Metrology and Quality Control_ Limit, fit, tolerance

Limit, fit, tolerance

1. Unit-2 Metrology and Quality Control

2. Limits, Fits, and Tolerances No two parts can be produced with identical measurements by any manufacturing process. M anufacturing process essentially comprises five m’s — M an , Machine , Materials , Money , and Management . No component can be manufactured precisely to a given dimension; it can only be made to lie between two limits, upper (maximum) and lower (minimum) . The difference between the upper and lower limits is termed permissive tolerance . For example, a shaft has to be manufactured to a diameter of 40 ± 0.02 mm. This means that the shaft, which has a basic size of 40 mm, will be acceptable if its diameter lies between the upper limit of 40.02 mm and a lower limit of 39.98 mm. Permissive tolerance is equal to 40.02 − 39.98 = 0.04 mm

3. Principle of Interchangeability Various components are manufactured in one or more batches by different persons on different machines at different locations and are then assembled at one place . To achieve this, it is essential that the parts are manufactured in bulk to the desired accuracy and , at the same time, adhere to the limits of accuracy specified. Manufacture of components under such conditions is called interchangeable manufacture.

4. Tolerance Tolerance can be defined as the magnitude of permissible variation of a dimension or other measured value or control criterion from the specified value . The basic purpose of providing tolerances is to permit dimensional variations in the manufacture of components

5. 1. Unilateral tolerance 2 . Bilateral tolerance Classification of Tolerance Unilateral tolerance When the tolerance distribution is only on one side of the basic size, it is known as unilateral tolerance . Tolerances (a) Unilateral (b) Bilateral B ilateral tolerance When the tolerance distribution lies on either side of the basic size i.e. the dimension of the part is allowed to vary on both sides of the basic size

6. Fits Manufactured parts are required to mate with one another during assembly. The relationship between the two mating parts that are to be assembled, that is, the hole and the shaft, with respect to the difference in their dimensions before assembly is called a fit . Three basic types of fits: Clearance fit Interference fit 3. Transition fit

7. Clearance fit The largest permissible diameter of the shaft is smaller than the diameter of the smallest hole . This type of fit always provides clearance The difference between the sizes is always positive Example , a shaft rotating in a bush.

8. Interference fit The minimum permissible diameter of the shaft exceeds the maximum allowable diameter of the hole. This type of fit always provides interference . Interference fit is a form of a tight fit . When two mating parts are assembled with an interference fit, it will be an almost permanent Assembly. The difference between the sizes is always negative. Example, pressing a bushing, bearing, or dowel pin into its mating component

9. Transition fit The diameter of the largest permissible hole is greater than the diameter of the smallest shaft and the diameter of the smallest hole is smaller than the diameter of the largest shaft . The combination of maximum diameter of the shaft and minimum diameter of the hole results in an interference fit, while that of minimum diameter of the shaft and maximum diameter of the hole yields a clearance fit. Since the tolerance zones overlap , this type of fit may sometimes provide clearance and sometimes interference ,

10. How to Choose the Right Fits in Engineering https:// www.youtube.com/watch?v=0QuVB012loc

11. Cont … In order to find out the type of fit, determine Maximum clearance, HLH − LLS and Minimum clearance, LLH − HLS HLH-Higher limit of Hole, LLS-Lower limit of Shaft LLH- Lower limit of Hole, HLS- Higher limit of Shaft If both the differences are positive, the fit obtained is a clearance fit, If negative, it is an interference fit. If one difference is positive and the other is negative, then it is a transition fit.

12. Allowance An allowance is the intentional difference between the maximum material limits, that is, LLH and HLS (minimum clearance or maximum interference) of the two mating parts . Allowance may be positive or negative. Positive allowance indicates a clearance fit, and an interference fit is indicated by a negative allowance. Allowance = LLH − HLS

13. System of Fits Fit system is the systems of standard allowance to meet specific range of basic size Hole Basis System In this system, the basic size of the hole is kept constant while the shaft size is varied according to the type of fit Lower deviation of the hole is zero i.e. the lower limit of the hole (LLH) is the same as the basic size. Shaft Basis System In this system, the basic size of the shaft is constant while the hole size is varied according to the type of fit The upper deviation of the shaft is zero, that is, the HLH equals the basic size.

14. Hole Basis System Shaft Basis System This type of system is widely adopted in industries , as it is easier to manufacture shafts of varying sizes to the required tolerances. Standard size drills or reamers can be used to obtain a variety of fits by varying only the shaft limits, which leads to greater economy of production. The shaft can be accurately produced to the required size by standard manufacturing processes, and standard-size plug gauges are used to check hole sizes accurately and conveniently. This system is not preferred in industries , as it requires more number of standard-size tools such as reamers, broaches, and gauges, which increases manufacturing and inspection costs. It is normally preferred where the hole dimension is dependent on the shaft dimension and is used in situations where the standard shaft determines the dimensions of the mating parts such as couplings, bearings, collars, gears, and bushings.

15. System of Fits

16. System of Fits A clearance fit has to be provided for a shaft and bearing assembly having a diameter of 40 mm. Tolerances on hole and shaft are 0.006 and 0.004 mm, respectively. The tolerances are disposed unilaterally. If an allowance of 0.002 mm is provided, find the limits of size for hole and shaft when (a) hole basis system and (b) shaft basis system are used. Numerical Examples

17. For the following hole and shaft assembly, determine (a) hole and shaft tolerance and ( b) type of fit. System of Fits For the following hole and shaft assembly, determine (a) hole and shaft tolerance and (b) type of fit

18. International tolerance grade ( IT): The ISO system of limits and fits comprises 18 grades of fundamental tolerances to indicate the level of accuracy of the manufacture and are are designated by the letters IT followed by a number. The ISO system provides tolerance grades from IT01, IT0, and IT1 to IT16 to realize the required accuracy. The greater the number, the higher the tolerance limit Tolerance class: It is designated by the letter(s) representing the fundamental deviation followed by the number representing the standard tolerance grade. When the tolerance grade is associated with letter(s) representing a fundamental deviation to form a tolerance class, the letters IT are omitted and the class is represented as H8, f7, etc . Tolerance symbols : These are used to specify the tolerance and fits for mating components. For example, in 40 H8f7, the number 40 indicates the basic size in millimetres ; capital letter H indicates the fundamental deviation for the hole ; and lower-case letter f indicates the shaft. The numbers following the letters indicate corresponding IT grades

19. Maximum and Minimum Metal Conditions Consider a shaft having a dimension of 40 ± 0.05 mm. The maximum metal limit (MML) of the shaft will have a dimension of 40.05 mm because at this higher limit, the shaft will have the maximum possible amount of metal. The shaft will have the least possible amount of metal at a lower limit of 39.95 mm, and this limit of the shaft is known as minimum or least metal limit (LML). Consider a hole having a dimension of 45 ± 0.05 mm The hole will have a maximum possible amount of metal at a lower limit of 44.95 mm and the lower limit of the hole is designated as MML . The higher limit of the hole will be the LML . At a high limit of 45.05 mm, the hole will have the least possible amount of metal.

20. https:// www.youtube.com/watch?v=YtS8zVPLaW0 Largest shaft Smallest shaft Smallest hole Largest hole MMC Vs LMC Maximum Metal Condition Least Metal Condition

21. Design of Gauges Gauges are inspection tools without a scale which serve to check the dimensions of the manufactured parts. Instead of measuring actual dimensions, the conformance of product with tolerance specifications can be checked by a "GO" and "NOT GO" gauges. These gauges represent the limit sizes of the work piece, as per the specified product tolerances. A "GO" gauge represents the maximum material condition of the product and conversely, a "NOT GO" gauge represents the minimum material condition Plane Gauges

22. Types of Go, No Go Gauge Following are the types of Go/No-Go gauges available to control part quality and ensure the interchangeability of the manufacturing parts: Plug Gauge Ring Gauge Snap Gauge Plug Gauge

23. Ring Gauge

24. Snap Gauge

25. PRINCIPLE OF GAUGE DESIGN Taylor's Principle of Gauge Design • According to Taylor, "GO" and "NO GO" limit gauges should be designed to determine the maximum and minimum metal limits. • Go limit gauge : A GO gauge corresponds to maximum metal condition . For example: upper limit of a shaft or lower limit of a hole GO and NOT GO limits of snap gauge

26. NO GO limit gauge: A NO GO gauge corresponds to minimum metal condition . • For example, lower limit of a shaft and the upper limit of a hole. • It should check only one feature of the component at a time. • The NOGO snap gauge corresponds to lower limit while the NOGO plug gauge corresponds to upper limit.