Mechanical Comparators 5.5.In these comparators, magnification is obtained by mechanical linkages and othermechanical devices.5.5.1.

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)(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)(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 toproduce indication. (Refer Fig. 5.3)

Fig. 5.1. Rack and pinion.

Fig. 5.2. Cam and gear train.

Fig. 5.3. Lever with toothed gear.

Fig. 5.4. Compound levers.

Fig. 5.5. Twisted taut strip.

Fig. 5.6. Lever combinedwith band wound around drum.(iv) Compound Levers. Here levers forming a couple with compound action areconnected through segments and pinion to produce final pointer movement.(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 ofstrip is displayed by the attached pointer.(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 rotationof the drum.(vii) Reeds combined with optical display. In this case parallelogram reeds are usedwhich transfer measuring spindle movement to a deflecting reed whose extension carries atarget utilised in optical path.(viii) Tilting mirror projecting light spots.5.5.2.

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 sothat 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.5.5.3.

The Johansson 'Mikrokator'.

This comparator was made by C.F. JohanssonLtd. and therefore named so. It is shown diagrammatically in Fig. 5.7. This instrument uses

the simplest and most ingenious method for ob-taining the mechanical magnification designedby H. Abramson which is called Abramson Move-ment. It works on the principle of a button spin-ning on a loop of string. A twisted thin metal stripcarries at the centre of its length a very lightpointer made of thin glass. The two halves of thestrip from the centre are twisted in oppositedirections so that any pull on the strip will causethe centre to rotate. One end of the strip is fixedto the adjustable cantilever strip and the otherend is anchored to the spring elbow, one arm ofwhich is carried on the measuring plunger. As themeasuring plunger moves either upwards ordownwards, the elbow acts as a bell crank leverand causes twisted strip to change its length thusmaking it further twist or untwist. Thus thepointer at the centre of the twisted strip rotatesby an amount proportional to the change inlength of strip and hence proportional to theplunger 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 thecentre line of the strip is straight even when twisted, therefore, it is directly stretched by thetension 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 itsneutral 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 verythin rectangular strip must be used. Further amplification can be adjusted by the cantileverstrip- which also provides anchorage. It increases or decreases effective length of strip. Finalsetting of the instrument amplification is made by a simple adjustment of the free length ofcantilever strip.A slit C washer as shown in Fig. 5.7 is used for the lower mounting of plunger. Thusthis instrument has no mechanical points or slides at which wear can take place. Themagnification of the instrument is of the order of x 5000.5.5.4.

Reed Type Mechanical Comparator.

In mechanical comparator, the gauginghead is usually a sensitive, high quality, dial indicator mounted on a base supported by a sturdycolumn. Fig. 5.8 shows the reed type mechanical comparator.The reed mechanism is frictionless device for magnifying small motions of spindle. Itconsists of a fixed block A which is rigidly fastened to the gauge head case, and floating blockB, which carries the gauging spindle and is connected horizontally to the fixed block by reedsC. A vertical reed is attached to each block with upper ends joined together. These verticalreeds are shown in the figure by letter D. Beyond this joint extends a pointer or target. A linearmotion of the spindle moves the free block verticallycausing the vertical reed on the floating block toslide past the vertical reed on the fixed block. How-ever, as these vertical reeds are joined at the upperend, instead of slipping, the movement causes bothreeds swing through an arc and as the target ismerely an extension of the vertical reeds, it swingsthrough a much wider arc. The amount of targetswing is proportional to the distance the floatingblock has moved but of course very much magnified.The scale may be calibrated by means ofgauge block (slip gauges) to indicate any deviationfrom an initial setting.Comparators using this type of linkage havesensitivities of the order of 0.25 micron per scaledivision.The mechanical amplification is usually lessthan 100, but it is multiplied by the optical lenssystem. It is available in amplifications rangingfrom x 500 to x 1000.

Fig. 5.8. Reed type mechanical comparator.5.5.5.

The Sigma Comparator.

Fig. 5.9 shows the constructional details of the SigmaMechanical Comparator.The vertical beam is mounted on flat steelsprings A connected to fixed members, which in turnare screwed to a backplate. The assembly provides africtionless movement with a restraint from thesprings.The shank B at the base of the vertical beam isarranged to take a measuring contact, selecting fromthe available range.The stop C is provided to restrict movement atthe lower extremity of the scale.Mounted on the fixed members, is the hingedassembly D carrying the forked arms E. This assemblyincorporates a hardened fulcrum (provided withmeans for adjustment of controlling the ratio of trans-mitted motion) operative on the face of a jewelled inserton the flexible portion of the assembly.

The metal ribboni^, attached to the forked arms,passes around the spindle G causing it to rotate inspecially designed miniature ball bearings. Dampingaction to the movement is affected by a metal disc,mounted on the spindle, rotating in a magnetic fieldbetween a permanent magnet and a steel plate. Theindicating 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 constantpressure over the whole scale range is achieved.The instrument is available with vertical capacities of 150 mm, 300 mm and 600 mmand magnifications of500,1000,1500,3000 and 5000. The scales are graduated in both Englishand 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 being

present in the moving parts.5.5.6.1.

Diagrammatic sketch showing the movement of sigma comparator.

Thevarious movements in the 'sigma' comparator discussed in Art. 5.5.6, will be very clear withthe 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 havefrictionless linear movement. A knife edge is mounted on it and bears upon the face of themoving member of a cross strip hinge. For details of cross strip hinge refer Fig. 5.11. The crossstrip hinge consists of the moving component and a fixed member which are connected by thinflexible strips alternately at right angle to each other. Thus if an external force is applied tothe 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. Ifthe distance of the hinge from the knife edge be a then the magnification of the first stage

Fig. 5.11. The cross strip hinge usedin sigma comparator.

(a) Comparator (b) Movement usedin comparator.Fig. 5.12. Mechanical comparator usingrocking prism.In order to adjust the magnification, distance a must be changed by slackening andtightening 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 followedby it, therefore, if too robust movement of the plunger is made due to shock load, that will notbe transmitted through the movement.(2) By mounting a non-ferrous disc on the pointer spindle and making it move in fieldof 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 bythe use of a magnet plunger on the frame and a keeper bar on the top of the plunger. As theplunger is raised the force required increases but the keeper

bar approaches the magnet and the magnetic attraction bet-ween the two increases. Thus as the deflecting force increases,the assistance by the magnet increases and total force remainsconstant.5.5.7.

Mechanical Comparator.

The diagram of a typewhich is used for comparative measurement of external sur-faces is shown in Fig. 5.12 (a). In this instrument the move-ment of the measuring tip attached at the end of the spindle istransmitted to the pointer through a mechanism shown in Fig.5.12 (b). The upper end of the spindle bears against a rockingprism (knife-edge). There is a frame member having two V-slots offset to each other by distance a. The end of the spindlerests against the first V-slot and its movement is transmittedto this frame through a prism. A knife edge which is stationaryrelative 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-slotsforms the shorter lever arm of the system, whereas, the longer

Fig. 5.13. Mechanical comparatorusing sector and pinion.lever arm is the distance L from the centre of rotation of the system to the other end of hand,which moves along the comparator scale. Thus the magnification of the instrument is L/a andis of the order of 1000. The contact pressure is of the order of 300 to 400 grammes and isprovided by a spring. The use of knife-edge pivots in the comparator movement excludes theinfluence 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 movementof the plunger (contact member) is transmitted to the pointer through an angular lever whichends in the form of a sector and pinion. It may be noted that a spring attached with the angularlever avoids the play in comparator.

Pneumatic Comparators 5.8.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 andair 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 inmass 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 asthey are being machined and take corrective actions.5.8.1.

Systems of Pneumatic Gauges

. Based on the physical phenomena on whichthe 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 agraduated 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 thevelocity 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. Magnifica-tion of the order of 500 to 5000 times is possible.5.8.1.1.

Free Flow Air Gauges (Flow or velocity type).

In this case the compressed airafter the filtering and pressure reducing unit flows through a tapered glass tube containing asmall metal float and then through a plastic tube to the gauge head having two diametricallyopposed orifices for air escapement into atmosphere (Refer Fig. 5.32). The position of the tubeis dependent upon the amount of air flowing through the gauge head, which in turn isdependent upon the clearance between the bore to be measured and the gauge head. Fig. 5.34shows a curve between the air flow and the clearance between the part and the orifice in gaugehead.

Fig. 5.33. Zero and magnification adjustmentin flow type pneumatic comparator.

Fig. 5.34. Characteristic of air flowversus clearance of flow.The flow velocity type pneumatic comparator with zero adjustment and magnificationadjustment is shown in Fig. 5.33. Magnification can be changed by passing some of the airsupply, using a screw at the inlet to the tapered glass tube. The float can be zeroed by a bleedvalve installed at the top of the tube. Size is measured by the velocity of air in a tapered glasstube 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 highamplification (10 : 1) and thus within the linear range, it is possible to read accurately uptomicrons depending upon scale length, or classify the sizes quickly and accurately. Theamplification can be changed by quick change of tube, float and scale. Air gauge amplificationand range are based on the tooling and instrument standards of manufacturer. The amplifica-tion and instrument are selected by considering the total tolerance spread and choosing theinstrument that covers the range. About 50 to 100 mm of column is usually allowed for theactual tolerance spread.In the gauging head, the air escapement orifices are recessed below its cylindricalsurface so that the orifices never contact the part being gauged. Thus the surface wear will notaffect the accuracy till it is worn down to orifice level. Also the orientation of gauge or the wayoperator holds the gauge is of no consequence and same readings will be obtained for givendiameter. 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 spindlealternatively and adjusting the float position for each master by turning the knurled knobs atthe base of the instrument.Free-flow column type gauges are usually assembled together side by side and thusmultiple interrelated readings can be seen at a glance. This is the big advantage of air gaugingthat the multiple dimensions and conditions can be inspected with great ease, accuracy andspeed.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 alsoto select parts to assemble to predetermined clearances or interference fits.5.8.1.2.

Back Pressure Gauges.

The basic principle and the theory of pneumaticgauging in the back pressure gauges is described below. (Refer Fig. 5.35)Air from a constant pressure sourceflows to the atmosphere through two orificesOc and Om in series.P is the pressure upstream of the firstorifice andp is the pressure between the twoorifices, both measured with reference tothe 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 Cbe 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 thedimensionless 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 employingBernouli'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 experimentalstudy 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 Pvarying from 2 to 75 pounds per sq. inch (0.13 to 5 kg/cm2) and inspection of any one of theseshows that within the range 0.6p/P to 0.8p/P, the curve approximates to a straight line, theequation 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 isclosely constant over the range of pressures investigated and for practical purposes, the valueof 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, itsnumerical value decreases as P increases and the limiting values found in the investigationare as under :b = 0.6 when P = 0.13 kg/cm2b = 0.4 when P = 5 kg/cm2.