ENGR1025 - Lecture 13

Topic Overview

• Understanding Dimensioning and Tolerances in Engineering Drawings
• Key Concepts: Nominal size, tolerance, limits, allowance of two mating parts.
• Types of Fits: clearance fit, interference fit, transition fit.
• Methods for dimensioning mating parts.
• Tolerances: bilateral, unilateral, limit dimensions.

Tolerance Fundamentals

• Tolerance Definition: Range of acceptable variation in dimensions necessary for the proper function of parts.
  • Facilitates fit between manufactured parts.
  • Tolerance = Maximum allowable variation.
  • Example: Decimal tolerance of $0.0040$ (from $1.6230$ to $1.6270$).
• Tolerances must be specified for each dimension for quality control.

Types of Fits

1. Clearance Fit: Internal part fits into an external part with space.

   • Example: Largest shaft < Smallest hole.
   • Always ensures movement without issues.

2. Interference Fit: Internal part is always larger than the external part.

   • Example: Smallest shaft > Largest hole.
   • Requires force to fit together, leading to tighter assembly.

3. Transition Fit: A mix between tight clearance and tight interference.

   • Example: Smallest shaft fits into largest hole (clearance) and largest shaft must be forced into the smallest hole (interference).

Limits and Allowances

• Allowance: Minimum clearance or maximum interference between parts.
• Limit Dimensions: Specify the upper and lower limits of a feature's size, providing precise tolerances.
  • Non-interchangeable parts might not need exact tolerance specifications.

Basic Hole and Shaft Systems

• Basic Hole System: Nominal size is maintained for holes, while shaft size is adjusted for fit.
• Basic Shaft System: Nominal size is constant for shaft and hole dimensions are adjusted.

Specifying Tolerances

• Methods of indicating tolerances on drawings:
  • General tolerance note for common applications.
  • Specific tolerance notes next to dimensions.
  • Reference documents for detailed tolerance standards.
  • Use of limit and bilateral tolerances.

Tolerance Notations

• Bilateral Tolerance: Variation allowed in both directions from the basic size.
  • Example: $57.20 ext{±}0.80$
• Unilateral Tolerance: Variation allowed in one direction only.
  • Beneficial when critical dimensions approach.

Angular Tolerances

• Generally expressed in degrees, minutes, and seconds (bilateral).

Tolerance Stacking

• Cumulative Dimensioning: Adds tolerances for features in a sequence (can lead to excess tolerance).
• Baseline Dimensioning: Relies on a common reference point to maintain tighter control over cumulative tolerances.

Metric and English Units

• Recognized standards for specifying metric limits and fits (ANSI B4.2-1978).
• International Tolerance Grades (IT) guide the degrees of tolerance application (e.g., IT grades from $IT0$ to $IT16$).
• Understanding preferred fits ensures proper engineering fitment across applications, e.g., H7/d9 for clearance fit.

Surface Texture Symbols

• Indicate the type and roughness of a surface.
  • Symbols like LAY, C, R, and P indicate the orientation and method of surface treatment needed.

Summary

• Understanding tolerancing, fits between mating parts, and dimensioning conventions is crucial in engineering design.
• Proper application of tolerances ensures parts fit correctly, function as intended, and meet quality standards.
• Knowledge of advanced aspects like surface texture and angular tolerances is essential for precise engineering.