Geometric - Engineering Design Graphics - Lecture Slides, Slides of Computer-Aided Design for Engineers

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Geometric

Dimensioning

& Tolerancing

Skill-Development Goal

• To learn how to effectively tolerance parts such that - The Parts Function Correctly - Fabrication Cost Is Kept To A Minimum
• Apply Geometric Dimensioning & Tolerancing (GD&T); in particuluar - Position, Size - Flatness, Circularity - Perpendicularity, Parallelism

Geometric?

• The “G” in GD&T refers to Geometric Forms
  • e.g., plane, circle, cylinder, sq, or hexagon
• Theoretically these forms are Perfect – but any REAL Form will be Imperfect
• In GD&T The Limits of Real Variation (tolerance) are Specified by the Diameter/Width of a Planer, Cylindrical, Annular, or Spherical Zone

History of Tolerancing

• In the 1800’s, manufacturing used the “cut & try, file & fit” approach.
• The plus-minus (or coordinate) system of tolerancing was next developed.
• In the 1900’s, the first GD&T standards came out to improve the quality & utility of engineering drawings.
• In 1966, the united GD&T standard was published → ANSI - Y14.5M

GD&T Definitions - 2

• Tolerance Zones
  • all geometric tolerances have imaginary tolerance zones that are the basis for acceptance or rejection of the product
  • have specific shapes depending on the geometric tolerance and feature being controlled
• Actual Local Size
  • the value of any individual distance at any cross section of a FOS

GD&T Definitions - 3

• Actual Mating Envelope (AME)
  • a similar perfect feature counterpart that can be circumscribed/inscribed about/within the feature so it just contacts the surfaces at the highest & lowest points
  • It is derived from an actual part
  • Used When Calculating a “Bonus Tolerance”
    • More on this Next Time

Limits of Size

• A variation in form is allowed between the least material condition (LMC) and the maximum material condition (MMC).SIZE DIMENSION

MMC

LMC

ENVELOPE OF SIZE

(2.003)

(2.007)

ENVELOPE PRINCIPLE

Envelope Principle defines the size and form relationships between mating parts.

Limits of Size LMC & MMC

• Clearance & AllowanceENVELOPE PRINCIPLE

LMC CLEARANCE

MMC ALLOWANCE

Limits of Size - Boundary

• No portion of the feature may be outside a PERFECT FORM BARRIER at maximum material condition (MMC).

Most Common

The Surface can also be ROUGH

GD&T Feature Control Frame

 From ASME Y14.5M-

• Some ACAD Feature-Frames from Y14.5-

GD&T – Form & Profile

INDIVIDUAL(No Datum Reference)

INDIVIDUALor RELATED FEATURES

GEOMETRIC CHARACTERISTIC CONTROLS TYPE OF FEATURE TOLERANCE TYPE OF CHARACTERISTIC SYMBOL FLATNESS STRAIGHTNESS CIRCULARITY CYLINDRICITY LINE PROFILE SURFACE PROFILE

FORM

PROFILE

14 characteristics that may be controlled

GD&T – Orient, RunOut, Loctn

RELATEDFEATURES (DatumReference Required)

GEOMETRIC CHARACTERISTIC CONTROLS TYPE OF FEATURE TOLERANCE TYPE OF CHARACTERISTIC SYMBOL

SYMMETRY

PERPENDICULARITY ANGULARITY PARALLELISM CIRCULAR RUNOUT TOTAL RUNOUT CONCENTRICITY POSITION

ORIENTATION

RUNOUT

LOCATION

14 characteristics that may be controlled

Basic Dimension

• A theoretically exact dimension used to locate features in GD&T - The Dimension From Which the Limits of Variation are Derived
• Basic dimensions are UNtoleranced - These NOMINAL Dims are THEORETICALLY Exact

 Basic Dims Identified by Enclosure in a FRAME

Std-Tol vs GD&T - 1

• Standard Tolerance

Not Well Known: Actual hole-ctr distances, angle of hole-ctrs