Notes on Geometric Dimensioning and Tolerancing (GD&T)

Introduction to Mechanical Design

• Designing and building mechanical systems is complex and involves considerations like:
  • Cost
  • Materials
  • Manufacturing techniques
• Key challenge: Ensuring parts fit and function as intended.

Importance of Tolerancing

• Tolerancing is crucial in mechanical design.
• Dimensional Tolerancing: Defines allowed deviations from nominal values, but has limitations in practical applications (e.g., does not account for surface flatness or perpendicularity).
• Geometric Dimensioning and Tolerancing (GD&T): Alternative approach that reflects the intended use of parts better.

GD&T Overview

• GD&T allows control of 14 geometric characteristics.
• Characteristics categorized into 5 groups:
  1. Form
  2. Orientation
  3. Location
  4. Profile
  5. Runout
• Tolerances are applied to features rather than dimensions.

Features of Size vs. Surface Features

• Surface Features: Individual surfaces.
• Features of Size: Defined dimensions (e.g., holes, slots).
• GD&T tolerances differ based on whether they apply to surface features or features of size.

Feature Control Frames

• Used to assign geometric tolerances.
• Structured with:
  • Symbol for the geometric characteristic
  • Tolerance value (size of tolerance zone)
  • Series of letters for datums
  • Modifiers for additional control

Geometric Characteristics Examples

Flatness

• Defines a tolerance zone between two parallel planes.
• Used to ensure even contact for mating surfaces (e.g., flanges).
• Inspection methods include dial test indicators and CMMs.

Straightness

• Applied to individual lines, defining a tolerance zone with parallel lines.
• Inspected by sweeping the probe along lines.

Circularity

• Controls roundness with two concentric circles.
• Inspected through rotation and measurement of deviations.

Cylindricity

• Similar to circularity but uniform along the full length of the feature.

Datums and Datum Features

• Datums: Reference surfaces for inspection; identified by a letter and triangle symbol.
• Datum Feature: Actual feature on the object that gets restrained.
• Datum Simulator: Actual imperfect surface used to approximate the datum.
• Establishes a datum reference frame for measurements.

Orientation Tolerances

• Parallelism: Closeness to parallelism with a datum (two parallel planes).
• Perpendicularity: 90-degree tolerance to a datum.
• Angularity: General angle control between a feature and a datum.

Location Tolerances

• Position: Defines maximum distance a feature axis can be located away from the true position.
• True position established with basic dimensions.
• Dimensional tolerances create rectangular zones; GD&T uses cylindrical zones.

Modifiers in GD&T

• Bonus Tolerance: An additional tolerance based on the actual size of the feature.
• Maximum Material Condition (MMC): Smallest allowable size for holes; adds bonus tolerance if feature is oversized.
• Least Material Condition (LMC): Largest allowable size; adds bonus tolerance if feature is undersized.

Envelope Principle (GD&T Rule Number 1)

• Surface of a feature must not extend beyond a perfect form envelope at MMC.
• Controls both size and form of the feature.
• ISO standard uses Independency Principle, which separates size and form considerations.

Profile Tolerances

• Versatile; control form, orientation, and location simultaneously.
• Profile of Surface: Tolerance zone follows the shape of the feature.
• Profile of a Line: Controls line elements of a surface.

Runout Tolerances

• Describes eccentricity of a surface relative to an axis.
• Two types: Circular Runout and Total Runout.
  • Circular runout uses concentric circles.
  • Total runout controls runout along the axial direction.

Conclusion

• GD&T is complex, but understanding its fundamentals is essential for mechanical design.
• Additional resources and videos available on platforms like Nebula and CuriosityStream for further learning.