How can you specify geometric features like holes, slots, and pockets using GD&T best practices?

1 Basic GD&T concepts

Before you start applying GD&T to your features, it's important to understand some basic concepts and terms. Datums are reference points, lines, or planes that serve as the basis for dimensioning and tolerancing. A feature control frame is a rectangular box that contains the GD&T symbol, the tolerance value, the datum references, and other modifiers. Features of size have two opposed or coaxial surfaces that can be measured by a size dimension like a hole, a slot, or a pocket. Geometric characteristics are symbols that indicate the type of tolerance that applies to the feature. Lastly, a tolerance zone is a region within which the feature must lie to meet the specified tolerance; it can be a cylinder, cone, plane, or complex shape.

3 Slot features

Slots are a type of feature of size that can have various purposes in mechanical parts, such as sliding, guiding, or adjusting. To specify a slot feature using GD&T, you need to consider the size and location of the slot. You can use a basic dimension to indicate the nominal size and location of the slot, and then use a feature control frame to specify the tolerance. Additionally, you need to consider the shape and orientation of the slot. You can use a form tolerance to control the shape of the slot, such as straightness or flatness. An orientation tolerance can also be used to control the angle of the slot relative to a datum. Lastly, you need to consider the surface condition of the slot. You can use a surface texture symbol to indicate the roughness or smoothness of the slot surface and a modifier to specify how the surface condition affects the tolerance.

4 Pocket features

Pockets are recessed features of size often used for weight reduction, heat dissipation, or accommodating other components. To specify a pocket feature using GD&T, the size and location of the pocket must be considered. A basic dimension can indicate the nominal size and location, while a feature control frame can specify the tolerance. For example, a profile tolerance can define the tolerance zone for the shape and size relative to a datum. Alternatively, a length symbol (L) or a depth symbol (D) can indicate the size tolerance and positional tolerance can define the location tolerance. Additionally, form tolerances can control the shape and orientation of the pocket, such as flatness or circularity, while an orientation tolerance can control the angle relative to a datum, such as perpendicularity or parallelism. Lastly, surface texture symbols can indicate the roughness or smoothness of the pocket surface and modifiers can specify how this affects the tolerance, such as maximum material condition (MMC) or least material condition (LMC).