 Welcome you all for the series of lecture on metrology, module number 10 and lecture number 2. In the previous lecture number 1, we started the discussion on comparators and we discussed a few mechanical comparators. In this lecture, we will continue the discussion on mechanical comparators. We will discuss about sigma comparator, Johansson microcator and we will also discuss the advantages and disadvantages of mechanical comparators. And then we will move to the discussion on electrical and electronic comparators in which we will be discussing about electro limit gauge, visual gauging head, electronic comparator and their advantages and disadvantages. Then we will move on to the discussion on optical comparators. Now let us discuss about the sigma comparator. Now we can see this photograph shows the sigma comparator. This is the sturdy base and then we have a column here on which the measuring head of the sigma comparator is mounted. We can adjust the height of this measuring unit by moving it up and down and then clamping at the desired height. So, this vertical movement of the measuring unit is needed to adjust the work pieces of different heights. Now you can observe here a replaceable table. Depending upon the work pieces, we can replace these tables and then they have to insert the work pieces between the surface of the table and the plunger. So, this shows the schematic diagram of the sigma comparator. I can see this is the surface of the table and then we have a job which is to be inspected. The work piece to be inspected is placed between the plunger and the surface plate. When the desired, when the work piece size is equal to the desired size, then the pointer shows zero. Initially using the slip gauges, we should adjust the gap between the plunger and the surface plate equal to the desired height. So, in that case the pointer will show, the pointer will show the zero reading. And then what is the upper limit and what is the lower limit of the work piece? So, accordingly we have to insert slip gauges equal to the upper limit of the work piece and then we can mark the upper limit of the work piece and then we should insert the slip gauge equal to the lower limit of the work piece and then we can adjust the lower limit of the work piece. So, these limiting pointers we can adjust by operating these knobs. Now, the construction of the sigma comparator is like this. We have a plunger which is connected to a rectangular block. So, this rectangular block is supported by flexor plates, two horizontal flexor plates. One plate is here, another plate is here and then we have a knife edge which is supported on the crossed strip hinge. The details of the crossed strip hinge are shown here. It consists of a fixed member and then a moving member with these two members are connected by flexible strips as shown here. And then we have a y-arm which is one end of this is connected to the crossed hinge, cross strip hinge, movable part of the cross strip hinge. The other part is bound over a drum of radius R. A pointer of length capital R is mounted on the drum and then there is a bronze band wound over the drum. Now, having studied the construction, now let us see how the comparator works. The plunger is attached to the rectangular bar that we can observe here. And a knife edge is fixed to the bar which bears on sapphire block attached to the moving member of cross strip hinge. We have this y-arm or four-arm of length y and you can see here at the distance between knife edge and this vertical cross strip is x. Then when the workpiece is mounted or inserted between the table and the plunger, depending upon the height of the workpiece, this plunger along with this bar, rectangular bar will move up and down and hence this part, this end of the forward arm will also move up and down. And then the other end of the forward arm will also move and hence the pointer will also move in this fashion and hence we can take the reading using this scale. The magnification can be calculated using this expression y divided by R, y is the length of the forward arm and x is this distance times R upon R where R is the length of the pointer and small r is the drum radius. The magnification possible with such a comparator it varies from 300 to 5000 and at least count of 0.1 micrometer is possible. Now the specifications of commercially available Sigma comparator are as follows. Range varies from minus 0.07 to plus 0.07 and the scale graduation is 0.002 millimetre. Magnification of 1000 and up to 5000 is possible and then we have interchangable work table. Depending upon the workpiece these tables can be changed and the contact tips the plunger end which makes contact with the workpiece. They are replaceable. We can have a flat end contacting or rounded end contact tip like this that depends upon the type of workpiece to be inspected. The workpiece height that is that can be inserted between the plunger and the table surface varies from 150 millimetre to up to 600 millimetre by adjusting the height of this measuring unit. And then we can also have the tolerance indicators which will indicate the upper and lower limit of the workpiece. Now we will move on to the discussion on Johansson microcator. The construction of the Johansson microcator is shown here. We have the two S shaped parts. These two parts are assembled by using this flexible slicked washer. Now in the middle we have the plunger which moves up and down depending upon the workpiece height. And then there is a bell crank lever. This bell crank lever one end of the bell crank lever is connected to this S shaped body and the other end is connected to the top end of the plunger. And then there is a metallic twisted strip. So here we have a cantilever strip between the cantilever strip and the bell crank lever. We have this twisted strip and then at the center we have a light point. Now let us see how this comparator works. When we insert the workpiece between the table surface and the plunger, the plunger moves up and down depending upon the height of the workpiece. Any longitudinal movement in either direction, upward direction or lower direction will cause the central portion of the strip to rotate. See when the plunger moves up and down, this bell crank lever will move in this fashion. So because of this, the length of this twisted strip changes and hence this pointer, the strip will rotate along with the pointer. Now we can see one end of the strip is fixed to an adjustable cantilever and the other end is fixed to this bell crank lever. The bell crank lever in turn is connected to the plunger at this portion. So which moves up and down when we insert the workpiece. Now the bell crank lever causes the twisted strip to change length when there is a movement in the plunger. This change in length will result in a proportional amount of twist of the metallic strip. The magnification can be varied by changing the length of this bell crank lever. Now what are the advantages of mechanical comparators? These are usually inexpensive when compared to other devices, other types of comparators. These do not require any external power supply such as electrical supply or compressed air supply. Usually the mechanical comparators have linear scale which is easily readable. These mechanical comparators are very robust and they are very compact and hence they are easy to handle. For ordinary workshop conditions these are suitable and are portable. Now what are the disadvantages of mechanical comparators? They have many moving parts than the other types. Due to this the friction is more and ultimately the accuracy is less. So this diagram, this photograph shows the various parts, December parts of dial indicator. Any slackness in moving parts reduces the accuracy considerably. The mechanism has more inertia and this may cause the instruments to be sensitive to vibration. The range of the instrument is very much limited as the pointer moves over a fixed scale. Now let us start the discussion on electrical comparator, a particular comparator named electro limit gauge. So in this we will study the construction of the measuring unit of electro limit gauge and working principle of electro limit gauge. I can see here the schematic diagram of the measuring head of electro limit gauge. The construction is shown here. This is the surface plate or table on which we have to place the workpiece. We have to insert the workpiece between the plunger and the table surface. Now the plunger depending upon the height of the workpiece, the plunger will move up and down. If the height of the workpiece is greater than the desired height, the plunger will move up. If the height of the workpiece is lower than the desired height, then the plunger will move down. And then we have an armature which is supported by metal strip springs here. So the other end is free to move up and down. When the plunger moves up and down, this armature will also move up and down like this. It will tilt like this. This being the hinged point. Now when the workpiece is removed, plunger will move back because of this spring force. Now you can see here we have two electromagnetic coils A and B housed in this enclosure. Now these two coils A and B, they form two arms of AC bridge unit. When the plunger moves up and down, the armature will also move. The movement of the armature between the coils so the armature will move in this fashion. So this part of the armature will move up and down in the gap between these two coils. Because of the movement of the armature, the coils sets up out of balance which is indicated by the display unit. So this display unit will directly show the amount of movement of the plunger which will indicate the deviation in size from the desired size. Now this view shows a complete arrangement of an electrical comparator. We have the table surface plate surface and then this is the work table on which we have to place the components or work pieces to be inspected and this is the plunger of the electro limit gauge. So this housing contains the two coils armature etc. etc. So this is the measuring unit. The height of this measuring unit can be adjusted by operating this wheel to accommodate the work pieces of different size. Now the plunger movement sets up an out of balance which is supplied to the recording head. So when there is out of balance because the movement of the armature now this pointer will move over the scale and that movement of the pointer indicates the deviation of the size from the desired size. Now this is the schematic diagram of visual gauging head. Now this is the work table on which we have to place the components. This is the plunger of this visual gauging head which is connected to rod C. So in between the plunger and the rod C we have the magnification device which can be a mechanical liver arrangement. Now we have two electrical contacts A and B. So in between there is a gap between A and B there is a gap in which the rod C will swing. Now the electrical contacts position can be adjusted by operating the micro meters. So depending upon the upper limit and lower limit the position of these electrical contacts is adjusted by operating the micro meters. Now when the rod C is in central position that means where it is not in contact with either here B then that indicates that the work piece size is between upper limit and lower limit and the green light will glow which indicates that work piece can be accepted. Now if the work piece size is greater than the desired size or if it crosses the upper limit then rod C moves to the right and makes contact with B. When the work piece height is greater than the upper limit the plunger will move up and then the rod C will swing and it makes contact with this contact B and then red light will glow indicating that the work piece is above the upper limit and it should be accepted. Now the work piece is other size that means it is lower than the lower limit then the rod C moves to left and makes contact with the contact A and then the yellow light will glow indicating that the work piece size is lower than the lower limit and it should be rejected. So we can see that in this type of comparator the actual size of the work piece is not indicated it will only indicate whether the work piece is acceptable or to be rejected. Now this shows multi-gaging device. In the previous case we have one set of contacts that means at any given particular time at any given point of time we can check only one dimension whereas in multi-gaging devices multiple sizes can be checked at a time. For example we have a work piece wherein we have four dimensions to be inspected this is the first dimension, second dimension, third dimension and fourth dimension. Four dimensions are to be checked in such cases we can have a multi-gaging device since four dimensions are to be checked we can have four gauging units. So the work piece to be inspected is placed on the table and the work parts will come in contact with these gauging units. So depending upon the actual size of the different dimension of the work piece the yellow light green light or red light glow depending upon the size and they will indicate whether the work piece can be accepted or not. Now we will move to the discussion on electronic comparator. So this diagram shows the general arrangement of an electronic comparator. So this is the contact probe or stylus which comes in contact with the work piece. The work piece is to be inserted between the contact probe and the table. So depending upon the height of the work piece this contact probe will swing. Now the electronic circuitry of this comparator is like this we have an we have oscillator and then we have regulated DC supply and there is provision for adjusting zero the range also can be adjusted different ranges can be selected and then magnification can be adjusted. Now these electronic comparators they work on principle of frequency modulation. When the work piece is placed on the table the oscillator frequency alters that is because of change in the dimension of work piece from the preset value. For the desired size of the work piece there is a particular frequency when the size of the work piece differs from the desired size then the frequency gets altered which is indicated by this display unit in terms of the linear dimension. So thereby we can inspect the work piece and then we can decide whether it should be acceptable or rejectable. Now let us study how we can use an electrical comparator. We can see these linkage bars which is used as a reference to set the comparator for the basic size. Now we are watching the electrical comparator LVDT fixed to the stand. We can see the indicator which indicates the deviation of the work piece from the basic size. We can see two knobs are there to set the limits upper limit and lower limit. We can connect two comparators at a time A and B and this is for adjusting the sensitivity of the comparator. Now it is set to 100 units 100 microns. So the highest sensitivity is 0 to 3 microns. Now you can see the knob is set to 100 microns. So the range is from 3 microns to 1000 microns. Now we can see the back view of the indicator. So we can observe the power connection. This is the power connection power cable and we can connect since a comparator to A and B at a time we can connect two comparators. Now the power connection to the indicator. Now you can see this is the LVDT the comparator, electrical comparator fixed to the stand, the rigid stand. You can see it is fixed to the bracket and the height of the comparator can be adjusted by moving it up and down and then you can clamp it using the knob. So this is the clamp to clamp the electrical comparator. Now you can see the object wave inspected. The basic size of the component is measured using the ordinary caliper. So basic size is 19 millimetre. Now the basic size of the workpiece is 19 millimetre. So I have to take two, I have to build a pile of slippage of height 19 millimetre. So I am taking two slip gauges. First one is 9 millimetre and second one is 10 millimetre thickness slip gauge. I am bringing the two slip gauges after cleaning them. Now we can see the slip gauge pile of 19 millimetre thick. So this is the basic size of the component. So I am inserting the slip gauge pile between the table surface and the plunger of the electrical comparator and then indicator is set to read 0 when the size is 19 millimetre. Now you can see the pointer is reading 0. There is a small parallax error. Now the reading is 0. Now I am removing the slip gauge pile and then I am inserting the workpiece to be inspected. Depending upon the actual size of the workpiece, the pointer will move to the positive side or negative side. If the size is greater than 19 millimetre, pointer will move to the positive side and it is reading 4. So we have selected a range sensitivity of 100. So we have to read the top scale. Now let us discuss some advantages of electrical and electronic comparators. Now when these type of comparators, the measuring unit can be at a remote place and display unit can be in a control room. So in the control room, we come to know whether the workpiece can be accepted or rejected or how many workpieces are rejected, how many workpieces are accepted in a particular shift can be, the details can be obtained in the display unit placed in the control room. Now there are very less number of moving parts and electrical and electronic comparators. Hence, the frictional losses are very, very less. So very high magnifications are possible as I have said 10,000 times. The measuring unit can be very small, very compact and hence it is easily portable. Now what are the disadvantages of electrical and electronic comparators? These types of comparators, they require external power supply, that is electrical power supply is needed for the operation of these comparators. If there is any fluctuation in the electrical power supply, it will definitely affect the results obtained from the comparators. And since in some cases, there are heating the coils, magnetic coils are there because of heating of these coils, the measuring unit may cause zero drift which alters the calibration of the comparator. So at regular intervals, we have to recalibrate the comparator to ensure that the comparators are working with the correct calibration. If the scale is fixed with a moving pointer, then the working range is very small with high magnification of the comparator. Generally these electrical and electron comparators are more expensive when compared to mechanical comparators. Now in these type of comparators, the measuring unit can be at a remote place and display unit can be in a control room. So in the control room, we come to know whether the workpiece can be accepted or rejected or how many workpieces are rejected, how many workpieces are accepted in a particular shift can be, that details can be obtained in the display unit placed in the control room. Now there are very less number of moving parts and electrical and electronic comparators. Hence the frictional losses are very, very less. So very high magnifications are possible as I said 10,000 times. The measuring unit can be very small, very compact and hence it is easily portable. Now what are the disadvantages of electrical and electronic comparators? These types of comparators, they require external power supply that is electrical power supply is needed for the operation of these comparators. If there is any fluctuation in the electrical power supply, it will definitely affect the results obtained from the comparators. And since in some cases there are heating the coils, magnetic coils are there because of heating of these coils, the measuring unit may cause zero drift which alters the calibration of the comparator. So at regular intervals we have to recalibrate the comparator to ensure that the comparators are working with the correct calibration. If the scale is fixed with a moving pointer then the working range is very small with high magnification of the comparator. Generally these electrical and electron comparators are more expensive when compared to mechanical comparators. Now let us move on to the discussion on optical comparator. Now let us study the working principle of optical comparator. We can see here we have a mirror which is pivoted at this point and at the distance of D we have a plunger. So this plunger moves up and down when we insert the workpiece between the plunger and the tables. So when the plunger moves up and down, the mirror gets tilted. Now when there is no movement, when the mirror is horizontal we have a light source here, this is the light source and this is the reflected light source and then we have a screen with a scale. Now the angle, this angle is equal to the angle that is available here. This is the normal to the mirror surface. This is the light ray and this is the reflected light ray. So this angle is equal to this angle. Now when the plunger moves up, when the plunger moves up by a distance h then the mirror gets tilted by an angle alpha. Now we can observe that the reflected light, the reflected light tilts by an angle 2 alpha. The mirror tilts by alpha, the reflected light tilts by 2 alpha and this is how we get magnification in optical comparators. Now this picture shows the general arrangement of optical comparator. We have the table surface and this is the workpiece. So depending upon the heights of workpieces, we have the input displacement x. So this is the deviation from the desired size. We have a lever and then there is a pipe out here and the one end of the lever is in contact with the workpiece surface and the other end is in contact with the mirror. So when the pipe out, when the pointer, the one end of the lever moves by a distance of x, the other end moves by a distance y because of this mechanical amplification. Now when the, this end of the lever moves by y, the pipe out. Again we have a mirror here and then a pipe out here. So because of the moment of this moment y, the mirror tilts by 2 theta and the reflected light moves by 2 theta. So if the moment at this point is a small x, the moment of the pointer over the scale is capital X. So the, hence we get the, this moment is magnified here. So we have mechanical amplification as well as optical amplification. Now the main advantage of optical comparator is that it is capable of giving the higher degree of magnification due to the reduction in moving, less number of moving members are there and higher degree of magnification is possible because we use two types of magnifications, one by mechanical amplification and further it is magnified by optical means. So far we discussed about the different kinds of comparators wherein the, there is a plunger in the comparator which comes in contact with the workpiece surface and then the plunger moves depending upon the size of the workpiece and the moment of the plunger is sensed and then it is amplified and the amplified value is compared with the size of, with the desired size of the workpiece. Now let us study about the another method of comparison wherein we use chart gauges. The chart gauges contain lines which denote the tolerance limits of the parts. I can see here we have a diagram here, this is the screen of the optical projector, optical comparator and then a chart gauge is fixed mounted on the screen of optical comparator. These chart gauges they contain lines you can see here this is a line corresponding to the upper, upper limit of the workpiece and then we have another line inside corresponding to the lower limit of the workpiece, low limit of the workpiece and this is the shadow or image of the workpiece. So image is made to fall on the chart gauge, the shadow of the part is made to fall on the chart gauge and if the shadow is within the tolerance zones that means you can see here the shadow the edges of the shadow are between upper limit and lower limit. Now if this is the case the workpiece is acceptable. Now I can see here how the comparison is made. This is the screen of optical profile projector wherein on the screen a chart gauge is mounted. You can see here we have different lines corresponding to different radius values and then we have circular scale using which we can measure the angle. So this is the workpiece image and this is the chart gauge mounted on the screen. So these apart from the comparison these chart gauges are used to check fillets, chamfers, threads, etc. For example, so we have a workpiece like this and then using the radius chart gauge we can measure the radius of this fillet that means the shadow of this work part is made to fall on the chart gauge. Say this is the shadow of this particular part and we can see to which radius line this shadow is coinciding and that particular radius will that particular line will indicate what is the radius of the fillet. Similarly, we can measure the chamfers and then we can also check the threads, screw threads using these chart gauges. Now how do we design the chart gauges? The following considerations are very very important in designing the chart gauges. The chart gauge should be made of a dimensionally stable material. A rigid material is preferred for normally plastic sheets. Clear transparent plastic sheets are used or the chart gauges are made on the soda line glass plates. The humidity and temperature coefficients of the chart gauge material must be considered. The lines on the chart gauge must be uniform in width with sharp edges. The lines must withstand the normal usage and cleaning operation. The chart gauges should be calibrated at regular intervals determined by the usage. The chart gauge master image should be produced with a precision plotter to 0.025 mm or better accuracy. The calibration fiduciary marks should be included for dimensional control. They should be stored. The chart gauges should be stored flat and they should never be rolled and they should be stored in a normal gauge room in the environment wherein the proper temperature and humidity is controlled. Now there are custom chart gauges depending upon the application or the customer usage. These custom built chart gauges are made by using a CAD file or a marked up print along with the chart size. And what is the magnification needed? She is also should be known to prepare the custom chart gauges. Now we can see here this is a custom chart gauge with the upper limit and lower limit of the work piece. Commercially standard chart gauges are available in the market and they are reproduced under controlled conditions from precision master on a dimensionally stable plastic sheet or soda line glass. Standard chart gauges are available in various line patterns at various magnifications like 10x, 20x, 50x. So different magnifications chart gauges with different magnifications are available in English as well as metric units. I can see some of the standard chart gauges. So this is a centerline chart gauge, radius chart gauge for example. So we have a work piece round work piece. So the round work piece is placed on the table of the optical comparator, profile projector and shadow is obtained on the screen which contains the radius chart gauge. The image is compared with the lines printed on the chart gauge. So with which line, with which circular pattern is coinciding that gives the radius value or the diameter value. Similarly thread screw thread chart gauges are available, protractors are available using which you can measure the angles. So this picture shows a combination chart gauge. You can see here one side we have which gives radius values and other side we have angle, angle can be measured, angle of work pieces can be measured. So this is a radius angle chart gauge. Now we can see a component which is having an angle. So it is placed on the table of profile projector. We wish to measure the angle between these two edges. Now we have selected an objective of 10x and using a chart gauge of 10x it is mounted on the screen of profile projector. So this angle chart gauge is used to check the angle on the work piece. Now we can see one edge the shadow of one edge of the work piece is coinciding with the zero line and the other edge of the shadow is coinciding with the 45 degree line. So the wedge angle on the work piece is 45 degrees. Like this we can use the chart gauges for measurement of angles. So this is the complete view of chart gauge. Radius can be measured as also angles can be measured using this particular chart gauge. Now what are the advantages of the optical comparators which uses chart gauges? So these optical comparators have a small number of moving parts. Now let us study what are the advantages of optical comparators. So these optical comparators have a small number of moving parts and hence less friction and higher accuracy is possible. In the optical comparators the scale can be moved past a datum line and thus have higher range and no parallax errors. It has a very high magnification and optical lever is weight less. So inertia effects are very very less. Now there are some disadvantages of optical comparators. As the instrument has a high magnification heat from the lamp and transformer etc may cause the setting to drift and electrical supply is necessary for the operation of optical comparators to light up the lamp. The operators are usually large and very expensive and compared to other types. When the scale is projected on a screen then it is essential to use the instrument at a dark room in order to take the readings easily. If there is too much of light in the room where the optical comparators are placed then taking readings becomes a bit difficult. The instruments in which the scale is viewed throw the eyepiece of a microscope. Such instruments are not convenient for continuous use since the operator is subjected to high fatigue. Let us conclude this lecture 2. In this lecture we discussed about the different kinds of mechanical comparators such as sigma comparator, Johansson microclator. We discussed the construction and working of these sigma and Johansson microclator. Also we discussed about the various advantages and disadvantages of mechanical comparators. Then we also discussed about the electrical and electronic comparators. In this section we discussed about the electro limit gauge, visual aging head, electronic comparator and various advantages and disadvantages. Also we discussed about the optical comparators. So in this section we discussed the working principles of optical comparators and we also discussed about the use of chart gauges, the different types of chart gauges and how to use them. And also we discussed about the advantages and disadvantages of optical comparators. In the next lecture we continue the discussion on the comparators. Thank you.