Lecture 18 - GD&T Overview

Unit 13 GD&T: Introduction

Importance of GD&T

  • Parts designed to meet functional requirements.

  • Ensuring fabricated parts function as designed is crucial.

Sheet Metal Bending

  • Bending Process: Sheet metal can be bent using a sheet metal roller.

    • Two or More Rollers: The gap between rollers determines the bend produced.

Roller Characteristics

  • Roller Shape Control: The shape of the roller is crucial for functionality.

    • Example Measurements: 7.510, 7.490, 2.510, 2.490.

    • Different profiles of rollers can have varying capabilities for functioning as intended.

Assessment of Roller Measurements

  1. Perfect Form: Meets size tolerance and has proper shape (e.g., 7.502).

  2. Bent Roller: Meets size tolerance but is bent (e.g., measurements: 7.502, 7.495, 7.494, 7.496).

  3. Non-Cylindrical Roller: Meets size tolerance, but not round (e.g., 7.502, 7.492).

  4. Non-Coaxial Features: Meets size tolerance, but features are non-coaxial (e.g., 7.502).

Functionality and Tolerances

  • Key point: All rollers meet size tolerance, but only one functions as intended.

  • The challenge is to determine how to fix functionality.

Increasing Size Tolerances

Solution #1: Reduce Size Tolerance

  • Tighter tolerances allows for less deviation in form, shape, etc. (Example sizes: 7.510, 7.490).

  • Result: Higher precision needed, but also increased cost and potential functional part rejections.

Example of Height Variation

  • Flatness Requirement: Surface needs a very flat dimension but height can vary significantly within limits (e.g., flatness of .0001”, height tolerance of .5”).

  • Scenario illustrates proper assessment of tolerances that balance operational needs.

    • Accepted Parts: Part A (20.001) and Part B (20.499).

    • Rejected Part: Part C (20.099).

Issues with Tightening Tolerances

  • Tightening height tolerance excessively restricts acceptable plate height.

  • Adequately flat plates can be rejected if too strict height controls are imposed.

Alternative Solutions and Introduction to GD&T

  • Essential to find new solutions without detrimental effects on functionality or costs.

Solution #2: Use GD&T

  • Definition: GD&T (Geometric Dimensioning and Tolerancing) is a symbolic language to define the function of features by specifying permissible imperfections.

  • Allows control over separate features of a part, addressing size and other characteristics through specific tolerances.

Geometric Characteristics Controlled by GD&T

  • Control of 14 Geometric Characteristics:

    • These include Form, Orientation, Profile, Runout, and Location.

Categories of Geometric Characteristics

  • Categories Include:

    • Form: Shape integrity.

    • Orientation: Angle placement of features.

    • Profile: Contour regulation.

    • Runout: Rotation precision measurement.

    • Location: Spatial positioning accuracy.

GD&T Implementation

Engineering Standards

  • Implemented through ASME Y14.5 standard.

  • Specific revision of the standard indicated in the title block or general notes of engineering drawings.

Feature Control Frame (FCF)

  • Classes of drawings utilizing GD&T include Feature Control Frames to define geometric tolerance characteristics.

  • Components of FCF:

    • Symbol, size value, datum references, and condition modifiers.

Datum Identification and Basic Dimensions

  • Datum Identification Symbol: Defines theoretical references critical for geometric tolerance establishment.

  • Basic dimensions specify theoretically exact sizes/locations of features outside title block tolerances.

GD&T Application on Components

  • The application of GD&T controls can adequately address specific issues related to parts, such as surface plates and rollers.

  • Different controls such as flatness, circularity, and cylindricity can be applied to enhance functional accuracy.

  • The need for precise measurements in maintaining correct tolerances and ensuring that components function as designed is critical throughout the manufacturing process.