ENGR1025 - Tutorial 7

Engineering Design: Tolerancing
Concept Review
  • General Overview: Understanding tolerancing is essential for ensuring the proper fit and function of mechanical parts in engineering design.
Fit and Clearance
  • Nominal Size: Refers to the intended dimensions of parts, such as holes and shafts in CAD models.
    • Example: A hole and a shaft might both have a nominal size of 10 mm.
  • Allowance: The hole's diameter must exceed that of the shaft slightly to allow smooth rotation.
    • Example: If a shaft diameter is 10 mm, a hole diameter of 10.1 mm provides sufficient allowance.
  • Tolerance: The allowable variations from the nominal size, which helps reduce manufacturing costs while maintaining functionality.
    • Example: A shaft with a nominal size of 10 mm might have a tolerance of ±0.05 mm, allowing it to be between 9.95 mm and 10.05 mm.
Types of Fit
  • Clearance Fit: Provides space between parts; allows movement and is the most common type.
  • Transition Fit: May allow some movement but can also cause tight fits; relatively uncommon.
  • Interference Fit: Parts are tightly fitted without any relative movement; very rare and often expensive to manufacture.
Tolerance Definitions
  • Tolerancing Limits:
    • Lower Limit: The smallest size that is acceptable based on mass.
    • Upper Limit: The largest size that is acceptable based on mass.
  • Tolerancing Methods:
    • General Notes: Tolerance specified in the title block.
    • Conditional Notes: Tolerance for specific features.
    • Direct Limits: Specific tolerances applied to dimensions.
    • Geometric Limits: Tolerances regarding geometric features, like straightness or roundness.
Tolerancing Methods Explained
  • Using Notes: Specify tolerance for most features in simpler designs.
    • Implementation: Place in title block as "ALL TOLERANCES ± XX UNLESS SPECIFIED OTHERWISE."
  • Conditional Notes: Tailored for specific features, used when parts have various shapes with simple feature patterns.
  • Direct Limits: Precise specifications for dimensions with either upper/lower limits or nominal sizes with ranges (symmetric/asymmetric).
Geometric Dimensioning (GD)
  • Usage: Provides detailed tolerance on part geometries, utilizing symbols located in a Feature Control Frame.
    • Steps:
    1. Define part functions by simplest features.
    2. Prioritize functions.
    3. Establish a datum reference frame.
    4. Select controls starting with the least restrictive.
    5. Calculate tolerances.
Practical Examples
  • Holes: Precise positioning relative to one another is critical; holes may only increase in size.
  • Arcs: Inside arcs can be larger; the outside arc may vary. The arc should maintain its linearity.
  • Basic Tolerances: For example, spaces like the following:
    • Hole Dimensions: Can allow larger holes but not smaller by +1 mm, 0.
    • Geometric Tolerances: e.g., arcs should be straight within ±0.05.
Design Project Overview
  • Project Tasks: Completion timelines for design tasks, with emphasis on creating detailed designs in SolidWorks and adhering to ANSI standards.
  • Marking Rubric: Detailed criteria based on:
    • Proficiency in SolidWorks (50%)
    • Adherence to ANSI Standards (25%)
    • Detail in Engineering Drawings (25%)
  • Manual Development: Requires producing effective brochures and assembly manuals.