage oversizedpieces for rework or to sort out for selective assembly, i.e., it is suitable both for variableinspection (measurement of size) and attribute inspection (GO and NO GO) gauging and limits.(i) The total life cost of the gauging heads in much less.(j') It is accurate, flexible, reliable, universal and speedy device for inspecting parts in

mass production.(k) It is best suited for checking multiple dimensions and conditions on a part simul-taneously in least possible time. It can be used for parts from 0.5 mm to 900 mm diameterhaving tolerance of 0.05 mm or less. It can be easily used for on line measurement of parts as they are being machined and take corrective actions. 

Systems of Pneumatic Gauges. Based on the physical phenomena on which the operation of pneumatic gauges is based,

these may be classified as :

(i) Flow or velocity type  (ii) Back pressure type.

Flow or velocity type pneumatic gauges operate by sensing and indicating the momen-

tary rate of air flow. Flow could be sensed by a glass tube with tapered bore, mounted over

a graduated scale. Inside the bore a float is lifted by the air flow. Velocity of air in velocity

type pneumatic gauges can also be sensed by sensing the velocity differential i.e.,

differential pressure across a venturi chamber. Such systems have

 

Fig. 5.32. Free flow air gauge.

quick response. These permit use of large clearance between nozzle and object surface,

resulting in reduced wear of the gauging members. There is less air consumption.

Magnification of the order of 500 to 5000 times is possible.

Free Flow Air Gauges (Flow or velocity type).In this case the compressed air after the filtering and pressure reducing unit flows through a

tapered glass tube containing a small metal float and then through a plastic tube to the

gauge head having two diametrically opposed orifices for air escapement into atmosphere

(Refer Fig. 5.32). The position of the tube is dependent upon the amount of air flowing

through the gauge head, which in turn is dependent upon the clearance between the bore to

be measured and the gauge head. Fig. 5.34 shows a curve between the air flow and the

clearance between the part and the orifice in gauge head.

 

Fig. 5.33. Zero and magnification adjustment

in flow type pneumatic comparator.

 

Fig. 5.34. Characteristic of air flow

versus clearance of flow.

The flow velocity type pneumatic comparator with zero adjustment and magnification

adjustment is shown in Fig. 5.33. Magnification can be changed by passing some of the air

supply, using a screw at the inlet to the tapered glass tube. The float can be zeroed by a

bleed valve installed at the top of the tube. Size is measured by the velocity of air in a

tapered glass tube which is measured by the height of the float in tube. The straight portion

of the curve is utilized for the measuring range. It provides high amplification (10 : 1) and

thus within the linear range, it is possible to read accurately upto microns depending upon

scale length, or classify the sizes quickly and accurately. The amplification can be changed

by quick change of tube, float and scale. Air gauge amplification and range are based on

the tooling and instrument standards of manufacturer. The amplification and instrument are

selected by considering the total tolerance spread and choosing the instrument that covers

the range. About 50 to 100 mm of column is usually allowed for the actual tolerance spread.

In the gauging head, the air escapement orifices are recessed below its cylindrical

surface so that the orifices never contact the part being gauged. Thus the surface wear will

not affect the accuracy till it is worn down to orifice level. Also the orientation of gauge or the

way operator holds the gauge is of no consequence and same readings will be obtained for

given diameter. On the gauge, knobs are also provided for adjusting float position and

calibration.Air gauge is set by placing masters for maximum and minimum tolerances on

spindle alternatively and adjusting the float position for each master by turning the knurled

knobs at the base of the instrument.Free-flow column type gauges are usually assembled

together side by side and thus multiple interrelated readings can be seen at a glance. This

is the big advantage of air gauging that the multiple dimensions and conditions can be

inspected with great ease, accuracy and speed.Pneumatic circuits can be arranged to

determine dimensional differences like taper (comprising the diameter of bore at different

points along a part), bore centre distance and also to select parts to assemble to

predetermined clearances or interference fits.

Back Pressure Gauges.The basic principle and the theory of pneumatic gauging in the back pressure gauges is

described below. (Refer Fig. 5.35) Air from a constant pressure source

flows to the atmosphere through two orifices Oc and Om in series.

P is the pressure upstream of the first orifice andp is the pressure between the two orifices,

both measured with reference to the atmospheric pressure as datum.

 

Fig. 5.35. Theory of pneumatic gauging.

The relationship between p and P will depend upon the relative sizes of the two orifices :

p being equal to P when Om is blocked and tends to zero as Om is increased indefinitely.

Let C be the geometrical area of Oe and M that of Om. Then if p and C are kept constant

while M is varied, the relationship between the dimensionless quantitiesp/P and MIC is of

the type shown in Fig. 5.36. (The general form of this curve is quite well predicted by an

analysis employing Bernouli's equation for flow of a compressible fluid.) We are interested

in linear portion of this curve. For design purposes we follow an empirical approach which is

based on an experimental study at N.P.L. (London) of the relationship between pressures

and orifices areas.The characteristics of p/P and MIC are determined experimentally for

pressure P varying from 2 to 75 pounds per sq. inch (0.13 to 5 kg/cm2) and inspection of

any one of these shows that within the range 0.6p/P to 0.8p/P, the curve approximates to a

straight line, the equation for which mav be written as

 

Fig. 5.36. Characteristic curve of pneumatic gauge.

Examination of the family of curves shows that constant A, the intercept on the p/P axis is

closely constant over the range of pressures investigated and for practical purposes, the

value of A = 1.10 can be adopted for any value of P likely to be used. The slobe b of straight

portion characteristics is however not independent of P, its numerical value decreases as P

increases and the limiting values found in the investigation are as under :

b = 0.6 when P = 0.13 kg/cm2

b = 0.4 when P = 5 kg/cm2.

Area of escape orifice.When the clearance between the surface and the nozzle face is zero, no air escapes from

the nozzle and the area of the escape orifice is zero.When the clearance between the sur-

face and the nozzle face is very large, the area of the escape orifice is ^ d2, where d is the

diameter of the nozzle.Between these extremes, especially where the clearance is small

and where airgauging can be employed, the area of the escape orifice is n dl, that is, the

area of the curved surface of the cylinder shown in Fig. 5.38.

Range of linear measurement.The condition 0.6 < p/P < 0.8 defines a section of the characteristics which experimental

investigation has shown to be linear to within 1%.

 

Fig. 5.37. Experimentally determined

characteristics for different operating pressures.

 

Fig. 5.38. Area of the escape orifice.

 

Fig. 5.39. Range of linear measurement.

Changing sensitivity (Magnification).A plot of pressure against escape orifice area for a number of different sizes of control

orifice will show that sensitivity increases as the diameter of the control orifice decreases,

i.e., for small control orifices the change in pressure is greater for a given change in escape

orifice area. Utilising this property, it is possible to set precisely the sensitivity

(magnification) by incorporating a variable control orifice.

Overall Magnification.In a practical pneumatic measuring apparatus the area M will be associated with the

measuring head and change in M will be the result of a change in the dimension which is

being measured e.g. a change in the separation L between nozzle and surface (Fig. 5.40).

The overall magnification of the apparatus, i.e. the ratio of the linear movement of the

pointer or index of the pressure measuring instrument to the change in the

 

Fig. 5.40. Variation of M w.r.t. L.

 

This condition requires the measuring head to be correctly designed. The final escapement

of the air from the nozzle to the atmosphere is taken as being through an area of the

curved surface of the cylinder of length L and diameter D, where L is the separation

between the nozzle surface and the surface to be gauged and D is the internal diameter of

the nozzle.

Response Speed.For a back pressure system the speed of response is not as fast as for free-flow type,

because some time is required for the pressure to build up. The speed of response

becomes of concern when the gauginghead is separated from indicating instrument

by long distance.A pneumatic measuring system will not correctly measure displacements

of frequency greater than about 2 cycles/second, because of its slow speed of response.

The response is considerably slower compared to the electrical system because of the

following reasons.Between the control orifice and the measuring head, there exists a closed

volume associated with the measuring instrument used to measure the pressure p. A

dimensional change, e.g., a displacement of the surface alters the flow so changing the

pressure p. The time needed to establish the new value of p depends on the total volume

and on the rate of air flow into and out of it. The latter in turn depends upon the operating

pressure P and the size of the control orifice and orifice in the measuring head. These

orifices are related in size and determine the pneumatic sensitivity, the smaller orifices

(corresponding to the higher magnification), having a more restrictive effect on the air flow

and so slowing the response.It has been examined theoretically and experimentally that

response is slowed by using a large operating pressure P and a large volume, but by these

high sensitivity (magnification) is obtained. In any practical pneumatic measuring system,

the overall response will be influenced by dynamic characteristics of the pressure

measuring device. Thus it follows that the use of a low operating pressure will not improve

the overall response if a low pressure measuring device of slow response is used to

measure the pressure changes. High sensitivity will inevitably be associated with slow

response and the only factor left to the designer is the volume, which should be made as

small as possible for quick response.Since gauge is always located at some distance from

the control unit, the effect of variations in the gauging position does not reach the control

unit instantaneously, though the size variations of the object will promptly affect the air flow

at nozzles. The time gap between the sensing and indication is known as response time

which depends upon : Length of air line between the nozzles and indicator, (ii) type of

gauge system, and (Hi) the design of control unit. Response in case of flow type pneumatic

gauges is relatively quick. Response in case of back pressure gauges is slow, the

compressibility of air also contributing to the delayed transmission of the variations sensed

at the nozzles.Response time of back pressure type pneumatic gauges can be improved by

utilizing following devices :

(i) Using filled system pressure gauge, thereby reducing volume of air.

(ii) Restricting the unimpeded escape of air through the orifices when the gauging head

is not in operation, by using a spring-charged cover sleeve around the gauge head.

(iii) Counteracting the unrestricted air escape by an auxiliary air supply relay whose

operation automatically discontinues as soon as a specific back pressure develops during

the actual gauging process.

(iv) Using a high speed relay to compensate for additions to the volume of the instrument

system. 

Zero Setting.It is accomplished by means of a bleed valve and consists in adjusting the indicating

element of the gauge to that marking on the scale which was selected to signal coincident

with the nominal limit size represented by a setting master.

Datum Control.If means be provided to change the pressure in the cavity (between control orifice and

measuring orifice) using a variable bleed to atmosphere, a datumor zero can be provided

which varies the pressure 'p' when the escape orifice area remains fixed. This addition to a

circuit provides means of accommodating small differences which inevitably occur in the

manufacture of gauge heads. Limited use of a datum control in the form of a bleed to

atmosphere has an insignificant effect on linearity.However, this system depends highly

upon the pressure regulator to maintain the supply pressure within very close pressure

limits. Thus the pressure regulator is a critical component in this circuit. This problem is

overcome in differential back pressure circuit in

 

Fig. 5.41. Practical back pressure circuit.

which accuracy is maintained regardless of some variation in the regulator performance

which controls supply pressure. Fig. 5.42 shows a practical flow-responsive system which is

in common use. This system employs variable orifices for sensitivity and datum control.

 

Fig. 5.42. Practical flow responsive system.

 

Fig. 5.43. Plug for pneumatic comparator.

plification Adjustment.This permits different range of gauge indications on same scale length and is carried out

with a precision valve of the control unit. Both zero setting and amplification adjustment

should be checked from time to time depending upon experience.

Jet Recession.It has already been seen that when the surface being measured is very close to the nozzle

face, equal increments of change in clearance do not produce equal increments of pressure

change. The system is not linear under these conditions.Because of this the faces of the

jets on air plug gauges are ground below the body diameter of the plug as shown Fig. 5.43.

This grinding back is called jet recession and it is the means by which the non-linear portion

at the high pressure (low flow), end of the pressure/clearance curves is avoided.

Measurement of Bore. [Refer Fig. 5.44 (a) and (6)]In the above discussion we have assumed a jet separation of the nozzle from the

flat surface. This is also applicable to measurement of bore, where L is modified as :

L = Lx + L2, Lx - Radial gap for one jet and L2 = Radial gap for other jet.Theoretically Li

should be equal to L2,but even if L\ * L2 the back pressure would be closely equal to that

corresponding to the condition L1 = L2 = L/2. This is a big advantage as operator need not

be very meticulous about orientation of the measuring head in the bore, and thus the

readings from operator to operator will be uniform as they do not depend upon a high

degree of operator skill or sense of feel.

Measuring Heads Measuring heads fall under two categories,viz. direct head [Fig. 5.45 (a) and (6)1, and indirect or contact head [Fig. 5.45 (c)

and (d)]. Tapered nose type direct head [Fig.5.45 (a)] is quite popular as it permits easy

access in constricted measuring conditions.The ratio of land diameter (overall diameter at

tapered small end) compared to jet diameter is twice in size. Bigger ratio would affect the

escapement of air and the characteristics of the system. Head at Fig. 5.45 (b) provides

good protection to the nozzle due to incorporation of guard ring and escapement holes. In

the case of indirect measuring heads, the jet is protected from accidental damage. The size

of air escapement is controlled by a needle valve or flat plate [Fig.5.45 (c) and (d)

respectively] which move due to movement of measuring plunger. By changing the taper of

needle valve, the range of measurement can be changed. Parabolic needle provides linear

response. Fig. 5.45 (e) shows the plug gauge used for measurement of diameter, lobing,

taper etc. The measuring side is made somewhat smaller than the bore so that it enters

much more conveniently.

Solex Pneumatic Gauge. (Fig. 5.46).This instrument is produced commercially by Solex Air Gauges Ltd. This is generally

designed for internal measurement, but with suitable measuring head it can be used for

external gauging also. It is obvious from the equation for sensitivity that in order that

sensitivity (magnification) remains constant, the supply pressure P must be constant.

Thus it is necessary to have a simple pressure regulator which may control the pressure

of air from the normal supply line, and if necessary the pressure might be reduced also. The

arrangement used in Solex gauge is to pass the high pressure air after filtering, through a

flow valve. There is a tank in which water is filled upto a certain level and a dip tube is

immersed into it upto a depth corresponding to air pressure required. In Fig. 5.46, it is

represented by H. Since air is sent at higher pressure than required one, some air will leak

out from the dig

 

tube and bubble out of water and the air moving towards control orifice will be at desired

constant pressure H. Now-a-days diaphragm type pressure regulators are readily available

in the market and they are better for regulating the pressure than the above device. The air

at reduced pressure then passes through the control orifice and escapes from the

measuring jets. The back pressure in the circuit is indicated by the head of water displaced

in the manometer tube. The tube is graduated linearly to show changes in pressure

resulting from changes in internal diameter of the work measured. This instrument is

capable of measuring to the accuracy of microns. It is very obvious from Fig. 5.46

that the diameter being measured at any instant is corresponding to the portion against two

jets. Now to find the concentricity (roundness of any job at any section), the workpiece may

be revolved around measuring gauge. If no change in reading is there, then it is perfectly

round hole. Similarly the diameter can be noted down at several places along the length of

bore and thus tapering of hole is determined. This method is, therefore, best suited for

measuring roundness and taperness of cylinder bores and gun barrel bores. By having

suitable measuring head this can be used for external gauging, and head in this case will be

as

 

(a), (6) Direct heads (c) Forward head

(d) Reverse head (e) Plug gauge.

Fig. 5.45. Measuring heads.

 

Fig. 5.46. Solex Air Gauge.

shown in Fig. 5.47.This can best reveal any lobing effect also.It is also possible to have

arrangement to measure the length of slip gauge by having the flat table and one jet at the

top.

Overall Magnification and Range.From equation dpIdM = 0.40 P/Mc, the pneumatic sensitivity of a pneumatic measuring

apparatus can be increased by increasing the operating pressure P, but controlled by the

length of scale of pressure measuring instrument corresponding to pressure changes from 0

to P. If this scale length is to remain of convenient magnitude, increasing the operating

pressure is not a suitable method for improving the overall magnification. The only effective

method for obtaining the higher magnification is, therefore, to reduce average separation

between nozzle and surface, which at the same time, of course, reduces the range of linear

measurement.

Limitations of empirical approach.From the view-point of air flow, the effective area of an orifice is not usually identical with its

geometrical area. If two orifices are made by producing holes of identical geometrical area

in two thin discs, their effective areas may be appreciably different as a result of edge

effects on the air flow arising at the peripheries of the orifices. Again the relationship

between effective area and geometrical area is unlikely to be the same for air flow through

an orifice and the jet of air from a nozzle. In the experimental determination of the p/P, MIC

characteristics the value of M and C used were the geometrical areas of orifices Om and

Oc. Therefore due to effective area being different from the geometrical area, the empirical

equation obtained by analysing these characteristics would not be expected to provide a

completely accurate numerical forecast of performance.Nevertheless, experience has

shown that they do give a first approximation sufficiently reliable to permit the required

performance to be obtained by a single-step corrective adjustment of the control orifice.

Differential Comparators.

A later development brought out the balanced circuit type of air-gauge. In this equipment a

differential pressure indicating mechanism,connected across the two air-paths and a built-in

gauge zeroing valve is provided. Such a balanced circuit is shown schematically in Fig.

5.48. An air gauge based on this balanced circuit is called 'Differential Comparator'.

 

Three equally spaced measuring orifices

(jets) reveal any lobbing effect also.

Fig. 5.47

 

Fig. 5.48. Differential Circuit.

Compressed air from a suitable source, after passing through air-drier and filter is regulated

for constant pressure by a pressure regulator. Now the air flows into two channels, each of

which has a control orifice Ocl and Oc2 (control orifices are also called master jets). From

the control orifice Ocl, air flows to the measuring head where it meets further restriction of

the workpiece or the master settings. The restriction of the workpiece builds up back

pressure as explained earlier. At the same time, other half of the air is flowing through the

other control orifice Oc2 to the reference jet Om. By closing or opening the valve of

reference jet Om, the pressure in the space between Oc2 and Om is regulated (adjusted) to

match the back pressure from the measuring jets, which is sensed by the pressure

indicating device fitted across the two channels as shown. At this adjustment of the

reference jet, the pressure indicator would indicate equal pressure in the two channels and

hence read zero on the scale. This zero setting (adjusting of reference jet Om) is done with

master workpiece whose dimension is exact nominal size. Now the variation of the

dimension at the measuring head would cause change of back-pressure in channel A. This

pressure would be different from the mean pressure which has been already set in the

channel B (by reference jet). Now the difference of pressure of the two channels would be

indicated by the pressure indicating device which can be directly calibrated in terms of

variation of dimension from the mean dimensions. Hence the instrument based on the

measurement of differential pressure is called Differential Comparator. If the dimension

causes a decrease in gap L as compared to La, this in turn decreases M and hence

increasing back pressure in channel A and vice versa. In these cases the pressure

indicator would show readings on both sides of zero corresponding to ± deviation from

mastersetting.

 

Advantages of Differential Circuit over Single Channel Circuit,(i) Effect of change of operating pressure P. The operating pressure may vary slightly from

the designed value. It can be shown that error due to change of pressure would be 0.6 to

0.8 times the change in pressure in single channel in case of differential circuit the error

would be 0.1 times the change in pressure.

(ii) Zero setting of master gauge is an extra advantage.

(iii) Rectification for control orifice. In a single channel system, the practical limitations

may not give the perfectly correct and accurate dimension of the control orifice as designed.

Sometimes it may go out of the useful range of the design and it may have to be discarded.

Therefore, in order to avoid the error of manufacture and also to take into account the

fact that the geometrical area is different from the effective area, we need a needle valve so

that area may be adjusted accordingly. But in the differential circuit which incorporates a

needle zero adjusting valve, the off-set of the actual size of the control orifice from the

designed value can be nullified by adjusting this valve.

Non contact and Contact Type Pneumatic Gauging Elements.Non contact tooling is best suited for automatic gauging applications because of the

advantages of no contact, clearing of oil or foreign particles from gauging area, etc.

In the case of non-contact air gauge tooling, only the air coming out of the air escapement

orifice touches the part to be measured, the air flow rate depending on the cross-sectional

area of the jet and the clearance between the jet and the part to be measured. It may have

a single jet, two diametrically opposite jets or more evenly spaced jets. Single open-jet

tooling can be used for checking outside diameter, height, depth, straightness, squareness,

etc. and Fig. 5.49 shows a few of such applications. Dual jet techniques can be applied for

determining true diameter, out or round, bell mouth, thickness, etc. The various gauges may

be designed either for presenting the gauges to part or vice versa.Many modern mechanical

assemblies demand that holes should be closely controlled for straightness as well as

diameter. An air plug gauge for gauging hole straightness is shown in Fig. 5.50.

In the contact tooling, a mechanical member is incorporated between the air escapement

orifice and the part to be measured. The air flow from the jet changes due to displacement

of this mechanical member when it contacts the part. The mechanical member could be a

ball, lever, plunger or blade. A big advantage of contact type air gauge is that a much

bigger measuring range (upto 2.5 mm) is possible i.e. it is suitable for wide range of

gauging. Another advantage is that it eliminates surface roughness from size. It may be

mentioned that open jet type method would be subject to error for rough surfaces because it

measures a combination of size and surface finish ; further its range of measurement is

limited. Ball jet spindle gives a point reading rather than the average over a small area

and is best suited for gauging inside diameter of soft or porous parts and for rough bores.

Leaf jet spindle can be used for checking laminated bores, blind holes in which keyways

etc. do not permit the use of open jet spindles at extreme bottom of blind holes etc. Blade

jet spindles are used for inspecting gun bores in which oil grooves, or slots do not permit the

use of ball jet or leaf jet spindles. Fig. 5.51 shows a small plunger type air gauging cartridge

which is highly efficient size-sensing element for wide range of gauging, tooling, fixturing,

and machine control applications. It essentially consists of a spring loaded plunger. The

spring tries to keep the plunger outwards and when the part to be measured comes in

contact with it or it comes in contact with the part it moves in and at the end restricts the

orifice, thereby increasing the back pressure. The maximum and minimum limits of the

plunger movement can be set with the help of masters. Such cartridges can be secured in

gauging position on various types of fixtures and used for measurements like height, depth,

flatness, concentricity, squareness, inside/outside diameter, etc. Fig. 5.52 shows the same

principle used in a test indicator which is very efficient for several applications. It has a tiny

stylus capable of entering into small holes, keyways, slots etc. and its movement causes the

tapered end to act as a precision valve stem to regulate the amount of air flowing through

an orifice. It is free of hysteresis or lag or drag in indicationswhen the stylus is moved in any

direction across the workpiece.

 

Fig. 5.49. Applications of single open jet tooling.

 

Fig. 5.50. Straightness measuring gauge.

 

Fig. 5.51. Air Cartridge.

 

Fig. 5.52. Pneumatic test indicator.

The contact type adjustable spindle kits and adjustable air snap gauges available in

market are found to be very suitable for handling new designs, altered dimensions and

various other varied applications.

Indirect Pneumatic Gauging Devices.The open jet has the disadvantage of small measuring range. It can be overcome by using

indirect pneumatic gauging devices by using a gauging cartridge. Such cartridge employs a

contacting stylus, the inner end of which is tapered and forms the restriction in an escape

orifice. The position of the stylus and consequently the position of the taper in the orifice

causes changes in the area of the orifice. Changes in the rate of taper change the

measuring range of the cartridge. Measuring ranges upto 3 mm can be obtained with this

type of cartridge.

Air gauging with electronic sensors.Air gauging system operate on either low or high air pressure. While low pressure systems

have greater sensitivity, quicker response time and minimal distortion when measuring

flexible components, the high pressure systems are self cleaning type and have a large

measuring range. Basically air gauging comprises air jet gauges—such as ring or plug, and

air operated liquid columns for multi-dimensional measurement. Now-a-days electronic

flowmeters are used in place of air operated liquid columns. The have the advantage of

measuring flow of air with the added benefit of electronic display. Such instruments can

easily have 2 or 3-range selection to give an extra magnification factor. Tolerance limit lights

can be incorporated to indicate whether parts are inside or outside manufacturing

tolerances. Response time increases many folds. The versatility of air gauging is enhanced

by the wide range of measuring tools like 2 and 3 jet non-contact air plug gauges, setting

rings and air jet ring gauges.

Multi-gauging Systems.Multi-gauging systems are used to measure a number of dimensions simultaneously. Parts

to be gauged are compared with a setting master which simulates the component. The

features gauged could be external/internal diameters, lengths, straightness, squareness,

ovality, run-out of faces, etc. The measuring head gauging fixture is specifically designed to

suit the component to be measured and may be completely special or it may be built using

a series of modular elements. It contains the means for sensing the dimensional difference

between the components and the master which may take the form of mechanical or

electronic probes or air jets connected to the means for amplifying the difference.

The amplifying and display of readings may take the form of dial gauges or some form

of electronic or air/electronic system. Display may be analogue, digital or graphic and may

be augmented by out of tolerance indication. Using electronic differential methods, the

relationship between different features can be related to a common datum.

The choice of system depends on number of factors like initial cost, dimensional

tolerances of the features to be measured, complexity of the component, complexity of the

features to be measured, number of features to be measured, speed of measurement

required, skill or otherwise of the user.

The reasons affecting the choice of displays are given below :

(i) Pointer displays—best where a rapid check of run-out or concentricity is required.

(ii) Columns—natural choice where a considerable number of dimensions are involved.

These are fastest and most convenient form of displaying the readings for every

dimension.

(iii) Digital—provides high accuracy over a longer measuring range and best for situa-

tions where it is required to work in drawing dimensions. Can be viewed without strain over

longer distances.

(iv) Graphic on VDU—the most sophisticated display essential when statistical process

control systems are employed.

Systems capable of dealing with very large number of inter-related dimensions can be

developed. Automatic inspection machines incorporate both automated loading hopper,

magazine,pick and place robot and—automated segregation of inspected components.

Automatic inspection is essential where the complexity of the component is such that

manual methods can not achieve the desired levels of accuracy. The results of inspection

could be fed to electronic computer based system which may also control the machine

operation. The use of such computer based processing also allows the results obtained to

provide a wide range of control facilities including feedback for control of the manufacturing

process. Coordinate measuring machines accommodate multidimensional inspection by

using a single point contact to take successive measurements over the component profile.

The contact movement and processing of the dimensional information is under computer

control, which can also provide similar facilities to those offered by multi-point gauging.

Multicheck Comparators Now-a-days modern trend in comparison is towards the inspection of all dimensions atthe same time, as it is economical procedure. It is particularly desired where various dimensions have some relationship with each other, e.g., diameter and concentricity measurement. This job is done by multicheck comparators, which are of the following types and incorporate the following systems of amplification.(a) Electrical (b) Air(c) Combination 'air-electric'. 

Electric Multichecks.This is combination gauge incorporating about twenty or more electrical-check heads to

measure simultaneously a number of dimensions of one part. For each dimension, there is

one measuring head and signal lights for each dimension indicate whether it is within

tolerance, undersize or oversize.The instrument is used for high production checking and

chances of any error in any dimension are less. As it will not be desirable to see all the

lights for various dimensions,a master signal light is used to integrate all the individual

lights, and inspector has to watch only one light. On seeing the master light warning

individual signal lights are examined. This instrument is well suited for all sizes of works.

Air Multichecks.In this instrument, group of air comparators are set up to check a number of dimensions. In

this type of instrument, initially some difficulty is experienced in setting the various

comparators to check various dimensions and to arrange them in compact form. But once

this is done things become very easy. In one setting all the dimensions can be checked and

the time of inspection is reduced considerably. Mass inspection becomes possible and high

quality can be ensured.

An air Electric Multicheck.This makes use of both types of comparators.It is very easy to use air comparator for

checking diameter and concentricity, so for this purpose air comparator is used, while for

other measurements electric-checks are used. 

Automatic Gauging Machines.These machines incorporate comparator amplifying methods and are similar to multicheck

devices. They eliminate manual inspection and parts are inspected for all the dimensions

simultaneously and segregated and classified automatically

Advantages of Mechanical Comparators

(1) These are usually cheaper in comparison to other devices of amplifying.

(2) These do not require any external supply such as electricity or air and as such thevariations in outside supplies do not affect the accuracy.

(3) Usually the mechanical comparators have linear scale which is easily understood.(4) These are usually robust and compact and easy to handle.(5) For ordinary workshop conditions, these are suitable and being portable can be issued from a store

(6) The electrical comparators have got small number of moving parts.(7) It is possible to have a very high magnification and the same instrument may have two or more magnifications. Thus the same instrument can be used for various ranges.

(8) The mechanism carrying the pointer is very light and not sensitive to vibrations.(9) As the instrument is usually operated on A.C. supply, the cyclic vibration substantially reduces errors due to sliding friction.

(10) The measuring unit can be made very small and it is not necessary that the indicating instrument be close to the measuring unit, rather it can be remote also

Disadvantages(1) The mechanical comparators have got more moving parts than other types. Due to more moving parts, the friction is more and ultimately the accuracy is less.

(2) Any slackness in moving parts reduces the accuracy considerably.

(3) The mechanism has more inertia and this may cause the instruments to be sensitive to vibration.

(4) The range of the instrument is limited as the pointer moves over a fixed scale.

(5) Error due to parallax is possible as the moving pointer moves over a fixed scale.

(6) As the instrument has high magnification, heat from the lamp, transformer etc. may cause the setting to drift.

(7) An electrical supply is necessary.

(8) The apparatus is usually large and expensive.

(9) When the scale is projected on a screen, then it is essential to use the instrument in a dark room in order to take the readings easily.

(10) The instruments in which the scale is viewed through the eyepiece of a microscope are not convenient for continuous use.