Measured and Constructed Features
What’s the difference between measured and constructed features? The vast majority of workpieces are made up of simple geometric elements created by machining or forming. These primary elements (planes, edges, cylinders, spheres, cones, etc.) are called features. When a CMM can measure these features directly, by touching the surfaces that make up the feature with a probe, the features are referred to as measured features.
Other features, such as distance, symmetry, intersection, angle and projection, cannot be measured directly but must be constructed mathematically from measured features before their values can be determined. These are called constructed features. In Figure 11 the centerline circle is constructed from the center points of the four measured circles.
The relationships between one feature or group of features to another feature or group of features are critical to manufacturing. For example, the intersect point between the cylinders on one side of an engine block and those on the other side determines how well mating parts fit (Figure 12). This intersect point is constructed from the two measured features (the engine cylinders).