GD&T - Part 3; Form Features
Geometric dimensions and tolerances (GD&T) allows you, the designer, to control the dimension, fit and form of a part in way that reflects the intended function of the part and/or assembly. These instructions, typically detailed with symbols on a 2D drawing, allow you to communicate which areas of the design are of the highest importance.
These geometric characteristics are assigned to features or surfaces using feature control frames. These frames communicate all the information required to fully control the size, shape, and location of the respective feature.
GD&T symbols are neatly grouped according to their function. In this post we are going to take a look at form functions. Form features include;
- Straightness
- Flatness
- Roundness
- Cylindricity
Form features make for a great foundation for any 2D technical drawing. When starting to dimension and tolerance a component, always think 'Form Features First'. Although they cannot control the size or orientation of a feature, instead they are used when we wish to control the variation or ‘waviness’ of a feature. For example form features are critical when we wish to control sealing features or provide an even amount of wear between mating parts.
Straightness
Straightness can be defined in two ways; a 2D tolerance zone bounded by two parallel lines, typically applied to an edge or surface, or a 3D tolerance zone bounded by a cylinder, when applied to an axis.
A tolerance zone is defined by the maximum allowable deviation of a feature bound by the geometry stated within the tolerance frame
Flatness
Flatness is defined by a 3D tolerance zone bounded by two parallel planes. Flatness is measured by a number of extracted points across the whole surface
Roundness
Roundness is defined by a 2D tolerance zone bounded by two concentric circles. Roundness is measured by a number of extracted points applied around a single cross section of a cylindrical, conical or spherical surface
Cylindricity
Cylindricity is defined by a 3D tolerance bounded by two concentric cylinders. Unlike roundness, cylindricity is measured by a number of extracted points along a rotated surface about a common axis of a cylindrical, conical or spherical surface.
