JNTU B.Tech III ENGINEERING METROLOGY Previous Question Paper (March - 2021 - R13/R15)

Interchangeability is a manufacturing philosophy where any component from a batch can be assembled with any other mating component without further selection or fitting. It requires high-precision control of dimensions and tolerances, ensuring that all parts are produced within specified limits. This allows for mass production, easy replacement of parts, and reduced assembly time. For example, in the automotive industry, a replacement spark plug from a store will fit perfectly into any engine of the same model without modification.

Selective assembly involves sorting manufactured parts into several groups based on their actual measured sizes. Mating parts are then selected from corresponding groups to achieve the desired fit. This method is used when the required tolerance for a fit is tighter than the manufacturing process can economically produce. For example, in the assembly of high-precision ball bearings, balls are sorted into size categories (e.g., small, medium, large) and paired with races of matching categories to ensure the required clearance is maintained.

Given: Nominal diameter = 70 mm.

Hole limits: Lower deviation (EI) = 0, Upper deviation (ES) = +8 microns (0.008 mm).
Hole size range: 70.000 mm to 70.008 mm.

Shaft limits: Lower deviation (ei) = -10 microns (-0.010 mm), Upper deviation (es) = -6 microns (-0.006 mm).
Shaft size range: 69.990 mm to 69.994 mm.

Allowance is the difference between the maximum material limit of the hole and the maximum material limit of the shaft. For a hole, the maximum material limit is the lower limit (70.000 mm). For a shaft, the maximum material limit is the upper limit (69.994 mm).

Since the allowance is positive (the smallest hole is larger than the largest shaft), the parts will always have a clearance between them. Therefore, the type of fit is a clearance fit.