Mechanical System Design and Tolerancing

Key Considerations in Mechanical System Design

• Complex Process: Involves cost, materials, manufacturing techniques.
• Parts Fitment: Ensuring parts fit together and function as intended.

Tolerancing in Design

• Importance: Essential to ensure parts meet functional requirements.
• Dimensional Tolerancing:
  • Defines allowable deviation in part dimensions.
  • Limitations concerning part functionality and usage.

Geometric Dimensioning and Tolerancing (GD&T)

• Introduction:
  • Controls tolerances based on function of the part.
  • Defines 14 geometric characteristics.
  • Categories: Form, Orientation, Location, Profile, Runout.
• Features vs Dimensions: Applies to surfaces, holes, slots.

Geometric Characteristics and Their Categories

• Form: Flatness, Straightness, Circularity, Cylindricity.
• Orientation: Perpendicularity, Parallelism, Angularity.
• Location: Position, Concentricity, Symmetry (Removed in ASME 2018).
• Profile: Profile of Surface, Profile of Line.
• Runout: Circular Runout, Total Runout.

Feature Control Frames

• Structure:
  • First Box: Symbol of the geometric characteristic.
  • Next Box: Tolerance value (defines tolerance zone).
  • Letters: Datums (reference surfaces).
  • Modifiers: Additional control over tolerancing.

Examples and Inspection Methods

• Flatness:
  • Tolerance Zone: Two parallel planes.
  • Inspection: Dial test indicator or CMM.
• Straightness: Applied to lines (surface) or axis (feature of size).
• Circularity: Radial distance between two concentric circles.
• Cylindricity: Uniform tolerance along feature length.
• Runout:
  • Circular Runout: Roundness of cross-sections.
  • Total Runout: Eccentricity along entire length.

Datums and Datum Reference Frames

• Purpose: Establish coordinate system for inspection.
• Application:
  • Defined using surfaces or features of size.
  • Immobilize part fully for accurate measurement.

Material Condition Modifiers

• Types: Maximum Material Condition (MMC), Least Material Condition (LMC), Regardless of Feature Size (RFS).
• Application:
  • MMC: Smallest hole, largest pin.
  • LMC: Largest hole, smallest pin.
  • Adjusts tolerance zones based on actual size.

Tolerance Examples

• Flatness of Surfaces: Used for even contact in mating parts.
• Position Tolerance: Location of holes, true position, cylindrical tolerance zones.
• Profile Tolerances: Shape controlled by following the feature’s profile.

Rule Number 1: Envelope Principle

• ASME Standard: Perfect form at MMC.
• ISO Standard: Independency Principle (size and form separate).
• Impact: Ensures functional fit of parts.

Inspection Techniques

• Dial Test Indicators: Used for flatness and runout.
• CMMs: Expensive but versatile for all geometric tolerances.

Summary

• Essential Understanding: GD&T’s importance in ensuring functional and manufacturable parts.
• Common Compliances: Familiarity with standards like ASME Y14.5.

Additional Learning Resources

• Nebula Streaming: Exclusive engineering content and ad-free videos.
• Bundle Deals: CuriosityStream and Nebula available for discounts.