 you know he did a wonderful job on two counts number one I think he taught you well because I learnt a lot along with you and number two teaching for three hours at a stretch is no joke you try it out teaching for three hours at a stretch is absolutely no joke and then doing that consecutively on Friday and on Saturday. So he deserves another round of applause please. So he will be seeing you again on this Friday and this Saturday we are can you hear me can you hear me ok. So he will be seeing you again on this Friday and this Saturday with one the problems that you are going to be solving with AutoCAD based on the experience that you have had the previous week ok. So you know I was looking at certain documents and I dug this thing out do you know what this is do you know what this is what identity card it says Indian Institute of Technology Bombay and incidentally it has my name here and this is something that you are going to be getting four years down the line when you are graduating. So this is my graduating degree B Tech ID Bombay well somebody told me that I need this to get my passport renewed tomorrow. So I am going to take it along with me anyhow let us get started if you recall quite a few lectures ago we had dealt with this problem. So we had this cube which was FCC'd on all sides six sides and there was a BCC void within the cube ok you recall that ok. So the front top and profile views were identical and in none of the views you could realize any information or you could know or you could learn any information about the void that was within the cube within the big cube. So there are ways to come around it and we are going to be discussing more these ways today section and assembly views ok. So that was one of the motivation this is the second motivation that I am going to be talking about. So let us draw a problem which is very similar to question one of your fourth homework. So let me start with the top view so you have two lines two semi circles on the left on the right in the top view a bunch of circles with center lines I am going to talk about this feature in a while and then you have some supporting structures called reds which I am going to talk about today. So let me start drawing the front view of this you see a rectangle take the projections of these cylindrical feature down this is the cylindrical feature take the projections down. So if you realize you have this hole which is a through hole it is going from top to bottom of the object and then you have some like this this feature and I want your attention here because this is something that I am going to be asking you in the exam this feature is called a countersunk counter sunk c o u n t e r s u n k countersunk what I will give you as information is this countersunk five something deep comma five something through once again countersunk diameter something some distance deep d e e p and then space of comma five something through. So this describes this entire feature we will continue the projections I will start drawing hidden lines pertaining to the circular voids in the top view something that should be familiar to you because you have drawn this in your homework problem I am going to go slow so that you realize what is going on alright. So at this time you see these hidden lines pertaining to the circular voids here hidden lines pertaining to these circular voids and then the corresponding center lines the same on the right hand side hidden lines pertaining to these voids and then the center lines and then this is a triangular rib R I B this is a triangular rib the corresponding projection here on the top view likewise you would see something very similar on the right hand side. So there are ribs on top and bottom in the top view and correspondingly you will be seeing those structures in the front view of course the third angle projection question one is this technically correct yes or no no yes maybe assuming that I have not made a mistake if you assume that I have not made a mistake is this technically correct yes or no no yeah this one yeah looks like there will be three looks like there will be three fine. So ignore this circle for the for the time being ignore this part for the time being there should be or there should not be there should be one more circle there should not be one more circle projection one projection two projection three you had me there for a while in an asymmetric you have drawn it right. So in the top view you will see three of these. So the innermost part corresponds to this through whole folks can I have your eyes on the screen please. So the innermost circle corresponds to this the middle circle corresponds to this and the outer circle corresponds to this technically correct no yes. Question two are you happy with the front view why is that it is difficult to distinguish which hidden line corresponds to which feature in the top view. So too many hidden lines yes no too many center lines the front view is confusing yeah can you do something better perhaps ok. So we discuss all these reasons and of course hidden lines center lines all those lines projection lines many lines they create a lot of confusion ok what do we do about this very simple trick very simple trick divide and conquer works everywhere yeah divide and conquer. So what we are going to do is we are going to take an eye section the object. So that the top view sectioned this is your section line this is how the section is depicted by convention take the bottom part out and view the object in this direction ok once again take a knife cut the object into two different pieces ok by convention this is how we represent the cutting operation ok we let go of the bottom part of the top view and we look at the object from this direction ok of course there will be certain changes in the front view and we will try to figure what projection lines back I want you guys to be very careful and follow me ok. Now that we have done this there is one thing that I want I would actually want you guys to keep in mind I would not want to show in my front view any hidden feature ok so long as you keep this rule in mind it is ok I would not want you guys or me for that matter to show any hidden feature ok having said that let us continue what do you see do you see this line is solid line it is basically you have what you have done is you have cut these cylindrical void into half. So, essentially this would represent one of the boundaries of that cylindrical void of this one in particular do you see this line yeah ok as I said I am going to be ignoring this cylindrical void because it is essentially hidden I do not want to be showing the hidden features likewise I am going to be ignoring this cylindrical void I would actually want to show what I see ok what I see do I see this line is solid line yes this thing is cut do I see this line this thing is cut do I see this line yes or no good do I see this line do I see this line do I see you I do do I see this rib now because that part is gone ok that part is gone with the part of the top view that was here ok again I am going to be letting go of these hidden lines because they correspond to the cylindrical void at the back here stay with me would I see this line this line these guys gone ok the corresponding center line is gone how about this line how about this line would I see this or would I not see this I would not see this ok what is the difference what is the difference the front view is a lot more clearer now is it is it vertical you mean this line this thing I am so glad you guys are thinking so far so good to show that you have actually taken a section of the object ok once again to show that you have actually taken the section of an object this is the only part of shading that is permitted in technical drawing ok by convention this is the only part of shading that is permitted in technical drawing this essentially means that you have material here no material here ok on section plane material here material here no material here material here the triangles I will come to that I will come to that I will come to that ok for now for now of course you would agree that front view is much clearer ok you would also realize that the entire information is not depicted ok. So, I am going to be making a statement here I am going to be making a statement here be careful section views are nothing but a trade off between clarity and information it is a compromise between clarity and information ok if you want more clarity let go of certain information if you want less clarity include as much information as you want ok something to keep in mind however it will be a nice idea for you to show the full top view ok. So, that much of information not all but much of information is something that you are able to retain ok section views is about relaxing once again relaxing rules of projection in favor of clarity ok. So, this is how the section plane is shown top view this is like the edge of knife and you are looking at these two directions and the bottom view in the front view you say section a a ok why did I not section the triangular ribs you have no idea and I expect that not a problem I will come back to it later ok. But to give you a clue by convention there are certain elements in the object which are supposed to be for support ok for example these triangular ribs they are essentially meant to support the cylindrical feature over this platform ok this need not be there in the object ok. But since this is there it is kind of ensuring that this cylindrical feature stays perpendicular to the base of the object. So, it is a supporting element it is not the machine part but supporting element it is called a rib ok and later I will tell you that there are certain supporting elements that you do not section you just ignore that the section plane is passing through those supporting elements it is called the full sectional view full sectional view you are taking the entire section ok there are different kinds of sectional views the second one is the half section view and for this example you can take advantage of the symmetry in the object ok again it is a trade off between clarity information you do not take the entire section but you just cut one quarter of the object take it off the part on the left in the front view stays the same as if you are taking the front view of the entire object on the left ok the part in the right is sectioned from the top view you realize that the object is symmetric ok this is where you depict the entire information this is where you depict the information after you have sectioned the object ok or the interior of the object right your top view will always remain full look at the way I have drawn the section line here dash dot dot dash dot dot pretty much like the hinge line you are the boss you can choose the cutting plane in whichever way you want ok it need not be straight it can go zig zag depending on the details of information that you want to depict for example I can choose the cutting plane to be like this ok starts from here up till here goes back starts from here up till here ok. So, the right part is actually showing the centre part of the object and the left part is also covering the features which are at the back of the rep. So, you can choose the section plane you want ok take this part off and start drawing the front view this is something that we have seen before let us start drawing the left hand side the projection lines you will see this would you see the edge fine you will see this edge of course no yes no what is wrong these guys should be and should not be hashed solid this part should not be hashed yeah it is called a rib it is called a rib. So, this line and this line would not be visible assuming that this is not there would be visible then. So, when you are sectioning just assume that supporting elements are not there section and then add the supporting elements later ok by convention I will tell I will talk about that a little more discussion time full section half section offset section the fourth one is the revolve section it is mainly used to show the cross section of elongated parts let us say if you have an object like this and if you want to show the cross section this is an object where the front view and the top view they are the same cut the object anywhere and show the cross section within either the top view the front view likewise the second example this is like a t section if you cut the object over here this is like a t section and the t section is going to look like this. So, just to show some more details in the object revolve section removed section it is very similar to the revolve section just that if you want to show the cross section at any point of the object show that section elsewhere cut the object go down do not show the section with the front view itself, but slightly down around ok likewise for this also this is important and this is something that all of us follow or try to follow diligently for clarity standard parts are not sectioned even when cutting planes pass through parts like shots nuts bolts ribs spokes I am going to talk about webs and lugs different examples this is an object with a rib and a lug if you want to cut this object in two pieces like. So, you do not section the rib you do not section the rib and you do not section the lug if you look at this carefully if you let go of this object you will not lose much sorry if you let go of this part you will not lose much if you let go of this part you will not lose much either. So, they are not the main or so speak defining features in this machine part they are just supporting elements they are just there for example, this thing is to help or this thing is to in a way ensure perpendicularity between the cylindrical feature and this platform. So, it is just there and let go of this so more matter much look at this example and look at this section view of this. So, it is pretty much like take the bolts out listen to me very carefully take the bolts out take the shaft out what you are going to be left with is this flange and this flange take the full section. So, these are two parts so this is the corresponding section for this flange this is the corresponding section for this flange since there are two parts they are hatched differently 45 degrees from the right and 45 degrees from the left. Once you have shown that section then bring in the bolts back and the shaft back with me think analyze which couple of this one well this is different from this no this is different from this with the ribs you cannot lock them with the ribs you cannot fasten them with the ribs can you with the ribs well by convention elements which happen to be of supportive nature elements which happen to be of fastening nature they are not section by convention. So, two kinds of pulleys are shown over here one is a pulley with solid web and one a pulley with spokes. So, the one with solid web is sectioned the web is sectioned and in this case these spokes are not sectioned. So, it is not so very easy to appreciate this when you are learning this for the first time. So, you need some practice and on my web page I have also uploaded a supporting document for you to kind of go through. So, this lecture along with another supporting document first example. So, you have fasteners over here. So, if your section plan is not passing through this do not worry about that just section it and show what you see. So, this part is getting sectioned this part is getting sectioned. So, the black part and so it is hashed in your drawing the red part is not sectioned. So, it remains white let me come to this example full section half section offset section what full section one slice do you know what a rib is he is on time he is dot on time. Thank you part one part two they are supposed to be perpendicular this part is called a rib it is a triangular feature it is called a rib. So, if I take a section I just take the rib off take the section hatch this platform and this platform and then. So, speak put this back I cannot foresee how you can fasten two flanges with this rib or maybe on Thursday you make a model like you did last time. So, this is what a rib is essentially is there to maintain perpendicularity between these two parts of the object full section half section quarter section what full half quarter this would be the full section half of that is half section what this would be the full section half of that is half. So, even though I have taken the quarter of this object out it is actually half section. Now, realize what I have done I have taken the bolts out I have taken away the bolts attach these guys together sectioned this part sectioned this part you can see these section lines red and white. And then I have placed the bolts back this is just for the sake of depiction because see if you start sectioning everything it comes very very difficult to follow you have to be clear in your drawings that is one of the reasons why they do not section standard objects this is another part what is this well in layman's language what I have done is I have taken a quarter of the object on the right out retain the object on the left I have not sectioned it the bolt stays there the shaft comes out maybe it also stays there shaft comes out I section this part and then place the shot back this is how you are going to be sectioning your drawings section the main parts section the main parts ignore the supporting elements ignore the fastening elements the elements like ribs spokes lugs section the rest of the object and then bring those supporting elements back a lot of scratch my head I would say is the half section of the right part of the object half section of the right part of the object the left part is intact I would actually call it still a half section but I would add some more information to differentiate between the fact that I am taking the section of the entire thing or just one part I will still call it a half section alright. So, aligned section so if you realize what we are doing is we are relaxing the projection rules we are going more into clarity and we are letting go of certain information. So, we are striking the trade off between clarity and information. So, this is another example where your projection rules are relaxed for better clarity features which are all in number are rotated to align with the sectioning plane. Let us say you have the top view of an object it is circular you have got three ribs you got three ribs 1 2 and 3 at 120 degrees and you have got three circles through voids again at 120 degrees this is the cylindrical feature let me try to draw the front view. So, if I section this object like so and if I try to look at what the front view looks like I will see a rectangle. So, there is this hollow feature over here at the bottom of the desk I will take the projections I will get the cylindrical feature I will get this rib and I will get this hole now look at the object what is your first impression what is your first impression yeah a second part of the second rib is also going to be visible you mean this one this one well why not this one the other one will be kind of hidden behind the front one this is the section okay fine. So, this part is off of course this one yeah so what you are saying is whatever information that is left you are trying to retrieve it now using projection rules yeah. So, you are going for that you are going for that so you are saying fine I mean this rib is something that we have not shown okay. So, let us use projections and try to show the rib yeah how about this hole yeah well how by projection by projection yeah. So, essentially you are going to be taking the guys hold on. So, what Kevin is saying is essentially you are going to be taking the projection lines over here and you are going to be trying to show the rib yeah and likewise for the holes also for this hole the hole won't be visible okay the rib will be visible okay you will be a wonderful teacher one day without the mic of course you sing you sing all right you see some symmetry in the top view symmetry in some sense in top view in some sense okay all right. But if you take the projection of the rib this will be in a way unsymmetric no yes or no yeah all right. So, this is the trick that people tend to follow okay fall away carefully I am going to be aligning once I do that I get to show this section plane over here does this hole over here okay folks I align this guy over here and I get to see this corresponding feature here which is in symmetry with this one thing I do the same thing with the rib I align it and I get to see this part okay. So, I am relaxing my projection rules significantly over here okay while in a way not compromising much on the information in a way not compromising much on the information with this alignment I am able to show the true dimensions of a feature okay once I do that I get to section one more time. So, far so good here so far so good imagine that the hole travels up to the section plane and participates in the sectioning correspondingly you will see this hole over here likewise imagine that the rib rotates itself to align with the section plane you will see this part and then you section the rest okay my projection rules are relaxed now in favor of information okay this is how you show the aligned section.