1. Chapter one Limits and Fits-1111.pptx

Limits and Fits

1. Tolerance, Limits and Fits

2. INTRODUCTION No two parts can be produced with identical measurements by any manufacturing process. In any production process, regardless of how well it is designed or how carefully it is maintained, a certain amount of natural variability will always exist. A manufacturing process essentially comprises five m’s— man, machine, materials, money, and management. Variations in any of the first three elements induce a change in the manufacturing process or manufactured part. All the three elements are subjected to natural and characteristic variations.

3. Natural variations (NV) are random in nature and are the cumulative effect of many small, essentially uncontrollable causes. Characteristic variations (CV) can be attributed to assignable causes that can easily be identified and controlled. Usually, variability arises from improperly adjusted machines, operator error, tool wear, and/or defective raw materials. It is therefore impossible to produce a part to an exact size or basic size and some variations, known as Tolerances , need to be allowed. Tolerance is the total amount a dimension may vary and is the difference between the upper (maximum) and lower (minimum) limits. The permissible level of tolerance depends on the functional requirements, which cannot be compromised.

4. It can only be made to lie between two limits, upper (maximum) and lower (minimum). Limits: The maximum and minimum permissible sizes within which the actual machined size lies are called limits. Maximum limit: Maximum permissible size for a given basic size. From the fig, the maximum limit is ϕ30+0.035 = ϕ30.035 mm. Minimum limit: Minimum permissible size for a given basic size. From the fig, the minimum limit is ϕ30-0.215 = ϕ29.785 mm. Tolerance: Is the amount of variation permitted to a basic size. It is the difference between the maximum and minimum limits of a basic size. From the fig., the tolerance is = ϕ30.035- ϕ29.785 = 0.25 mm.

5. Deviation: It is the difference between the limit sizes (either maximum or minimum) and the corresponding basic size. ✓ Upper Deviation: Algebraic difference between the maximum limit and the basic size = ϕ30.035-ϕ30 = 0.035 mm. ✓ Lower Deviation: Algebraic difference between the minimum limit and the basic size = ϕ29.785-ϕ30 = -0.215 mm. ✓ Actual Deviation: Algebraic difference between the actual measured size and the basic size = ϕ29.925-ϕ30 = -0.075 mm.

6. Classification of Tolerance Tolerance can be classified under the following categories: 1. Unilateral tolerance 2. Bilateral tolerance 3. Compound tolerance 4. Geometric tolerance

7. Unilateral Tolerance When the tolerance distribution is only on one side of the basic size, it is known as unilateral tolerance. In other words, tolerance limits lie wholly on one side of the basic size, either above or below it. Unilateral tolerance is employed when precision fits are required during assembly.

8. Bilateral Tolerance When the tolerance distribution lies on either side of the basic size, it is known as bilateral tolerance. In other words, the dimension of the part is allowed to vary on both sides of the basic size but may not be necessarily equally disposed about it. This system is generally preferred in mass production where the machine is set for the basic size.

9. Compound Tolerance When tolerance is determined by established tolerances on more than one dimension, it is known as compound tolerance. For example, tolerance for the dimension R is determined by the combined effects of tolerance on 40 mm dimension, on 60 o , and on 20 mm dimension. The tolerance obtained for dimension R is known as compound tolerance. In practice, compound tolerance should be avoided as far as possible.

10. 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 . An ideal fit is required for proper functioning of the mating parts. Three basic types of fits can be identified, depending on the actual limits of the hole or shaft: Clearance fit Interference fit Transition fit

11. 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. Small clearances are provided for a precise fit that can easily be assembled without the assistance of tools or force. When relative motions are required, large clearances can be provided, for example, a shaft rotating in a bush. In case of clearance fit, the difference between the sizes is always positive.

12. Interference fit: The minimum permissible diameter of the shaft exceeds the maximum allowable diameter of the hole. To assemble the parts with interference, tools, heating or cooling may be required. When two mating parts are assembled with an interference fit, it will be an almost permanent assembly. In an interference fit, the difference between the sizes is always negative. Interference fits are used when accurate location is of importance and also where such location relative to another part is critical.

13. Transition fit: 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. Precise assembly may be obtained with the assistance of tools, for example, dowel pins may be required in tooling to locate parts.

14. In a clearance fit; Minimum clearance = low limit of the hole (LLH) - high limit of the shaft (HLS) In transition fit; Maximum clearance = high limit of the hole (HLH)- low limit of the shaft (LLS) In an interference fit; Minimum interference = HLH - LLS Thus, in order to find out the type of fit, one needs to determine HLH − LLS and LLH − HLS. If both the differences are positive, the fit obtained is a clearance fit, and if negative, it is an interference fit. If one difference is positive and the other is negative, then it is a transition fit.

15. 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. It is the prescribed difference between the dimensions of the mating parts to obtain the desired type of fit. 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

16. Hole Basis and Shaft Basis Systems To obtain the desired class of fits, either the size of the hole or the size of the shaft must vary. Two types of systems are used to represent the three basic types of fits, namely clearance, interference, and transition fits. They are (a) hole basis system and (b) shaft basis system. Although both systems are the same, hole basis system is generally preferred in view of the functional properties.

17. Hole Basis System In this system, the size of the hole is kept constant and the shaft size is varied to give various types of fits. In a hole basis system, the fundamental deviation or lower deviation of the hole is zero, that is, the lower limit of the hole is the same as the basic size. The two limits of the shaft and the higher dimension of the hole are then varied to obtain the desired type of fit. a. CF, b.TF, c. IF

18. 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.

19. Shaft Basis System The system in which the size of the shaft is kept constant and the hole size is varied to obtain various types of fits is referred to as shaft basis system. In this system, the fundamental deviation or the upper deviation of the shaft is zero, that is, the HLH equals the basic size. The desired class of fits is obtained by varying both limits of the hole. A. CF, b.TF, c.IF