 Hello friends, I am Mr. Sanjeev B. Naik working in Mechanical Engineering Department of Walton Institute of Technology, Singapore. In this video, I am going to explain about the design and natural tolerances which are used in engineering practice so that at the end of the session, learners will be able to understand the role of design and natural tolerances which are being considered to decide upon the manufacturing of the product. We know that the design tolerances are being decided by the designer when he is designing the product for intended functional requirement of the customers and that is why he has to set certain limits of variability based upon the functional need of the product and the manufacturing process which has been selected by that designer to manufacture the part considering the economical constraints. So, it becomes a decision making under combination of different constraints such as a functional requirement of the product or part considering the manufacturing process, capabilities and corresponding economy of manufacturing. So, all these will decide what specific difference in the dimensions or variations in the specified parameter are going to be accepted. Based upon that, the design tolerances are being specified by the designer as specification limits as an arbitrary set value. However, the natural tolerances they exist with the process. So, whatever the manufacturing process is going to be adopted, it will have its own inherent variations produced on the parts by the process and that is called the process capability constants. So, this is inherent with particular process and that has been provided as some specific limit of variation between upper and lower dimensions or sizes and that we call as a natural tolerances. So, anything is going to be manufactured for certain functional purpose or intended function to reach to the customer as a product will have two considerations. One is a design tolerances which has been thought properly by the designer and the product and the tolerances which are been accepted by its manufacturing process called as a natural tolerances and that must be always kept in mind while going for tolerance analysis and decision making. Now, you just think for a while the use of limits, fits and tolerances which are used in engineering practices for the products, for the machines, for the equipments which is going to be used in engineering practice. Now, based upon the analysis of design and natural tolerances, there can be three conditions which may arise in actual practice. So, we know that the design tolerance is set arbitrarily as a variation limit as a upper limit of dimensions and lower limit of dimensions or specified characteristics of the system or a parameter of the product set by the designer because of the constraints. Functional constraint, economic constraint, manufacturing process constraint and another difference in the dimensions which is going to be accepted because of its process called as a natural tolerance which refers its process capability. The condition may exist that the design tolerances or the variation of the dimensions or variations of the specified parameter decided by designer is less than the natural tolerance. So, process capability tolerance available is greater than what is been restricted by the designer. So, designer tolerance is less than natural tolerance or natural tolerance is greater than design tolerance. So, this is a picture shown over here as a curve. The tendency is a normal distribution curve as we discussed so far and that is why natural tolerance is shown between plus 3 sigma and minus 3 sigma limit over here which is greater than the specification limit set by the designer. So, designer has set this as a lower limit of dimension, particular dimension and this is the upper limit of dimension. So, the design tolerance limits as a lower limit and upper limit are comparatively less than the lower limit and upper limit of natural tolerance. That means that because of this process on which actually the product or part is produced may have dimensions which vary from this lower limit to this upper limit. However, from the point of view of designers requirement as a quality of the product and satisfactory performance of the product is concerned it is only allowed between this minimum limit and this maximum limit means that if the dimensions is different than this upper limit towards this side then the quality is not accepted that means there is a rejection from the point of view of designers requirement functional requirement product cannot work and that is why it shows a rejection. However, it is possible by manufacturing that is why it is manufactured but not accepted by the customer or designer and that shows a rejection. Similarly, if the minimum dimension is brought down than this limit till this limit then there is a same way there is a rejection. So, in this case indicates that whenever design tolerance is less than natural tolerance there is inherent rejection that some of the product produced by that process is going to be rejected as degrading quality that it will not work satisfactory with its intended function and that is why this consideration in the design to keep design tolerance less than natural tolerance is a matter of accepting definite certain rejection. So, it involves the cost of rejection. So, one has to think critically that what level of rejection can be accepted and then one can decide the design tolerance and natural tolerance in the process and also it is a matter of economy under consideration. Rejection cost is more but the process capability is poor that is why the cost of process may be less. So, it can struck a balance of total cost. So, it is also a case sometimes occur in actual engineering practice whether we are considering the case as a natural tolerance greater than design tolerance. The second case is what natural tolerance equal to design. So, one of the ideal case where a designer is specifying certain limit of lower and upper limit of acceptance as a quality of the product is same as what the process can produce. So, the process is perfectly compatible for the design specifications and that is why the wastage of process capability is not there and whatever design is expecting the same is going to be produced by the process and that is why if they are same there is no case of rejection can be shown. So, it reaches normally the economical constant as a desired level and that is why this can be considered to be a better or ideal case while deciding upon the natural tolerance and design tolerance. However, the threat is that because of any of the reasons if this natural tolerance goes on increasing then it once again results in what you can say rejection. So, it is a critical combination that natural is equal to exactly design. So, few times it occurs reality one can accept it as per the need of the application and overall objective of the design. The third case naturally is natural tolerance is less than design that means the process capability is higher where the variation which are actually maintained on the product is having low variation but allowed variation by the designer is large and that is why 100% all the products produced on particular manner is accepted by the designer. So, there is no deterioration of quality which has been decided by the manufacturer say designer as a specified limit of variation. So, 100% parts produced on this process as its process capability here are accepted so there is no question of rejection at all. So, sometimes rejection is not affordable or it goes delays in the system or there are many drawbacks of rejection if at all it is been expected not to have then this case arises. However, the process capacity being higher order higher level cost of process may be more as required by the designer. So, designer may expect the poor process capability than this still there may not be any rejection. So, it depends upon finally what application one is going to expect accordingly natural tolerance can be considered less than design tolerance. So, this is another way we can analyze the design tolerance and natural tolerance and we can club it to struck a balance and get a best possible outcome as per the interest of the designer is there or the product is there. This is my reference. Thank you.