Introduction to Mechanical Design and Tolerancing

Key Aspects of Mechanical Design

• Complexity in designing mechanical systems.
• Considerations:
  • Cost
  • Materials
  • Manufacturing techniques
• Ensuring parts fit and function together is crucial.

Importance of Tolerancing

• Tolerancing ensures parts fit together after manufacturing.
• Defining tolerances via dimensional approach:
  • Dimensions allowed to deviate from nominal values.

Limitations of Dimensional Tolerancing

• Does not reflect intended use of parts.
• Cannot specify requirements for surfaces (e.g., flatness for sealing).
• Lacks control over geometric relationships (e.g., perpendicularity).

Introduction to GD&T (Geometric Dimensioning and Tolerancing)

• GD&T as a complementary approach to dimensional tolerancing.
• Controls 14 geometric characteristics to reflect part function better.

Categories of GD&T Characteristics

1. Form
2. Orientation
3. Location
4. Profile
5. Runout

Features in GD&T

• Surface features: Individual surfaces.
• Features of size: Defined dimensions (e.g., holes, slots).
• Different tolerances apply to different feature types.

Feature Control Frames

• Structure of feature control frames:
  • First box: Symbol for geometric characteristic.
  • Second box: Tolerance value (defines tolerance zone).
  • Following boxes: Datums and modifiers.

Example: Flatness Tolerance

• Tolerance zone defined by two parallel planes.
• All points on the surface must be within this zone.
• Commonly applied to mating surfaces (e.g., flanges).

Inspection Methods for Flatness

• Dial Test Indicator: Measures high/low points on surfaces.
• CMM (Coordinate Measuring Machine): Measures geometric tolerances using probes.

Additional Form Tolerances

1. Straightness: Applies to lines on surfaces.
2. Circularity: Controls roundness of surfaces with concentric circles.
3. Cylindricity: Uniform tolerance along the length of the feature.

Datums in GD&T

• Used to locate features and immobilize parts during inspection.
• Differentiation:
  • Datum Feature: Feature on the object restrained.
  • Datum: Theoretical perfect surface.
  • Datum Simulator: Real surface that approximates the perfect datum.

Degrees of Freedom

• A part has six degrees of freedom in space.
• Restraining datum features reduces degrees of freedom and aids in accurate inspections.

Orientation Tolerances

1. Parallelism: Closeness to being parallel to a datum.
2. Perpendicularity: Closeness to being perpendicular to a datum.
3. Angularity: Angle control between feature and datum.

Location Tolerances

• Position: Defines maximum allowable distance from true position.
  • Cylindrical tolerance zone around true position.
  • Primary datums chosen based on functional importance.
• Modifiers for Position Tolerance:
  • MMC (Maximum Material Condition): Bonus tolerance based on feature size.
  • LMC (Least Material Condition): Similar function, less commonly used.

Envelope Principle (GD&T Rule Number 1)

• Regular features of size must not exceed an envelope at MMC.
• Ensures parts fit together properly.
• Contrasts with ISO standard's Independency Principle.

Profile Tolerances

• Versatile tolerances controlling form, orientation, and location.
• Profile of a Surface: Follows the shape of the feature.
• Profile of a Line: Controls individual lines of a surface.

Runout Tolerances

1. Circular Runout: Eccentricity relative to a datum axis.
2. Total Runout: Controls runout along the axial direction.

Conclusion

• GD&T is complex but crucial for ensuring functional design and manufacturing.
• Offers enhanced communication and control over design aspects.