 Hello friends. Let me introduce you. I am Professor Prakash Sarunke from Mechanical Engineering Department of Valshan Institute of Technology, Solapur. Last time we had discussed a topic limit tolerances and fits from the subject metrology in which we have studied the basic concepts. Now we shall continue our discussion further and move into more details. At the end of this session, students will describe types of tolerances and will be able to distinguish between them. Students will identify various grades of tolerances and they will be able to calculate the values of tolerances for given size and specified grade. Let us see. Last time we have seen the tolerance zone. So it is the difference between maximum and minimum limit. When it is represented graphically it is called as tolerance zone. Now these tolerances are two types. One is called as unilateral and another is called as bilateral tolerance. When the tolerance zone is placed only on one side of the basic size or zero line it is called as unilateral tolerance. For example both limits maximum and minimum they will be either above plus plus or minus minus that is unilateral tolerance. While bilateral tolerance is one where one limit is above and another limit is below the zero line or basic size. So basic size line is contained between the within the tolerance zone. So that is known as bilateral tolerance. Now how much amount of tolerance should be specified that is always the question for a designer production engineer. And for having some standardization Bureau of Indian Standards had specified certain grades of tolerances and they have also given the way how to estimate their values for different grades of tolerances. You can see there are 20 grades of tolerances specified by Bureau of Indian Standards and which are designated as IT01, IT0, IT1, IT2 etc. Up to IT18 and these are specified for dimensions up to 500 mm. Now for each of these grade we can estimate the value of tolerance the procedure like this. Value of tolerances are normally calculated by using the following formula I is equal to 0.45 cube root of D plus 0.001 D. In this formula I is called a standard tolerance unit and D is the geometric mean of the two steps of the basic size class in which the actual basic size lies. Let us try to understand what we mean by this. So these are the basic size classes table 2.1 shows. So first is 0 to 3 mm 3 to 6 mm 6 to 10 mm likewise say the size sizes are specified in I mean classified in steps. Now the actual basic size in which class it lies for example if it is having a diameter diameter of 15 mm so it lies between 10 to 18 this class. So for this basic size the two limits are one is the 10 another is 18 so that capital D value will be calculated as square root of this 10 multiplied by 18. So that way we can calculate for each basic size and the upper step is contained in that particular class means if it is 18 so that we will consider 10 to 18. If it is 30 then that will be considered lying in this class 18 to 30. So that way we calculate for the value of D substitute the value of D in the formula of I and then we calculate the value of I. Now while D is expressed in mm the value of I will get that will be in microns. Now this is standard tolerance unit for getting the actual value of tolerance say we have to use again table 2.2 which specifies the formula for it 5 to 18 in terms of I. For example it 5 the amount of tolerance will be 7 I, it 6 10 I like that in terms of I the amount of tolerance is specified. So from which we can calculate. Now this is for it 5 to it 18 for grades which are lower than it 5 there are different formulae this table 2.3 gives the formulae for it 0 1 it 0 and it 1. So the formulae are like this 0.3 plus 0.008 D like this and for it 2 to 4 this can be calculated a geometrical means of it 1 and it 5. It 1 value we can calculate here it 5 we can get here for it 3 it 3 will be root of it 1 multiplied by root of it 5. Once we get it 3 then it 2 can be calculated from it 1 and it 3 as its geometric mean. For it 4 calculation we shall take it 3 and it 5 values and we shall calculate its geometric mean. So that way we can for it 0 1 to it 18 the grades of tolerances are specified I mean value of tolerance can be calculated. Now these values are also specified like this that first basic size is given and tolerance grade is expressed as its suffix suffix suffix to basic size. When grade and basic size is given we can find out the value of tolerance by using the table 2.1 to 2.3 and formula for standard tolerance unit I that we have seen. These values are also given in table form in IS code 91993 this table is shown in our next slide so this is the table. You can see here so these are the basic sizes in mm say and these are the classes so this is the lower limit and this is the maximum limit say 0 to 3 3 to 6 like this. Now here we get the various grades it 0 1 to it 18 and say here these are values are specified in terms of microns. Now let us see how to use this again coming to some size say if we go for this class say 30 to 50 the basic size is this class. And then grade of tolerance say that we can see if it is IT7 for example then the value here that will be say 25 microns. So that way for each basic size we can first determine the basic size class then see what is the grade of tolerance and we can arrive at the same in correct value from this table. Now this table is used as reference we can also calculate say for all these topics where I have used references IS 91993 this is ISO system of limits and fits say part 1 specified by the Bureau of Indian standards. So it consists of basis of tolerance and deviations and fits. Apart from that I also used the book of Mahajan metrology Dhanpatray and Sons edition 2012. Okay I hope you have understood if you have any doubt you can contact me anytime. Thank you.