When designing components, Geometric Dimensioning and Tolerancing (frequently abbreviated GD&T) is crucial for ensuring part consistency and communicating both design intent and tolerance requirements to engineers and injection molding manufacturers.
In this article, we’re going to cover GD&T and its primary purpose in the design of plastic injection molded parts. We’ll walk through what Geometric Dimensioning and Tolerancing is, the primary purpose of GD&T, its standards, basic guidelines, and commonly used symbols by injection molding companies.
What Is Geometric Dimensioning & Tolerancing?
Geometric Dimensioning and Tolerancing (GD&T) is a standard system for defining design intent and communicating the engineering tolerances that a certain part requires.
In short, GD&T is a way to make manufacturing consistent and ensure that injection molding manufacturers are optimally controlling any possible variations in the manufacturing process.
GD&T helps ensure both the functionality and accuracy of parts without the usual increase in cost that comes with tightening tolerances. With plastic injection molded parts and general mold design, GD&T ensures that the resulting parts meet specifications. This is especially important for rapid injection molding, where as long as the functional GD&T tolerances are met, the part will be good enough to proceed with.
Without GD&T, plastic injection molding companies risk producing parts with surface and feature variations. For some applications where precision is critical, this would result in a part that doesn’t meet quality control standards.
With GD&T, engineering drawings specify the tolerances for manufactured parts and ensure consistency that still meets quality control specifications.
What Is the Primary Purpose of GD&T?
The primary purpose of GD&T is to establish maximum and minimum acceptable limits on various dimensions within engineering drawings. For example, if a part is specified to be exactly 500 millimeters in length, a tolerance of 5 millimeters could be applied, indicating that the produced part would pass quality control as long as it is between 495 millimeters to 505 millimeters in length.
When it comes to plastic injection molding and rapid prototyping of plastic parts, GD&T ensures that manufacturers have specific tolerances where a part would still meet specifications. Tight tolerances are often difficult to adhere to in plastic injection molding processes and can result in substantial costs if a manufacturer has to scrap parts that might not fit a tight tolerance scheme.
There are a wide array of standards related to Geometric Dimensioning and Tolerancing. Some of the standards we see used today include:
- ISO TC 10 Technical Product Documentation
- ISO/TC 213 Dimensional & Geometrical Product Specifications & Verification
- ASME Standards
- ISO 10303 GD&T Standards for Data Exchange & Integration
These documents and standards ensure that engineers use consistent naming conventions, labeling schemes, and standards to communicate dimensions and tolerances using GD&T.
Basic GD&T Guidelines
It is essential to understand that GD&T when applied to drawings:
- Does not tighten tolerances
- Helps create clear and functional drawings and blueprints
- Has a general tolerance listed on a drawing or blueprint and also indicates tighter or looser tolerances near specific components and/or parts
- Does not explicitly label 90-degree angles, as they are always assumed and do not need to be written in a drawing
To describe various tolerances on parts, surfaces of parts, or other part features, GD&T utilizes multiple types of symbols that help define a part’s: shape/form, profile, orientation, location, datum variations, and runout.
We’re going to describe some of these commonly used GD&T symbols and what they help define on a part drawing or blueprint.
When it comes to defining the shape or form of a part on an engineering drawing, there are four major symbols that are used.
Flatness refer to how flat an object is.
Circularity refers to how close an object should be to a true circle.
Straightness refers to the variance of a surface within a specified line).
Cylindricity refers to how closely an object needs to conform to a true cylinder.
GD&T can be used to define the profile of an object through three symbols.
The Profile of a Line
The profile of a line describes a tolerance zone around a line within a feature (usually of a curve).
The Profile of a Surface
The profile of a surface describes a 3D zone around a surface (typically an advanced shape or curve)
An unequally disposed profile applies unilateral or unequal tolerance zones to a specific profile within a part.
The orientation of a part can be defined in GD&T through parallelism, perpendicularity, and angularity.
Parallelism describes the parallel orientation of a feature to a datum line or surface.
Perpendicularity describes the tolerance of a feature for forming a 90-degree angle.
Angularity describes the tolerance for the angular orientation of one feature to another.
GD&T can define the location of various parts’ features through the symbols pertaining to concentricity, symmetry, and position.
Concentricity determines the tolerance of the central axis of a feature to the datum axis.
Symmetry is a three-dimensional tolerance that ensures two features on a specific part are uniform across a particular datum plane.
Position describes the positional tolerance of a particular feature, always used as a feature of its size.
For datum variations and runout, GD&T has symbols relating to runout and total runout. Runout describes how features vary with respect to a different datum when that part is rotated around its datum axis 360 degrees. Similarly, total runout describes how much an entire feature or entire surface varies with respect to a datum when the part is rotated around its datum axis 360 degrees.
Learn More About Injection Molding From MSI Mold
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If you have questions about designing plastic injection molded parts, or would like to request a consultation, please contact us today.