GD&T Lecture Notes

Introduction

• GD&T (Geometric Dimensioning and Tolerancing) is a language of symbols used to communicate design intent.
• Four major concepts to cover:
  1. Datums
  2. Feature Control Frames
  3. Material Condition Modifiers
  4. Basic Dimensions

1. Datums

• Definition: Theoretical point, line, or plane used as a reference for measurement.
• Identification: Features on the part serve as datums, simulated with datum feature simulators (e.g., surface plate, gauge pin).
• Importance: Datums specify the exact surface for measurements, reducing interpretation mistakes.
• Examples:
  • Implied Datum: Older drawings may indicate parallelism without explicit datums, leading to ambiguity.
  • Explicit Datum: Clearly identifying a surface or centerline to measure from.

2. Feature Control Frames

• Purpose: To reduce interpretation errors by using symbols instead of written notes.
• Structure: Box containing:
  • Geometric characteristic symbol
  • Tolerance
  • Datum references
• Examples of Tolerances:
  • Form Tolerances: Flatness, which does not need a datum reference.
  • Profile Tolerances: Can function like flatness or locate surfaces; may include datum references.
  • Position Tolerances: Generally more complex; involve multiple datums.
• Datum References: Identified by letters (A, B, C), establishing inspection setup.
  • Three-Two-One Rule: A method to immobilize the part during inspection.

3. Material Condition Modifiers

• Types:
  • MMC (Maximum Material Condition): Largest size of an external feature or smallest size of an internal feature.
  • LMC (Least Material Condition): Smallest size of an external feature or largest size of an internal feature.
  • RFS (Regardless of Feature Size): Default requirement for features of size; no symbol needed.
• Application: MMC and LMC allow for more tolerance based on actual sizes of parts.

4. Basic Dimensions

• Definition: The theoretically exact values applied to size or location of features controlled by a feature control frame.
• Characteristics:
  • Not included in tolerance stack-ups.
  • Must be associated with feature control frames and originate from datums.
  • Allow for clear interpretation of feature location and tolerances.

Conclusion

• Recap of the four major concepts of GD&T.
• Encouragement to learn and engage with future videos on the subject.