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Module 1: Tribology, Surface Roughness in Machining

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Tribology in Machining

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Video 1: Chip-Tool Tribology
So, currently what ah I am going to deal in this class is a chip tool tribology. So, majorly ah the class all revolve around a chip tool tribology, how do you measure the experimentally ah by using the tribometers, and all those things ah tool workpiece tribology is ah not discussed much, but; however, I will just brief you sticking and sliding zones which are nothing but the part of a chip tool tribology types of lubrication. So, coming to this one chip tool tribology basically came into existence are the first ah they started in a 1970's where a interactions are studied ok. So, you can see this one and the during that ah what they have done is they have taken the transparent sapphire tool. Normally, the sapphire is one of the hardest materials. So, they have taken it as a tool ok. Since the hardness is most important in cutting tools. So, they have taken it. Normally nowadays if you see the sapphire is commonly used as a nozzles for abrasive jet machining are some of the advanced machining processes ah abrasive processes ok. So, they studied using this one because if it is the transparent. So, they can understand how the chip flow, on how the ah stick slip phenomena takes place and all those things ok. So, if you look at this picture, and we have 2 zones one is a sticking zone another one is a sliding zone ok. So, this comes under the zone one that is called a sticking zone and comes the sliding zone ok, there are the 2 zones ok. So, these zones in the sticking zone chip, material stick to the rake surface of the tool. So, normally it will have a perfect contact, ah I am not saying that perfect behind the person, but ah it is a having a good contact and a sliding zone. It slides across the rake surface this is 2 surfaces one is the sticking zone another one is a sliding zone ok. So, if you can see, how the sticking zone and sliding zones are ah differentiated from this point of view. So, this is a experimental which we have recently done some of the work in this area, for understanding the basics of the chip tool tribology. So, if you can see the cutting tool machining the workpiece in this case ah carbide tool is machine in the stainless steel . So, for understanding if you see the ah primary shearing zone. If you this is the experimental one, the experimental picture. So, this is a schematic one, how we are analyzing a schematic one. If you see this one, what will happen this is primary shearing zone, and ah followed by the secondary shearing zone, which is most important in this case? Primary shearing zone is because of the plastic deformation, in the secondary normally we deal with . If you see this ah area, zooming we have a sticking region and followed by the , it represents a height of deposition that is taking place. So, the red color and ah white color that you are seeing at this position. This stands for the peaks or the height that is forming assume that this is my surface, how the things are piling up on top of it. So, built up it formation, and ah the all these things are studied there ok. So, that is why this is a green region height is slightly less . So, that is what the ah beneath the chip material is adhered with the tool particular. If you see the sticking zone surface roughness and sliding zone surface roughness how it will changes and all those things. If you see in the previous slide, you have seen the sticking and sliding regions, if you take the surface roughness of ah the sticking zone ah normally you will have a very rough surface ok. So, you can see the 3D profile, a 3D roughness profile normally this we will taken from the non-contact surface profilometers. So, this is a 3D surface roughness profile. So, you can see the completely it looks like a some craters are formed, and some deposition is taken place. This is a completely random. This is corresponding to my sticking region. In the sliding region basically, they would not be proper contact ok. In the proper , if there is no proper contact what will happen it will slide, the chip will slide on my workpiece. What was happening here is, if I have a workpiece, and ah my tool is there, chip will move like this ok. So, the first portion is ah this is sticking zone. The next one is a sliding zone. So, there is a gap it is forms like a wedge wedge type of thing what will happen this is the ticking region, and ah in between you will have a sliding region. There we will have asperities will rub against and ah it will form it is one lines ok. That is why this is called a sliding region. So, slide these are forms the sliding marks can see the sliding marks. So, what you are seeing these lines are ah some of the textures that we have done, but it is nothing to do with ah the current physics what we are talking about. These are the marks ok, that that is about the thing. See, now we will see the surface morphology, how the surface morphology, because the surface morphology plays a major routine ok. The surface morphology means how the surface look like on the surface, that is called as surface morphology, surface morphology includes surface roughness ok. So, coming to the sticking region, if you see the surface, it is completely rough. If I am taking a this one is a 3D surface profile , surface roughness profile . And this is 2D surface profile. So, this is 2D surface profile if you see the sticking region, if I am measuring in this region. Normally some of the average region if I take, any randomly some of the region that I will take normally, these will have a ah peaks and valleys peaks and valleys. This is a peak, and these are valleys ok. So, this is a sticking region. So, this is too rough at the same time if you see the sliding region, sliding region the beauty about the sliding region is we will have a gradual flattening ok. If you see here any surface in the sliding region, normally these are the sliding region. So, you have a rough surface here, rough surface rough surface and it is smoothly ok that is what you can interpret here. You can see the surface it is a rough zone here, but it is this roughness is ah much lesser than the sticking region. Then normally it will become partially flat region ok, is called a flat region ok. So, that means, that ah there are marks on the surface, and ah wave marks then it will become flat ok. So, that is the beauty about the sticking region and sliding region. The surface roughness and sticking region is much much higher compared to the sliding region. And also, you can ah um see that ah if the sliding region to the start of a sliding region to the end of the sliding region you will have a a very rough to the flattened surface. And then onwards you can see the completely flat surface, because there is no interaction between tool and chip ok. That is that is about the surface roughness ah in the tool chip interface ok. So, normally if you see a commonly question. So, why the forces are high in when mission with a weared tool ok. So, if if I ah ask you a simple question. Like, I have a 0-rake angle tool. This is a 0-rake angle tool. So, I am a having a positive rake angle tool, rake angle is alpha equal to 0, alpha is positive Normally here force equal to F 1 here force equal to F 2. If I am giving you a 0-rake angle tool ok with crater wear basically. So, crater wear looks like with in terms of ah the crater depth we measure. So, we measure in terms of this one ok. In that circumstances, what will happen it looks like a positive rake angle ok. Here force is F 3 . In generally what ah we study about the forces. Normally, F 2 less than F 1 and F 3. But if you see the geometry wise. If I just ah remove this portion, weared portion what will happen? So, you should get F 3 also like F 2, normally the if the rake angle is 0 my forces are F 1 if my positive rake angle forces are F2. Normally positive rake angle experiences less forces, because chipping flow will be uniform, and there is no it is aerodynamic and all those things. In terms of 0 rake angle, the fraction of forces is very high because the chip will move parallel to the rake surface That is why normally F 1 is higher than F 2 ok . If the tool wear takes place, that is crater will takes place, in that circumstance the force basically experimentally it will increase. But geometrically if you see, it is resembling like your positive rake angle. These 2 are looking alike in that circumstances F 3 should be less than F 2, but this statement is wrong why. So, for to answer this one, we will see the things how do handle it. If you see the surface roughness is sticking zone. Normally whatever the surface roughness that you are putting in a tool, what will happen this is completely rough. At the same time if you see the tool wear, tool wear is starting slightly ahead of cutting edge. It is not from the cutting edge. Normally, whatever the angle that will give is starting of this one. So, that it will be having a smooth flow, but here the roughness is very high, at the same time it is start away. From this cutting edge these are the 2 reasons ah there are many other reasons ok. So, these are the 2 reasons that why normally F 3 is much much higher than your F 2 and F 1 ok. Because tool wear may be geometrically same, but it is not. So, ah practically ah the forces arevery high. Because of this reason as a sliding region also it will have the negative effect, because of the sticking and sliding zones surface roughness. So, obstruction will be there if the obstruction is there the frictional forces will goes up. Normally, there are ah 2 mechanisms whenever; I already told you, when I am talking about the tool wear mechanisms. So, there are tool a tool types of abrasion takes place in the the chip tool interface, one is a 2-body abrasion. So, 2 body abrasion means when the 2 mating surfaces are abraded each other ok, because of the relative motion ok. One is moving in this direction another one is moving in this direction. Because of relative motion between 2 bodies, if there is a wear and tear takes place, that is called 2 body abrasion. So, if the mechanism is abrasion. So, the 3-body abrasion means, whenever 2 bodies are abrading each other are moving related to each other, and if there is a third body which can rotate about it is own axis is in between them.Normally what I mean to say is that if the chip tool interface is only interacting; that is, called 2 body abrasion. If I some people nowadays are using the nano fluids as the cutting fluids. So, nano fluids will have particles. At the same time, there is a possibility of the tool which is made up a powder metallurgical. If the binder is loosens and goes off what will happen it may releases the particles into the chip tool interface region. This also has one probability, that the particles come into the chip tool interface. Because of this you will have this particles are no more adhere to the tool material ok. So, they can rotate abouts their own axis because they are independent enough. In that circumstances it is called a 3-body wear. Why I am talking about a 2-body wear and 3 body wear is, nothing is how we have to measure 2 body abrasion and 3 body abrasion  approximately using experimentally, because tribology is itself is a big subject ok. We have to correlate with respect to machining. So, there are some standards ASTM standards that one can follow So, however, the abrasive wear resistance normally is a function of  hardness, if the tool is much much harder what will happen. So, ah what will happen normally the wear resistance will be more ok. Hardness is nothing but resistance to penetration. If the penetration is less what will happen abrasion is less ok . Now you understood what is the difference between 2 body abrasion and 3 body abrasion ok. So, 3 body abrasion takes place when there is a chance whenever you use the nano fluids as a cutting fluids ah , solid lubrication if we are using. Or whenever you are using ah whenever your tool got ah whenever temperature of the tool goes high and binder goes ah loosened then the particles of the powder metallurgy tool , some of the particles may come and ah in between chipper tool may have their independency in that circumstance of 3 body abrasion ok. So, this is about the 2 body abrasion as well as 3 body abrasion ok. So, because of a these ah 2 body abrasion as well as 3 body abrasion, there will be a lot of a frictional heating takes place ok. So, the frictional heating normally the frictional heating will takes place in terms of a in the region where a chip tool interface region This is the region. So, the rate of energy input in the friction is the product of frictional force and the sliding velocity ok. So, this is the multiplication or the if you multiply frictional force, and sliding velocity normally you will get the frictional energy ok. So, the normally perhaps 5 percent is consumed or stored in the material. This frictional energy what will happen if it is stored in the material what ah the temperature goes If the temperature goes up what will happen micro structural defects; that means, that if in your earlier courses, if you see, what will happen there is a core cold holding and a hot holding in the metal forming processes. If the temperature goes up, what will happen? My microstructure will change. For example, if you see black smithy if at all you want to make a sharpen the knives , will happen you just put into the ah fire then after 10 minutes or 15 minutes, just you will take out the orange red one. So, then because the number of a atoms in the knife is same, but the thing is that whenever you take out after sometime what will happen it might have slightly bulged enough, why? Because the micro structural changes taking place because of the temperature. Then what you will do you will take on the edge and you will hit it. So, that it will become sharp then you will put into the water. So, that again it will go to it is original microstructure. May not be 100 percent original microstructure. It will go. So, the sharpness will increase what the bottom line of change it is microstructure. So, because why the microstructure is taking micro structural change is taking place, because of the temperature if the temperature is stored what will happen? The microstructural defects or microstructural changes will takes place That is about if the temperature is still more what will happen that will be defects cracks and all those things will takes place ok. So, this is about the frictional heating drawbacks ok. So, frictional heating is dissipated as a heat. Normally whatever if the frictional energy whatever these ah stored energy. You can it can dissipate as a heat under certain conditions. And ah melting of the sliding interfaces also can take place ok. So, if the temperature is up. These are the sliding interfaces. This is the sliding interface, let me show you. This is the sliding interface ok ok. What will happen? This may melt ok. So, normally that mostly the workpiece is much lower hardness than the tool, at the same time if the temperature is high; normally the temperature that is carried away by a chip is always higher that is 80 to 80 4 percent. So, beneath the chip always is ah melting tendency ok. Not in all workpiece materials, those workpiece materials which are soft enough whose melting point is ah low, these type of things will have a melting tendency ok. So, this is about the frictionality.
Video 2: Problems with Chip-Tool Tribology
So, what are the basic problems with the chip tool tribology? Ok, why we have to study? We have already studied this, ah what I am going to show you, but it is a interlinking ok. Everything you have to interlink ah whenever you study in the past ah in this course. So, some of the metal cutting course and metal fluids having it is a single subject. So, always some things analysis you have to take from there and you have to correlate and all those things ok. So, why you want to study is the if the tribological conditions are bad; that means, that if the um tribology is very high, then your ah input energy will be very high; that is, already I have shown you , you in the previous class, where I am telling about the forces in the machining operation and all those things. You have seen the tan beta that is called a coefficient of friction is nothing but F by N. So, it your N is high what will happen frictional force is why they high, then your ah waste ah energy will be very high if you see. The complete that normally work done or energy input if you see what will happen Fc into v or the energy input, shearing force which is ah multiplied by shearing velocity this is called useful energy frictional force multiplied by chip velocity. This is called ah that will go as a waste that we have already seen. So, if my frictional force or the tribological conditions are bad frictional force will increase if my frictional force increases. What will happen? This particular part of my energy will goes up. If it goes up, what will happen? My input also will goes up ok. So, for that purpose you should be always careful and you should ah study the rheological things ok. So, that is why we have to coefficient of friction and all those things. So now, coefficient of friction you have already seen just ah and I have just want to give you glimpse F by N and ah previous class if if you see, how you calculate F, F is F sin alpha plus Ft cos alpha is 1. And N is Fc cos alpha minus Ft sin alpha. So, if you place both what will happen? You will get a final equation which is called ah mu equal to coefficient of friction. It is Fc sin alpha plus Ft cos alpha by Fc cos alpha minus Ft sin alpha. This is what ah you are going to get ok. So, determination of friction coefficient you have seen, and ah the basic assumptions normally what we consider ah in the this one is, normally F and N which are ah they on the tool rake surface. This is the rake surface this is the region ok. So, ah these are all uniformly placed ok, but; however, this is an assumption is not ah so a realistic ok. So, that is why that what what I want to say is that ah applied forces that is F and N are uniformly distributed over the chip tool interface. That is is the assumptions normally whenever will be we want to derive some equations like a equation or leash of us ah some of the equations, whenever you want to measure. The we want to calculate the shearing angle and all those things. For that purposes normally, we take F frictional force on normal to the frictional force coefficient of friction and all those things ok. So, we assume that these are all uniformly distributed on the rake surface; however, this assumption is not so. So, we will see why it is not so, though in terms of stresses distribution on the metal cutting you will see, because of sticking zone region stresses are different, I mean to say F and N and the sliding zone F and N are different ok. So, this is the chip tool interface this is the cutting tool. And the chip is coming here ok. So nowadays the contact pressure will be maximum at the tip point. And gradually it will decrease. This is a stresses ok; however, we have seen in a previous ah slide the assumption that it is uniform. But it is not uniform, if you can see it is or the tip it is maximum, and it will become a minimum as a the in moves from the tip ok. That is what ah we are ah seeing here. We can see the primary shear zone, which you have already seen rubbing action that is between the workpiece, and the flank face of the thing that is called tertiary normal shearing. And second is the shearing which we are talking about ah and we are talking in this one also. So, rubbing zone basically this will takes place between workpiece as well as the flank surface. So, ah this is not that much ah important; however, I said in the ah starting of the this class, that ah we just give you the glimpse that is nothing but if my frictional rubbing between the workpiece is very high, what will happen? The surface roughness that you are generating and the final product will destroy. I mean to say the surface roughness will increase. If it is increases, what will happen? It may not qualify the quality check ok. So, for that purpose always you try to avoid by putting the cutting fluid or any tool coatings or something ok. So, since normally if you have given a relief angle at a flank angle properly. So, that can itself is a good thing from from the rubbing action against the workpiece. That mean I mean to workpiece means I mean to say the final product that is coming out If you can provide the lubricant jet across the workpiece and a tool interface, that will also find. So, in order with the ah this will help the rub workpiece and tool flank surface interface ok. But ah not much a study has been done in this area, but some of the researchers they have done ah in this area by putting multijets. One jet on the tool rake surface, where the cooling property of the cutting fluid is much better. Another one they are putting on the workpiece that is a final product, and a tool flank surface they will put where a lubricating nature of cutting fluid is good. This type of a things they will do ok. So, people if somebody is interested to take the research in this area. So, there are some of the papers who talks about the a multi jet based a turning processes, where they will use as I said one jet which is having a cooling better cooling properties. Better cooling properties means water-based coolants on the chip tool interface, and the less water content are mineral iron content ah cutting fluids on the flank surface. You can try and in the advance materials, some people who are PhD students who are seeing, or who are watching my slides can take up a multi jets, assume that if I want a machine the titanium with the see the N or ah coated carbides. So, you can use similar technologies, even you can use some other technologies like, ah a cryogenic on one side and another side mineral ion based and all those. But you should be very careful because ah how to recycle it. Ah because once it go back to the cutting fluid how tank, then how to recycle it and all those  thing. One we should be take care and care about the ah lathe bed or mission tool bed and all those thing because cryogenics also involved here for the better cooling ability and all those things ok. If you see normal forces on the coefficient of friction is ok. So, the actual distribution of shear in normal stresses and the rake surface of the tool is probably from the shown figure. So, if you can see the normal stress, this is a normal stress, and this is the ah shear stress ok. So, this is a sticking region, and this is a sliding region ok. So, the stresses you you can see here this is a normal stress as and this is the shear stress ok. The steer shear stress is constant in the sticking region, but; however, in the sliding region it is ah gradually decreasing ok. As ah you cansee here, this is a constant domestic in region and it is decreasing gradually to the minimum from the maximum of a sticking region to a minimum in the sliding region. However, if you see the normal ah stress, that is ah N normal to the this one. So, which is gradually it is maximum of the tool tip surface, this is called a tool tip. To maximum here and it will gradually decrease to the minimum degree at the end of this sliding region beyond which there is no tool ah chip interface is not there ok. If you see some point see the normal stress is very high and the metals adhere to the tool tips resulting in the sticking friction conditions. This this is the normal stress normally if the normal stress is very high; that means, that normal force is very high for the same area. So, what will happen? This causes the sticking in the region, because of which there is a built-up edge formation will takes place and all those things. From C to D where the chip calls smaller normal stresses are update and given the results sliding friction conditions ok. C to D which we are studying in this is a sliding region. Because of the normal stresses are low here, what will happen? There is a sliding action takes place. And since there is no not much stresses on the tool by the chip. And if you have a cutting fluid application also in this one, if there is a cutting fluid and much difference. But there is a difference, what will happen? There is a just a slight rubbing action will take place wherever the stresses are very high there sticking at sticking will takes place with the normal stresses are low there is a sliding reaction will take place. That is what this normal stress and a shear stress distribution want to tell ok. If you take the analysis in the ah contact surfaces, basically Ar is the real area of contact, and the A is the apparent area of contact. If you see the normal force versus frictional force that is called F verses N ok. So, if your ah normal force in this region, region 1, what will happen? Normally your Ar by A is approximately 0 ok. Now, see what is mean by a real surface area of contact, and what is apparent area of contact. If you see here , this is a area is complete apparent area contact and real area of contact Ar ok. You real area of contact in chip tool interface. I am talking about petal cutting that is why correlating to the chip tool interface. So, Ar only it is at a certain points basically if this is the point, and this is the point, and this is the point. If your area of contact is approximately apparent area of contact is in mm normally, because it is a complete area that we are talking about. But real contact is some points that mean that it may be some few nanometers or micrometers. If you take the ratio of a real area of contact to the apparent area of contact where a nanometer by millimeter, millimeter is a big area where it is ah some about assume that it is more than thousand, 10 thousand times, that is why it is apparently 0 in this way ok. Slope is nothing but your ah quotient of friction. But if you see in the another region, where this is the region if you see, if my area of real area of contact is equivalent to area of apparent area, then it is a relation is one contact between ok. In this region normally, it is one ok. Because the contact area is ah same. For that purpose, normally, lower conditions will be high and velocity will be very low. In this condition velocity will be very high and load will be normally low ok. This is about the ah 2 forces and ah real area and this contact area and all those things ok.
Video 3: Experimental Measurement of Tribology
So now, how to study this ah tribological aspects? Tribology of metal cutting in experimentally,  can be um some machining tests or how to do ok. So, for that purpose there is one standard is a ASTM G99 one standard one of this there are many standards, first I am telling you about one standard. Standard method for where testing within a pin on disc apparatus, we sitting on disc apparatus is another tribal method ok. So, this is ah if at all I want to um test it, what will happen This is a simple pin on disc ok. I mean to say, there is a pin is there on disc is there, this is the disc ok. You can see this is the disc. And this is that pin ok. This is a holding position is there, but; however, you can see a pin here. This is the pin ok. This pin will be touching the surface like this, this will be ok. If at all I want to wish in a mild steel with respect to hss. How I have to do? Ok, so now, what will happen? I will make a steel disc and I will make a pin ah hss. You can make hss pin by using edm process basically, you take a hss block, then you use the wire edm process. And you can cut normally the size of the pin that ah cylindrical pin that one can use in the as for the standard is ah 6 mm diameter by 15 above 15 mm, you can use. Normally, what ah people take ah in the range of above 15 mm 6 to 8 mm normally, we will a people will take ok. So, this pin you will hold here.You are holding a pin here. And you are giving some input conditions. What are the input conditions? That you are going to give is normal load. If you see here there is a load cell is there, and you have to put this is the one that ah is a cantilever beam type it is holding. So, you can put a load connecting to it here load on other side which is not visible here. So, you can put a load. So, that is ah how much load. You have to put sliding sliding speed ah is nothing but how fast you are rotating your disc ok. So, whether you are rotating at a 60 meters per minute or 30 meters per minute wear track diameter. So, I am just using the reciprocation. So, how a reciprocation you can increase the lamp I can do this. So, I can get the different different loads, and scanning speed, if you do what will happen the scanning ah the shearing of the tool surfaces will goes off. It can be tested for the virgin tools, or it can be tested for the cuticles also. So, in that circumstances. You can see where the delaminating the in terms of tool coatings you can see at what load and what speed the delamination of the tool coating system taking place that you can study ok. So, in this way, one one can study is the pin on disc experimentally you can study the 2-body abrasion 3 body abrasion, if at all I want to study the de lamination of the coatings you can go do the scratch testing. This scratch testing is also one of the ways of a doing the tribological test ok.