Geometric Dimensioning and Tolerancing GD&T for Engineering

In this lesson emphasizes the significance of GD&T (Geometric Dimensioning and Tolerancing) in accurately reflecting the function and installation of features on a part. The designer should focus on the part's operation rather than the manufacturing process. Using an example of a part with holes, I will illustrate how dimensioning from the edge of the part can be misleading and increase the part's cost for the manufacturer. Instead, GD&T allows for defining the exact theoretical positions of each feature and adding tolerances to the whole features in relation to each other, ensuring that the part is dimensioned correctly for its intended use.

In this lesson I will show that GD&T symbols may seem complex but they serve to define the acceptable variation of features in a part. This is achieved by establishing the ideal form or dimension of a feature and allowing for deviations within a specified zone. Three main types of notations are used to define perfect geometry and allowable variation: feature control frames, datum features identifiers, and basic dimensions. Feature control frames determine the size and shape of the tolerance zone and reference a frame of reference using datum references. Datum features identifiers identify the features on the part used as a datum, which can be any surface, axis, or point providing a frame of reference for interpreting the feature. Basic dimensions represent the exact theoretical definition of a feature in relation to its datum references.

In this lesson I will discuss the format of Geometric Dimensioning and Tolerancing (GD&T) notation. GD&T uses a feature control frame, which is a horizontal rectangle divided into three sections. The first section contains the tolerance symbol, defining the type of tolerance zone, such as a perpendicularity symbol with an upside-down T. The second section specifies the allowable tolerance, for example, 20 microns. The last part in the feature control frame indicates which other features of the part the subject is referenced to, known as datum references. Datum features are identified by boxes and letters with leaders attached to the appropriate part. Basic dimensions, identified by a rectangle around the dimension value, are theoretically exact dimensions with no tolerance on their own, but the tolerance on the feature they point to is defined elsewhere on the drawing in the feature control frame.

In this lesson I will discuss the use of Geometric Dimensioning and Tolerancing (GD&T) for positional tolerances and diameter measurements. For positional tolerances, the process involves defining a frame of reference using datum identifiers, defining the theoretical exact position, and allowing for deviation using a crosshair symbol. Position tolerances are always between datum features and the subject feature, providing manufacturers with additional tolerance. I will also cover the concept of virtual conditions (VC) and position tolerances for diameter measurements. The VC zone, which is the area where a feature will never encroach, is calculated by subtracting the position tolerance from the maximum material condition (MMC) diameter. The VC zone is located on the position defined by the basic dimensions. Using the example of cylindrical tolerance zones and pins with specific diameters, then Finally I will demonstrate that any two parts with these specifications will always fit together, even with variations in size.

In this lesson I will discusses the concept of bonus tolerances in relation to a drilled hole with a nominal dimension of 250 microns that exceeds the minimum material condition (MMC). A hole larger than MMC can have its axis outside the 14 micron tolerance zone without violating the virtual condition (VC), and the difference between the actual part and MMC is considered a bonus tolerance for the hole's position. The tolerance zone diameter for a 250 microns hole would be 14 microns plus 10 microns, resulting in a tolerance zone of 24 microns. This flexibility in manufacturing allows for optimized processes and cost-effective production of high-quality parts. However,  notes that this calculation can be complex and introduces the concept of a functional gauge as a simpler solution for checking part dimensions against the VC. The use of a functional gauge is not possible if the feature control frame does not contain the M modifier.