 I welcome you for the module 5 lecture 2 in the series of lectures on surface finish measurement. In this lecture we will be discussing about the various parameters which are used to specify the surface roughness and we will also study about the different methods of surface finish measurement like contact methods and non-contact methods. And also we will study about the various elements of stylus type instruments. We will discuss about the types of skids and the design of stylus and the out of phase condition of the skid. And then we will also learn how to select the measuring parameters like cutoff length and then finally we will see some experiments. Now we will start with this parameters roughness parameters. So first one is RA roughness average which is also known as central line average. I can see here after getting the profile the workpiece surface is kept on the table of the measuring instrument and the stylus will move on the surface and finally we get the profile as output from the measuring instrument. Now we have to analyze this profile. In order to analyze this profile we can see at we have to select the length, sampling length L and we have to draw the ordinates h1, h2, h3, h4 up to hn at equal distance and then we have to measure the heights of these ordinates and then finally we should find the summation of heights like that is h1 plus h2 plus h3 up to hn divided by number of ordinates. So this will give the RA value. If ordinates are measured in terms of micrometers then we get the RA value in terms of micrometers. So another way of, see in the previous case you can see here when we select the ordinates at regular intervals some of the important ordinates we may miss. For example here we have this peak. So this peak will be losing similarly if we draw the h7 here. So we will be losing this valley depth. So we get a value which is somewhat erroneous in order to and also it is difficult to draw too many ordinates at very close spacing and it becomes difficult to measure. So we have another parameter RA which can be analyzed by means of measuring the areas. You can see here we have the main line or central line and below the central line we have areas a1, a3, a5 etc and above the central line we have areas a2, a4, a6 up to an. Now these areas can be measured using the planimator so that measurement of ordinates is avoided. So accurately we can measure the areas using planimator and then we can measure, we can calculate the summation of all the areas divided by the length. So this will give us RA value. So what are the advantages of using the RA parameter? This is most commonly used parameter to monitor the production processes. So this is the default parameter on a drawing if the parameter is not specified. So this RA parameter is available in almost all commercially available surface finish measuring instruments and this is statistically very stable and repeatable parameter. And this is very good for random type of surfaces such as grinding ground surface, a good parameter where a process is under control and where the conditions are always same like cutting tapes, plates, plates etc are same then this parameter is better to use. And disadvantage of RA is not, this is not a good discriminator of different types of surfaces. So no distinction is made between peaks and valleys. Now another parameter is RQ, root mean square roughness. So in this case instead of taking only h1, h2 values the squares of ordinates are taken like this h1 square plus h2 square plus h3 square like this divided by number of ordinates under square. So this will give us RQ. RQ is typically 11 to 15% higher than RA and RQ is more sensitive to peaks and valleys than RA because the amplitudes are squared. And this is used to control very fine surfaces in the scientific measurements and statistical evaluations. Now the next parameter is RY, this is also known as RMAX, you can see here this is the profile that is obtained over a sampling, a particular sampling length. Now the gap between the peak and the valley within the sampling length is termed as RMAX or RY. So it is the vertical distance between the top of highest peak and the bottom of deepest valley within the sampling length. Now this is RZ, so we have the evaluation length which is divided into 5 equal parts. So we have 5 sampling lengths RZ1, RZ2, RZ3, RZ4 and RZ5 and in each sampling length we have to find the peak to valley distance, now this is RZ1, similarly peak to valley in the second sampling length there is RZ2, RZ3 like this. Now after finding RZ1, RZ2 up to RZ5 the average peak to valley profile is calculated, so that is RZ. Now this is more sensitive than RA to changes in the surface finish as maximum profile heights and not averages are being examined. RMAX is useful for surfaces where a single defect is not permissible, for example a seal with a single scratch, if there is a scratch and a seal that also will be considered. So if it exceeds, if the RMAX exceeds a certain limit then that particular seal is rejected. And then RZ and RMAX are used together to monitor the variations of surface finish in production processes. So similar values of RZ and RMAX indicate a consistent surface finish while a significant difference indicates a surface defect in an otherwise consistent surface. Now here we can see a profile in which we have considered 5 sampling lengths RZ1, RZ2, RZ3, RZ4 and RZ5. We can see here in RZ1 value is 3.2 micrometer that we can see here this is the peak in this particular sampling length and this is the valley. So this distance is about 1.6 micrometers and this is about 1.6 micrometers. So when we add these two we get 3.2 micrometers. Similarly the RZ value for this particular sampling length is this one. So here this is 2 micrometer and here it is 2 micrometer. So peak to valley distance is 4 micrometers. Similarly RZ3 is 4.1 and RZ4 is 2.9 and RZ5 is 3. So when we calculate the average of these 5 values we get this RZ of 3.5 micrometer this is the average RZ value. Now coming to the next parameter this is RP. Now this is the gap this is the highest peak the maximum distance between the center line and the highest peak is termed as RP within the given sampling length. Also the distance between the center line and the lowest valley is known as RV value and then RT this is maximum peak to valley height the absolute value between the highest and the lowest peaks. So RT is equal to RP plus RV when we add these two we get RT value. So next one is RTM mean RT mean peak to valley roughness. Now we can see here we have this evaluation length which is divided into 5 sampling lengths and this is the first sampling length in which we have peak to valley distance it is nothing but Z1. Similarly the second sampling length we have Z2 this is the peak to valley gap similarly Z3 Z4 Z5. So when we calculate the average of these 5 values we get RT mean peak to valley roughness and then this is Pt value total peak to valley profile. So over the evaluation length the gap between the top most point and the peak peak and the lowest point and this gap is total peak to valley profile height. And we have another parameter 10 point average 10 point height average is also known as RZ value RZ parameter. So again in the evaluation length we have to consider 5 peaks you can see here in this profile P1 P2 P3 P4 and P5. Highest peaks are considered within the evaluation length similarly 5 lowest valleys are considered you can see here P1 P2 P3 P4 and P5 and now the average absolute value of the 5 highest peaks and the 5 lowest valleys over the evaluation length will give us RZ value that means we have to add the ordinates P1 P2 P3 P4 P5 and minus the summation of ordinates V1 V2 V3 V4 V5 then if we feed these values in this expression we get RZ value 10 point average height average value. So now coming to the waviness parameters so this is a waviness is a larger component of surface texture upon which roughness is superimposed like we have the surface roughness and then we have this waviness. So this profile will give us waviness profile and the parameter that is used to specify this waviness is waviness height that is the maximum height of waviness data within the evaluation length this is the evaluation length and this is the peak and valley so this gap is known as waviness height and it is specified by the symbol wt. So this waviness height parameter is used where in addition to roughness waviness is also critical. Now moving to the spacing parameters we have this parameter PC we have this profile wherein we have peaks 1, 2, 3, 4 etc and valleys 1, 2, 3, 4, 5 etc. So this PC it is nothing but peak count or number of peaks included in the analysis of the profile a peak is defined as a data point whose height is above a software selected bandwidth this is the software selected bandwidth and we have some peaks above this bandwidth so these are considered for the calculating the PC. So if we consider an area of surface then the number of peaks per unit area that is termed as peak density and other term is yes this is the average spacing between local peaks over the evaluation length. Now we can see here in this profile we have the evaluation length and we have peaks here one peak and second peak this is the gap between two peaks similarly we have considered one more peak here and another peak here this distance between this peak and this peak is s2 and similarly distance between this peak and this peak is s3 and so like this we have to measure the gaps these gaps that is spacing between the peaks and then we should find the average spacing using this expression. So varying s is equal to s1 plus s2 up to s6 divided by if we consider 6 peaks then s1 plus s2 up to 6 divided by number of peaks considered so this will give the average spacing. So now having understood the various parameters used to specify the roughness we will move to the measurement of surface roughness how we can measure the surface roughness how we can get the various parameter values and what are the kinds of measuring methods used what are the different types of measuring instruments so those things we will study. So roughness may be measured using any one of the following methods so we can measure the surface roughness of the component in the offline method that is after the production is over the part is taken out of the machine tool and it is de-oiled and deburred and then it is kept in the proper atmospheric temperature condition and then appropriate instrument is selected and then the surface is measured to get the various parameters. So in the offline methods we have different we have contact type and non-contact type measurement methods so in the contact type so we have stylus probe type instrument and macrin 3 instrument and in the non-contact type offline method we have optical systems like tool makers microscope can be used to analyze the surface and then we have laser based system which works on reflected light intensity method and then image processing systems are also available and then we can also use some comparison standards. Using comparison standards we can check the surface under question so we can use the visual examination or tactile examination so we will also learn about the comparison standards and then there are in process measurement method that means when the workpiece is being machined we can measure the surface finish without unloading the workpiece from the machining setup. So many methods are being used to measure the surface roughness during machining is in process so we can use machine vision system or inductance method in the machine vision the light source is used to eliminate the surface with a digital system to view the surface and the data being sent to computer for analysis. The digitized data is then used with a correlation chart to get the actual roughness values that means initially we should use some standard specimens whose surface roughness is known and then using those standard specimens we can calibrate the machine vision system that means for a particular surface roughness what is the digital data we get like that we can calibrate and then we can use the component for which surface is to be tested we should keep them under machine vision and then what is the digital signal we obtained and then that should be compared with the digital data obtained from the standard specimen. So that is the method used in machine vision system and inductance method is also used in which pickup is used inductance pickup is used to measure the distance between the surface and the pickup this measurement gives a parametric value that may be used to give a comparative roughness so it will be something like this we have the workpiece which is being machined and then we can always use in inductance pickup as the surface roughness varies this gap will be varying so that gap is sensed and the signal is sent out by the inductance pickup which is supplied to the computer for analyzing the surface roughness and ultrasound systems are also used a spherically focused ultrasonic sensor is positioned with a non normal incidence angle above the surface so it is something like this so this is the workpiece and then ultrasonic sensor is positioned in non normal incidence angle ok which is not normal to the surface and then the reflected sound wave is sensed by another receiver and the signal is sent to the computer for analyzing the surface. So when the due to the asperities the reflected sound wave characteristic will change so which is used to calculate the surface roughness and then we have pneumatic method so this we will study we will discuss after sometime now let us study some of the stylus probe instruments or the design of stylus probe and what are the various elements of stylus type instruments I can see here we have the surface being measured we have asperities peaks and valleys etc etc and then we have a stylus here a pointed stylus so that pointed stylus is made to move on the surface under question and because of these asperities the stylus will move up and down ok and then because of this up and down moment here we have a pickup so the design and construction of these this pickup will study later the stylus will move up and down and the pickup will give the signal which is sent to the computer for analysis purpose. This stylus tip it has a very small radius of 5 to 10 micrometer and normally it is made out of diamond material to resist the wear now pointed probe is drawn slowly over the surface again the movement is made probe is made to move at a particular slow speed and then the variations due to variations stylus will move and then the signal is digitized and it is sent for the computer sent to the computer for analysis purpose in the commercially available instruments there are magnification switches depending upon the surface whether it is rough turned surface or shaped surface or milled surface or ground surface or lapped surface so depending upon the machining process used so magnification switch magnification can be selected for lapped surface very high magnification is used whereas for rough turned surface very low magnification is enough also we can specify what is the cutoff length or sampling length depend again this is again depending upon the machining process that is selected if it is grinding process lapping process so where in the we get fine surface cutoff very small cutoff length of less than 0.8 millimetre is used so if it is rough surface turned surface or milled surface we can use cutoff length of 0.8 or 2.5 like that so the cutoff length selection we will discuss again after sometime the instrument will display the various surface finish parameters like RA, RP, RT etc etc there is a display device also we can get the print out of the profile the profile print we can obtain at different magnification values now here you can see a commercially available instrument so you can see the various buttons this the button to start the instrument switch on the instrument and then for starting the movement of the stylus and you can see this is the display device and the cutoff length selected is 2.5 millimetre and you can see it is now it is showing RA value 0 micro meter so different by operating this data point different parameters we can select and the value of that particular parameter is displayed here and we can see the probe here the pick up pick up we can see here so which will move on the surface and then the data that is obtained is analysed and then the particular parameter is calculated and then it is displayed so in this case there is no printer it is only display device now what are the various elements of a stylus type instrument I can see here this is the surface under question which is to be measured and then we have stylus here a diamond point and then we have pick up here the details of pick up will study after some time and there is a traverse unit or in other words drive unit so this stylus should be made to move on the surface at a predefined speed so we need to have a driving unit so this is the driving unit to move the pick up and the signals given by this stylus they are amplified and an appropriate filters are used for example if we want only waviness that can be indicated here if we want only the roughness parameter we can filter out the waviness and sometimes we have to eliminate the form error also so for such a thing different kinds of filters are used and finally the value is indicated or the roughness parameter is indicated or waviness parameter is indicated depending upon the requirement and here you can see there is a cutoff length selector depending upon the machining process we can select appropriate cutoff length also we have a switch to select the appropriate magnification so if the surface is very fine lapped surface then we should use very high value of vertical magnification if it is rough surface we can lower the vertical magnification and you can see the printer attachment is also there so we can get a print of the profile now you can see a commercially available setup here this is the workpiece for which surface finish is to be measured and then this is the instrument stylus type instrument and this is the probe and now you can see depending upon the height of the workpiece we have to adjust the height of this instrument otherwise there will be error in the measured value so for adjusting the height you can see a stand is provided so depending upon the height of the workpiece we have to adjust the height of this instrument and then the it is connected to a printer and display device so here various buttons are there to select the appropriate magnification cutoff length and what type of what are the various parameters needed etc. can be selected here and then the value the selected parameter is displayed as well as the profile print we can obtain now how the data is processed you can see here we have a flow chart here so the real surface profile now this is the real profile surface which is to be measured which is to be analyzed and then we have to trace the profile that means the stylus of the surface finish measurement instrument will should be moved on the surface to be tested you can see here we have a conical stylus and then radius tip so this radius will be like 2 micrometer or 5 micrometer or 10 micrometer and this stylus or probe tip will move on the surface now you can see this is the center of this probe curved probe and now when we move this stylus now we can see this is the locus locus of the center point this point okay now we can see the locus we have a sharp point here and here also we have a sharp point and then we have a peak here so at this place you can see instead of getting a peak we have a radius here so a sort of filtering effect will be there because of this radius of the stylus probe so we get the traced profile like this so which consists of the various elements like form, waviness, roughness etc etc that means the total profile which is a combination of form error, waviness and roughness we get and then using appropriate filters low pass filters and high pass filters different filter Gaussian filter different filters are used to get the required profile so if we pass this data obtained in this total profile via this low pass filter then we get the primary profile okay you can see here primary profile parameter so which has waviness as well as the roughness the form error has been removed and then again this data should be passed through high pass filter so that waviness can be removed and now we get the roughness profile you can see here all the waviness has been removed here only roughness parameters we get here roughness profile parameters we can obtain and if you pass this profile data primary profile data via the band pass filter then we get only waviness profile and we can eliminate the roughness parameters so the profile what we get will be something like this so its roughness element micro irregularities has been removed and we get only waviness profile parameter so like this by passing the data obtained via the various filters we can get the required parameters now how do we select the sampling length or cutoff length now you can see here we have listed different machining process milling boring turning grinding planing etc and then we have sampling length for milling we can select 0.8 millimeter sampling length or 2.5 millimeter or 8 millimeter 10 millimeter again this depends whether the milled surface is fine or very rough if the milled surface is very rough we can select the cutoff length of 2.5 or 8 if the milled surface is very fine by adjusting the fine machining parameters if we get fine finish then we can go for 0.8 millimeter cutoff length similarly we know that you can see here milling in the milling process they normally we get an RA value of 0.8 to 7 micrometer so and in the case of grinding we get very fine surface finish that is RA value of 0.025 up to 1.6 micrometer so better fine finish we get here so in the case of grinding we can select lower cutoff length like 0.25 millimeter 0.8 millimeter something like that now this is by knowing the machining process we can select appropriate sampling length or if we know roughly what is the surface finish that is obtained then also we can select the cutoff length for example if we have a surface if we know that the surface roughness on the surface under question is between 0.02 and 0.1 then we can select a finer cutoff length of 0.25 millimeter or if the surface finish is in the range of 2 to 10 micrometer then we can go for a cutoff length of 2.5 millimeter okay so by knowing approximate roughness value we can select cutoff length or by knowing the process also we can select the appropriate cutoff length now we should know one more thing that some ratio of sampling length to evaluation length now you can see here normally evaluation length is divided into 5 sampling parts or if we know the sampling length for example say 0.8 millimeter is the sampling length 5 times of this will give us the evaluation that means 0.8 into 5 that is 4 so 4 is the 4 millimeter is the evaluation length similarly if the cutoff length is 8 millimeter 5 times of this is 40 millimeter now what is the effect of cutoff length selection on surface finish measurement I can see here this is the profile obtained without any filtering and then when we select a cutoff of 0.8 millimeter okay then the finish surface finish value obtained will be 3.5 to 4.2 micrometer but instead of selecting 0.8 millimeter cutoff if we select a very finer cutoff length of 0.25 millimeter then the roughness value is 1.8 to 2.2 micrometer RA similarly if you go for very finer surface finer cutoff length then you can see here this is 0.08 millimeter cutoff is selected then the RA value obtained will be 0.95 to 1.05 micrometer that means if we change the cutoff length the RA value will also change so we should exercise a lot of care while selecting the appropriate cutoff length. If we select wrong cutoff length then the measurement data that is obtained will be useless so that is why the standards specify for the different machining process what should be the cutoff length that is to be selected or specified in various standards. Now we should understand one more thing that is distortion ratio so I can see here we have 4000 divided by 200 so 4000 is the vertical magnification so I can see here 4 parts are there this is 1 and this is second part and this is third and this is 4 part so each is having a value of 1000 so total height is 4000 units so this is the magnification 4000 times the magnification the roughness is magnified so and the horizontal magnification is 200 times now when we select this particular distortion ratio this is the profile that is obtained. Now if we change the distortion ratio then the profile the surface graph gets distorted we get another different kind of surface graph you can see here the distortion ratio is 4000 divided by 50 4000 by 50 so the vertical magnification remains same whereas horizontal magnification has been reduced by 4 times here it is for 200 units and here it is 50 units that means the length is compressed so this profile is compressed so just by observing the profile graph we cannot say so if we change the distortion ratio and the look of the profile will change we do not get similar graphs similarly we have one more example here this is the ground surface profile with distortion ratio of 10250 you can see here for milled surface okay vertical magnification that is selected is 4000 whereas the ground workpiece the magnification selected is 10000 since the surface will be very fine the very high magnification we have to select to get the profile. Now the same ground surface when we change the distortion ratio now you can see vertical magnification is the 10000 only whereas the horizontal magnification is changed to 200 then we get a profile like this and now let us study about the stylus and datum so there are two types of stylus instruments one with true datum we say skidless instrument and other with surface datum wherein some skid is provided to establish the reference reference. Now this picture shows true datum or skidless instrument I can see the probe which is moved on the surface and the datum is provided by the instrument external to the surface so the datum is not obtained by the surface itself it is provided outside so that is known as skidless instrument or true datum instrument you can see another version of true datum instrument here we have used an optical flat very flat surface is used as true datum and this is the pickup unit and this is the stylus unit and when we move the pickup you can see the stylus will move up and down depending upon the surface roughness and waviness. Now the advantage of true datum is that the resulting graph is nearly a true representation of the surface along the line showing form error waviness and roughness so it gives a total profile the disadvantage of this system is that it is very difficult to set up the instrument. Now we can see here the datum true datum height should be properly adjusted so that the it is parallel almost parallel and in line with the surface in question so very precise alignment the surface is required particularly if surface of tapered component is to be measured now you can see the mounting of tapered component when the tapered components are to be measured how do we adjust the datum in the case of true datum system so you can always use a work table with ball joint you can see here a work table with a ball joint so depending upon the taperness we can always align this we can change the inclination of this table so that the surface becomes parallel to the stylus movement or the probe movement like this so we have a tapered component like this and then we have the stylus and then externally we have the datum so the movement of the stylus should be parallel to this datum so in that case this line should be made this generator should be made parallel to this datum so in such cases we can use the work table with ball joint we can tilt the work table so that the generator becomes parallel to the movement also another provision is we can tilt the drive unit itself you can see here there is a knob here for adjusting the height of this that means to change the inclination of the movement and also there is provision for moving the total drive unit up and down for vertical height adjustment now let us conclude this lecture in this lecture we studied the various parameters which are used to specify the roughness like roughness parameters, waviness parameters and spacing parameters and also we learnt about the different methods of measurement of surface finish also we started discussion on stylus type instrument we discussed about this kid and then the stylus and then tip radius of the stylus etc now we will conclude this session in the next class we will continue with the discussion of the stylus type instruments thank you.