Limits, fits,tolerances

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Metrology - Limits, fits

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General Terminology in Fits, tolerance:

General Terminology in Fits, tolerance

Slide 3:

Tolerance Tolerance is the difference between maximum and minimum dimensions of a component, ie , between upper limit and lower limit. Depending on the type of application, the permissible variation of dimension is set as per available standard grades. Tolerance is of two types, Unilateral bilateral

Slide 4:

Allowance It is the difference of dimension between two mating parts. Upper deviation It is the difference of dimension between the maximum possible size of the component and its nominal size. Lower deviation Similarly, it is the difference of dimension between the minimum possible size of the component and its nominal size.

Slide 5:

Compound Tolerance For example, tolerance for the dimension R is determined by the combined effects of tolerance on 40 mm dimension, on 60º, and on 20 mm dimension.

Slide 6:

Geometric Tolerance Geometric tolerances are used to indicate the relationship of one part of an object with another.

Form tolerance:

Form tolerance

FITS:

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. When two parts are to be assembled, the relation resulting from the difference between their sizes before assembly is called a fit. A fit may be defined as the degree of tightness and looseness between two mating parts.

Types of fits:

Types of fits Clearance fit Interference fit Transition fit Clearance In a fit, this is the difference between the sizes of the hole and the shaft, before assembly, when this difference is positive. The clearance may be maximum clearance and minimum clearance. Minimum clearance in the fit is the difference between the maximum size of the hole and the minimum size of the shaft. In clearance fit, an air space or clearance exists between the shaft and hole as shown in Figure. Such fits give loose joint. A clearance fit has positive allowance, i.e. there is minimum positive clearance between high limit of the shaft and low limit of the hole.

Slide 11:

Clearance Fit In this type of fit, the shaft of largest possible diameter can also be fitted easily even in the hole of smallest possible diameter. Transition Fit In this case, there will be a clearance between the minimum dimension of the shaft and the minimum dimension of the hole. If we look at the figure carefully, then it is observed that if the shaft dimension is maximum and the hole dimension is minimum then an overlap will result and this creates a certain amount of tightness in the fitting of the shaft inside the hole. Hence, transition fit may have either clearance or overlap in the fit. Interference Fit In this case, no matter whatever may be the tolerance level in shaft and the hole, there is always a overlapping of the matting parts. This is known as interference fit. Interference fit is a form of a tight fit.

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