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Mechanisms of Tool Wear and Tool Life

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Video 1: Basic Causes of Tool WearWelcome to the course. So, this is about the tool wear and tool life. So, the tool wear,normally if you see the tool wear it describes the gradual failure of the cutting tool dueto regular machining because it is a progressive wear basically. So, it describes graduallyhow atom by atom or ah molecule by molecule are the amount gradually it will ah goes offthat is what the progressive tool wear is. Just it if you see to the introduction tothe progressive tool wear. There is a another tool wear that is called catastrophic toolwear which I am not a discussing it catastrophic tool wear is normally sudden breakage.So, that will happen basically in the brittle and ceramic materials , but the gradual toolwear or the progressive tool wear takes place in the other materials like a HSS, carbidetools and all those things ok. So, we are here we are seeing about the crater wear andthe flank wear these are the two varieties of the progressive tool wear that one cansee here. At the same time how to represent schematically if you see the schematical representation,this is the rake surface, and this is the flank surface. So, the wear that is takingplace on the flank is a flank wear and the rake surface the various taking place is craterwear ok. So, now the crater wear represents normallyin terms of crater wear depth this is crater wear depth, you can see the in terms of craterwear depth and in terms of flank wear it is normally represent in the flank wear landthis is a flank wear land, so on the flank face ok. So, these are the two wears commonlyobserved in the tool wear in the machining processes ok. There are other wears like notchwear edge wear many other wears are there, but ah since this is the introductory courseto the machining process. So, we are concentrating mostly on the two wears that is a crater wearand then the flank wear ok. This is a progressive wear, now you can seehow it takes place on the machining process we have seen previously. So, now, you seethis is the cutting tool basically, in this cutting tool because of the chip motion youcan see this is the crater depth that takes place because of the workpiece interactionnormally abrasion there there comes the flank wear. This is called the flank wear. Normallyit represents in terms of flank wear land this is represent in terms of flank wear land; that means, indirectly what it mean is it is a width of the flank wear and in termsof crater wear it represent in terms of crater wear depth ok. So, this about and if you wantpictorially you can see in the next slide. So, this is the crater wear depth normallythis is the crater wear which takes place on the rake surfacethis is a flank wear. So, I just talked about your flank wear ok.So, that is nothing, but this is the flank wear land or flank wear width ok the otherterm is width. This is the practically what we observe in the machining process if youwant to represent schematically the flank wear represent like this and crater wear representslike this ok, the other view that you can see is the crater wear this is the other viewok. So, one noting point that you have to alwayssee here is just I will come to some question here which I will explain you from the pointof questions. Just you have to see what is it starts slightly away from the cutting edgethat is what you have to observe ok. So, I will ask you one question and I will alsotell you the answer also. The basic mechanisms there are three mechanismsare the causes by which the tool wear takes place in the machining. So, whenever we talkabout the output that is if there is a research paper on the machining process. So, peopletalk about different different wears whenever you have a different wear you should talkabout the physics of it what is the physics behind it and how this process is taking placewhat are the basic mechanisms that are the people explained in the textbooks, that arethe mechanisms that explained in the research papers and all those things ok. This is thefirst one is abrasion, the second one is adhesion and diffusion ok.So, just we see this abrasive wear. So, these are the three hard particles microscopic variationson the bottom surface ok. So, this is about the explanation. So, microscopic variationon the bottom surface of the chip against. This is normally there is two types of abrasionswill takes place one is a 3D, another one is a 2D practically speaking it is a 3D, so,for better understanding whenever we do the modeling or whenever we want to understandthe things some people understand it is a 2D.The adhesion normally adhesion is nothing, but the joining. Abrasion is nothing, butit is like abrading the surface whenever a kabaddi player falls on the sand in a villagesyou see the players. So, the sand if it is here so that will abrade the skin that isnothing, but the abrasion and adhesion normally is ah just ah joining that is two materialsof different materials. And diffusion is a starting point of adhesion; that means, higherconcentration to lower concentration if the material transfer is gradually taking placethat is called the diffusion. Anyhow I will come in detail about all thesethings that the mechanical loss of material by micro cutting action normally if you seethis is to explain you what is abrasion wear. We have just taken the example of the microcutting action. So, whenever you have a two body abrasion;that means, that there are involvement of only two bodies in our present scenario thesetwo bodies are one is the workpiece material another one is a tool material ok. So, ifthere is a interaction or wear between two bodies the abrasive wear between two bodiesthen it is nothing, but the two body abrasion or two body wear.In term if you have a third body normally for example, this is commonly two body abrasionis commonly in terms of machining process, where in a tool single point cutting toolis there and you are removing a material assume that you are machining a copper with respectto the one of the hard tools like a this is are carbide tools normally you will have twobody abrasion. The three body abrasion will takes place when there is a third body comesinto picture like you can see that this picture.
Video 2: Tool WearNow, we will go into the tool wears. So, thepossible tool wears in orthogonal metal cutting process are one is crater wear we have alreadyseen and the another one is the flank wear. So, this is also I have shown in the previousslides which is a crater wear and the flank wear. To show you the isometric view justwe have given you a glimpse here on a single point cutting tool this is the crater wearand this is the flank wear ok. So, this is about the flank wear ok.So, normally flank wear I already told you flank wear width is the one flank wear landis the one another wear is nothing but the notch wear, you can see the notch wear whichI am not going to talk much about this one. Just I am talking about the introduction tothe tool wear. So, I talked about only tool wears, but however, you can see at the endof the flank wear there is a wear called the notch wear. So, that is another type of wear.So, there is another one wear is nothing, but the nose radius wear, you can see hereon the nose radius ok, this is the nose radius and this is the sharpness radius which isok. So, these are the glimpse that schematically I can show you ok.So, then a crater ware, the crater ware if you see crater wear this is the crater warethat I have already shown you the crater ware wear scars on the rake surface of the toolpossible mechanisms. Now, we have to see what is the possible mechanisms.We have seen three mechanisms that is abrasion, adhesion and diffusion. Among which, whichcauses more in terms of crater ware which is high in terms of flank wear; that means,which is a most responsible mechanism in the crater ware which is the most responsiblemechanism in the flank wear that we have to see, ok.See the possible mechanisms sequential order I am just saying about the crater ware. Thecrater wear means if my workpiece is there and the tool is there if my chip is movingon top of it ok. So, the chip is carrying lot of temperature and it is imparting intothe tool that what I have said and the basic mechanism here is welding and rupture. WhyI am saying is welding and rupture. If you see this picture where my chip is moving,so the bottom line surface of the chip gradually welds on the the surface the surface of theworkpiece and when the thermal softening takes place this will takes away . This particularportion is goes off ; that means, that the basic mechanism dominating mechanism hereis adhesion and followed by abrasion because it has a sticking zone as well as a slidingzone ok. So, the crater wear takes place in the interms of sticking as well as sliding, but the sticking it is dominating and slidingslightly less. So, sliding region sliding region is mainly because of the abrasion andthe diffusion also plays at very high speeds ok. The basic mechanism that one can tellis adhesion is the dominant mechanism of tool wear in terms of crater wear, ok.So, if you see the crater wear, normally crater wear represents in terms of this these arethe representations one is the distance to the middle that is K M, this is the K M. Anotherone is K B that is a width this is the width of the crater wear, depth of the crater wearthat is a K T which normally you can say is as creator wear depthand K L is distance from the start. So, this is nothing, but a K L, from the starting pointto the cutting edge a starting point ok. These are the notations and the abrasion area isthis one. So, crater wear normally represent in termsof crater wear depth that is what I mean to say is K T crater wear depth which is representok. Now the question assume that I am taking only three types of tools, one is 0 rake angletool positive rake angle tool ok. First let me take to wear normally forces will be veryhigh basically speaking assume that here is if it is F 1, this is F 2.So, F 1 will be slightly higher than F 2 because positive rake angle is there assume that thereis a crater wear an a 0 rake angel tool. What will happen? This is my 0 rake angle tool.So, I have a crater wear. So, how it look like? It look like similar to my positiverake angle where my forces is F 3, from the visibility from the what we are seeing itshould give F 3 F 1 should be greater than F 2, ok.So, normally it should be from the geometry what has a visibility from this point normallyF 1 should be greater than F 3 also ok from the visibility point of view, but it is not.So, the, but the case is F 3 will be greater than F 1, greater than F 2 this will be thecondition. Now, question is why, ok. So, the question is simple I have a 0 rake angle toolwear the force for the same conditions for the same depth of cut, speed cutting speedand all those things I have a 0 rake angle this is 0 rake angle this is positive rakeangle this is a wear tool wear that is crater wear take place, ok.So, in that circumstances F 3 should be wear tool normally will experience more forcescompared to 0 rake angle, but geometry wise it looks like a positive rake angle. Now,the statements are contradictory basically the person who is doing experiment with experienceF 3 greater than F 1 ok. From the visibility point of view from this slide it should resemblelike a positive rake angle ok. The answer as per the partial answers I can say is normallythe tool ware will not starts from the cutting edge like the positive rake angle ok. So,it has a slightly distance will be there. Second thing here it is a smooth cut in apositive if you see it is a smooth cut here the tool wear is random and it is not proper.So, the chip that moves will have lot of disturbances and all those things ok.
Video 3: Tool Life
So, now we will go to the tool life as I saidwhen I am explaining you that I am explaining you to the flank wear. Why I am explainingthe flank wear? Since the similar mechanism is there in crater wear also just. So, youcan go through that some basic textbook. So, you can get it. And the tool life if I willsay which wear is can be treated as a tool life criteria ok whether it is a flank wearor crater wear and why. Thus I said you when I was discussing aboutflank wear mechanism and all those things ok. Which wear? So, can anybody guess, itsnormally the flank wear ok. So, why the flank wear? Whenever the machiningoperation is taking place crater wear takes place on the rake surface and the flank weartakes place on the flank surface and the flank surface is the surface which will be in contactwith my final product. So, whenever customer ask me I have to give you a product to himin a good size, good tolerance good surface finish and all those things. If my flank wearis disturbed or the flank wear is high in that circumstances I cannot give him the goodproduct because the product comes out will be in a not good shape or in an acceptableform for a customer. So obviously, my chip will be in contact withrake surface which causes the this wear crater wear the chip I do not bother because thechip is the one which is not useful for me. I cannot say useful, but you can recycle intoworkpiece material and all those things, but I am worried about the final product, if atall I want to give a good product my flank wear land should be within the limits. Ifmy flank and flank wear is the criteria if my flank wear I can keep the criteria as mycriteria I can give you good product that is why always or the most of the time flankwear will be the criteria. Some of the times crater wear will also a criteria that is wheneveryou are speaking about high speed machining and all those things where the temperaturesare goes high. In that processes crater wear is also a criteria,but however, for the normal conditions of the machining process the flank wear is thecriteria that one can follow. So, the tool life , tool life normally canbe defined in many ways. You can see the length of the time that cutting tool can functionalproper before it begins to fail ok. So, it is performing its own function, it is a machiningoperation. So, if it is fake; that means, that my I have to change my tool; that means,that this is my about my tool life one wear. The tool life criteria can be related to theblunting of the cutting edge due to unavoidable wearing process ok. Blunting of the cuttingtool if the my cutting edge is gone what I mean to say is that I have to replace becauseotherwise lot of vibrations will takes place which hampers the accuracy of the productand all those things. The third one is the criteria of the toolfailure in the virtue of wear land which can easily measure under then most crater wearis using tool failure criteria I said that in the high cutting speed that is high speedmachining basically ok. Normally here crater wear will be criteria ok. So, that is whatI want to convey. Now, what are the variables that affects a tool life? If at all I wantto see what are the variables that affect the tool life of my any tool that I have takenwhether it is a HSS or carbide diamond or something. So, the variables affect is cuttingconditions, tool geometry, tool Materials, workpiece material, type of cutting fluidand cutting fluid application techniques. If you see some other things some are alreadygiven that is cutting conditions, tool geometry, workpiece material and all those things theseare you can find in some of the textbooks. However, cutting fluid technique and cuttingtype of cutting fluid these are all you may find in some of the new textbook wheneverwe prepare our slides or our notes normally we follow multiple textbooks. So, for thebasics we follow the old books and some of the pictures that we never we take we takefrom our own research since we are doing some of the research in these areas we also takewith that is why a research and teaching goes hand in hand ok.So, whenever we teach we also teach some of the advances that are found during the researchok. So, the first one which we can see is the cutting conditions.So, the cutting condition the rate wear and the cut is proportional to the cutting speedthat is V T power n equal to constant this is called Taylor tool life equation. So, wheneveryou see this Taylor tool life equation V T power n equal to constant where the V is cuttingspeed, T is the tool life and n and C are the constants ok.So, V is the cutting speed, as I said T is a tool life and the n and C are parameterthat depend on workpiece and tool material that is nothing but workpiece material andtool material combination n and the C depends ok. It is not depend on as a workpiece materialor the tool material it will always depend on the combination whether you are using HSSversus steel HSS versus cast iron or carbide versus cast iron or mild steel or somethingok. Normally the typical values for the tool materialswhenever you are machining the one of the workpiece materials like high speed steelwhenever you want to machine non steel work pieces it is n equal to 0.125 C equal to 120ok in terms of meters per minute; in steel work pieces 125 and 70. Cemented carbide ifyou see 0.25, 900; steel work pieces 0.25 and 900. Ceramics it will be if you see thevalues the values of n and C are gradually increasing with respect to tool materialsok. The basic problem if you see in the Taylortool life equation is it is considering only cutting speed as the criteria, but if yousee that is not the practically acceptable theorem.That is why there is a modified Taylor tool life equation there it is incorporated withrespect to a depth of cut as well as speed ok. So, V T power n equal to constant wasa Taylor tool life equation now feed and depth of cut is incorporated and constant is there.So, n and the C material constant x and y also have similar material combination constantwhere d is the depth of cut and f is a feed rate ok. So, now it is practically acceptableok, that means, the tool life majorly depend on my cutting velocity and followed by feedfollowed by depth of cut. Some of the research papers they show thesethat for the normal conditions of speed depth of cut and feed basically the contributionof the Taylor tool life equation or the tool life cutting speed will contribute more followedby the feed followed by depth of cut. In some circumstances it may be like a depth of cutalso plays a major role compared to the feed.
Video 4: High Speed Steel and Uncoated CarbidesComing to the HSS that is a high speed steel the competition goes 18 percent tungsten,4 percent chromium on 1 percent vanadium and sorry it is 0.6 to 0.8 ok. This was foundthe early 90s, very high early used for a hard ally steels can be hardened to variousdepth ok good wear resistance high toughness good for positive rake angles and there aretwo basics. What I mean to say is the best thing about this material is it is havinghigh toughness, compared to other materials this is the toughest material because it canabsorb the impact and all those things. So, from among what I am teaching is the highesttoughness to cutting tool is HSS, hardest is diamond basic ok. So, there are two basictypes one is molybdenum that is M series, another one is tungsten series that is T series,high speed steels are there. T series is same as what we have seeing thatis 18 is to 4 is to 1, tungsten chromium and vanadium ok. M series normally 2 to 10 percentwill be molybdenum, in 18 percent instead of 18 percent tungsten ok.So, if I am going to use 2 to 10 percent in 18 percent. So, I am using another assumethat 8 percent I am using 10 percent I am using tungsten ok. The chromium vanadium tungstenand cobalt, cobalt will be the normally binder better abrasion resistance less expensiveand less distortion and 95 percent of HSS industry are M series one ok.So, one of the question is if we have tungsten series that is T series why we have to gofor M series. The reason for this one is it is less expensive ok. This is the reason whymolybdenum is M series the molybdenum series the molybdenum cost is low, but the propertiesare approximately same. So, that is why we are going for the M series.So, another one is normally these are manufactured by casting and powder metallurgy technique.So, the applications it can be turning tool, taps, gear cutting, drills and all those thingsok. So, we are going for the uncoated carbides.The second variety is most of the HSS the basic problem is high temperature hardnessis very less; that means, that hot hardness at elevated temperatures or the hot hardnessis low. So, if at all I want a machine a hard material where machining temperature goeshigh in their circumstances the HSS will fail ok.So, the low life for high speed machining, normally if at all I want to go for high speedmachining in that circumstances it is very less. That is why we go for the next versionslightly higher version that is called uncoated carbides. Uncoated carbides are uncoated cementedcarbides class of hard tool material based on tungsten using powder metallurgy. Thereare two basic types in this one, one is tungsten carbide based that is called a non steel cuttingrates and the titanium carbide and tantalum carbide based tungsten carbide tools. Theseare the two varieties of cutting tools are there.First we will see the tungsten carbide. The particle sized 1 to 5 microns are pressedsintered into the desired shape and percentage of cobalt may vary. Cobalt is a binder basicallyok. So, tungsten carbide is also component sometimeswith the tantalum and improves the hard hardest. To improve the hard hardness normally tantalumand titanium are used. Basically these are developed by the powder metallurgy techniqueswhere we will used the tungsten carbide particles tungsten carbide particles are used and thebinder is added to it and they will do the ball mixing and all those things then theywill do the compaction sintering and they will from.The normally the cobalt is a binder. So, it will like assume that if you what to buildthe hole normally brick by brick we will do, so in between we will put the cement. So,this is nothing, but the cobalt this is nothing, but cobalt and this is nothing, but your tungstencarbide ok. This is how this will work ok. Next we will go for titanium carbide ok. So,in the titanium carbide tic nickel molybdenum matrix will be used. This is steel grades,normally previous one tungsten carbide which we are using is a non steel grades where ifat all we want to use the nonferrous based those can be used. So, titanium carbide asa base material and nickel molybdenum is the matrix is used good wear resistance and poortoughness as I said toughness is good in HSS, good for machining the steel and the highspeed then tungsten carbide. So, you can use for higher speed than the tungsten carbide.So, when the temperature goes up it can stays at the same time it is very good for the steelsok. The high compressive strength normally generalproperties if you see high compressive strength, but low to moderate tensile strength thisis problem, but you you have a compressive strength is very high. So, normally it willbe very good. So, high hardness and good hot hardness I said at a elevated temperatures.So, it possesses its own good hardness. So, wear resistance is very good.So, it is normally do not wear at less time, so high thermal conductivity. So, the temperatureit can conduct easily that means, from the cutting edge it can conducted to the nextother parts of the tool that is the benefit of high conductivity. Elastic modulus is inthis range and toughness lower than high speed steel as I said the high speed steel willhave higher toughness ok. We see the coated carbides, normally coatedcarbide what is the difference between coated carbide and uncoated carbide. Normally theseare the coated ones ok. How what is the difference normally if you see? This is the difference,this is the uncoated one this is the coated one ok. So, you can see normally it is thegold color normally the coating is done on this one ok.So, why the coating is required? Cemented carbide insert recorded with one or more layersnormally tungsten carbide tools are not good for the steel based workpieces and all those?If at all I want to use it what people will do is TiC, TiN this is titanium carbide andtitanium nitride or ceramic Al 2 O 3. Coatings will be done there are various techniquesthat is chemical vapor deposition. physical vapor deposition radiofrequency sputteringthere is a laser coatings are there may many techniques are there. So, ok normally thesethicknesses ranges from 2.5 to 13 microns. Nowadays very good nano coatings also cominginto the market. Applications normally you can use for the steels and other simplestapplied for high speeds and dynamic shock and the thermal shock and whenever the minimumif you want you can go for this coated carbide tools ok. Cermets normally what is meant bythe name itself says ah cermet means ceramic plus metal.So ceramic will be the base material and metal will be the binder basically ok. The compositionif you see 70 percent they will be in ceramic that is Al 2 O 3 and 30 percent will be liketitanium or something has a binder they will use ok.So, since there is a ceramic is there, there is a metal is there, that is why these arecalled ah cermets ok. The properties very high temperature hardness that is hot hardnessis very high abrasion resistance is very good and more chemical stability because ceramicis are there, ceramics are highly stable materials. So, this ah if the ceramics are very highstable materials, so the chemical stability will be very high and there will not be anychemical reaction and all those things. Less tendency for adhesion, so there is a lesschance of buildup edge. Good surface finish while machining the steel and cast iron.So, put only the problem here with this one is it is having poor toughness. So, if atall I want to use for intermittent cutting it cannot be used. For intermittent cuttingyou need good toughness in the tool material ok. So, this is very good from the chemicalpoint of view because this is having a dominating ceramic ok, so ceramic is highly stable material.The next one is cubic boron nitride CBN, whatever you can say it is a CBN is commonly knownas next to diamond and the hardest material that below the diamond and 0.5 to 1 mm polycrystallinecubic boron nitrides normally. High wear resistance brittle in nature this is a only problem itis brittle in nature and it is very hard material ok. So, if it is brittle normally intermittentis cutting is the problem and catastrophic value will takes place it cannot be used forintermittent catastrophic will failure will takes place.So, manufacturing techniques normally powder metallurgy this ah produced for the machininghardened steel high temperature wear, wear for example, nickel based alloys and all thosethings. You can see how it look like. So, completely the tool insert cannot be madefrom the this one that is why they will make a small bit and they do the brazing operationok. So, whenever we stake about the brazing see you may get down that ah if the it isbraced the normally brazing is done at very low temperature welding process or the joiningprocess, brazing comes out.