Courtesy: Xometry

GD&T, stands for geometric dimensioning and tolerancing, and is a system of symbols and standards that designers use to communicate to manufacturers through engineering drawings. Moreover, GD&T defines engineering tolerancing and relationships of parts in a design. In this article, we will discuss the basics of GD&T, geometric tolerancing vs size tolerancing, and categories of GD&T symbols.

Basics of GD&T

Designers use GD&T to express intentions that coordinate dimensioning and tolerances alone cannot show. For example, in designing a mirror, ensuring that the surface is not wavy is necessary to avoid image distortion. If using coordinate dimensioning, the designer can attempt to achieve this by specifying tight thickness tolerance in the drawing. However, the mirror could pass a thickness inspection test with measurements from several points, but it could still end up with a wavy surface. On the other hand, using GD&T will address this by specifying tolerance using the flatness symbol as the figure below shows.

Flatness symbol used to focus on the function of a part when using GD&T.
Courtesy: GD&T Basics

Because this approach focuses on the function of the part, other tolerances that are not important such as the thickness could be loosened. As a result, the part will be cheaper and easier to produce while still meeting its function requirements.

GD&T vs Size Tolerancing

Generally, tolerancing involves following either the size approach or the geometric approach. In comparison to size/dimensional tolerancing which regulates each dimension on a part, geometric tolerancing (GD&T) offers more sophistication. This is because it encapsulates the regulation of shape, position, tilting, parallelism, run-out, and other factors. For example, a designer will specify instructions for a sheet part using size tolerance as follows:

Sample size tolerance of a sheet part.
Courtesy: Keyence

Despite the part remaining within the set tolerance, this drawing could still lead to fabrication errors such as sloping or wavy surface.

Possible errors when using size tolerance.
Courtesy: Keyence

To avoid such defective products, GD&T would specify tolerance for geometric characteristics including parallelism, flatness, and the reference plane.

GD&T approach provides tolerance for geometric characteristics.
Courtesy: Keyence

Another difference between these two methods is the measuring instruments and inspections methods of parts. Size tolerance often involves taking measurements of two points, so instruments such as calipers and micrometer gages can satisfactorily inspect parts. However, geometric characteristics are more complex to measure so devices such as roundness measuring instrument and coordinate measuring machine come in handy.

GD&T Symbol Categories

Like every other dimensioning and tolerancing system, GD&T has several symbols. Classification of these symbols depends on the product feature that the designer intends to control.

Profile Symbols

GD&T has two symbols that describe the three-dimensional tolerance zone around a model. Moreover, this description could be with or without a datum. The symbols are:

  • Line Profile: This compares the two-dimensional cross-section of a part to an ideal shape. Generally, two offset curves are used to define the tolerance zone because it usually serves for features with curved shape.
  • Surface Profile: Surface profile is similar to line profile in that it uses two offset features to define a tolerance zone. The difference is that it describes surfaces rather than lines.

Form Symbols

Form symbols or controls specify the shape of features in a drawing and include:

  • Straightness: This provides control for the deviation of a line on a surface or an axis within the tolerance zone.
  • Roundness/Circularity: Describes how close an object should be from a true circle.
  • Cylindricity: Specifies how accurately circular and straight a feature should be. Measuring this is relatively expensive because it involves assessing straightness, roundness, and taper.
  • Flatness: This defines the straightness of a surface in multiple dimensions on the basis of measurements between the highest and lowest points on the surface.

Orientation Symbols

Orientation tolerance symbols determine the orientation of a feature in relation to a reference. They include parallelism, perpendicularity, and angularity.

Orientation symbols of GD&T
Courtesy: GD&T Basics

Location Symbols

Location tolerance symbols define the true position of a feature in relation to a reference. Moreover, they specify features using linear dimensions and includes position, concentricity, and symmetry.

Runout Symbols

Runout tolerance symbols define the amount by which a feature can vary with respect to a reference. These include:

  • Circular Runout: Circular runout is used to account for many different errors such as in ball-bearing mounted parts. It is a two-dimensional version of total runout.
  • Total Runout: This is measured on multiple points of a surface to describe the runout of an entire surface.
Runout Symbols of GD&T
Courtesy: GD&T Basics