Understanding Tolerance Limits in Engineering Drawing

The question asks what is shown by the notation 50.3 ± 0.05 mm, which represents a tolerance limit on a dimension.

Let's break down the components of this notation:

• Basic Size: The nominal or theoretical size from which limits of size are established. In this case, the basic size is 50.3 mm.
• Tolerance: The total permissible variation in the size of a dimension. It is the difference between the upper limit and the lower limit of size.
• ± 0.05 mm: This indicates the permissible variation from the basic size in both positive and negative directions. This type of tolerance where the variation is allowed on both sides of the basic size is called a bilateral tolerance.

From the basic size and tolerance, we can determine the acceptable range of size for the part. This range is defined by the limiting dimensions.

• Upper Limit of Size: The maximum permissible size of a dimension. It is calculated by adding the positive tolerance to the basic size.
• Lower Limit of Size: The minimum permissible size of a dimension. It is calculated by subtracting the negative tolerance from the basic size.

Calculating the Limiting Dimensions

Given the basic size is 50.3 mm and the tolerance is ± 0.05 mm:

• Upper Limit of Size = Basic Size + Positive Tolerance
• Upper Limit of Size = \( 50.3 \, \text{mm} + 0.05 \, \text{mm} = 50.35 \, \text{mm} \)

• Lower Limit of Size = Basic Size - Negative Tolerance
• Lower Limit of Size = \( 50.3 \, \text{mm} - 0.05 \, \text{mm} = 50.25 \, \text{mm} \)

The acceptable range for the dimension is therefore between 50.25 mm and 50.35 mm. These values (50.25 mm and 50.35 mm) are the limiting dimensions.

The notation 50.3 ± 0.05 mm is a standard way to express the basic size and tolerance from which the limiting dimensions are derived. Therefore, this notation is directly related to defining the limiting dimensions.

Analyzing the Options

• Type of fits: This refers to the relationship between mating parts (e.g., clearance fit, interference fit). The given notation is for a single dimension's tolerance, not a fit between parts.
• Limiting dimension: This refers to the upper and lower bounds of the acceptable size. The notation 50.3 ± 0.05 mm directly defines the range that includes the limiting dimensions (50.35 mm and 50.25 mm).
• Unilateral tolerance: This is when the tolerance is specified in only one direction from the basic size (e.g., \(50.3^{+0.05}_{-0.00}\)). The given tolerance (\(\pm 0.05\)) is bilateral.
• Upper limit tolerance: This might refer to the upper limit of size or just the positive tolerance value. However, the notation specifies the full tolerance range which defines both the upper and lower limits of size. The term "limiting dimension" encompasses both upper and lower limits.

Based on this analysis, the notation 50.3 ± 0.05 mm shows the information from which the limiting dimensions are determined, making "Limiting dimension" the most accurate description of what is being conveyed.

Revision Table: Key Concepts in Tolerance

Additional Information on Dimensioning and Tolerances

In engineering and manufacturing, it is impossible to make parts to exact theoretical sizes. Tolerances are used to specify the permissible variation in dimensions while ensuring that parts can assemble correctly and function as intended. Understanding tolerance limits is crucial for quality control and interchangeability of parts.

• Bilateral Tolerance: As seen in the example \(50.3 \pm 0.05 \, \text{mm}\), the tolerance is specified in both positive and negative directions from the basic size. The variation can be equal in both directions (\(\pm 0.05\)) or unequal (e.g., \(50.3^{+0.04}_{-0.02}\)).
• Unilateral Tolerance: The tolerance is specified in only one direction from the basic size (e.g., \(50.3^{+0.05}_{-0.00}\) or \(50.3^{+0.00}_{-0.05}\)). This is often used when the variation is desired in only one direction to maintain a specific assembly condition or critical dimension.
• Fits: When two parts are intended to be assembled, the relationship between their tolerance zones determines the type of fit (clearance, interference, or transition). This requires considering the limiting dimensions of both mating parts (e.g., a shaft and a hole).

The notation 50.3 ± 0.05 mm directly provides the necessary information (basic size and tolerance) to calculate the limiting dimensions (upper limit and lower limit), which define the acceptable size range for the manufactured part.