To explain the principle, if you simply break a 3D curve into equal length lines, the deviation of the lines from the curve would change as the curvature became larger and smaller. However, using a chordal deviation, the operator defines the maximum distance between the chord and the original geometry. The major benefit of using a chordal deviation to control the 3D mesh is that you have a consistent, predictable deviation.
You may then ask why not a set of very small chordal tolerance? In theory you can but this does effect the data size and also produces extra CNC code when it may not be necessary. You also have to be aware that on older machines without high speed controllers, the extra movements could choke the machine if the feed rates are not reduced. Dropping the feed rate is non-productive and reducing the accuracy means more time polishing and a less accurate part.
Okay, with the problem explained, the following tip shows how it is possible to improve this situation by defining a different set of chordal parameters based on face type.
When defining the piece parameters, the general chordal tolerance and angular tolerance is applied to the complete model. However, there is a face classification box that open an additional set of parameters that separates the tolerances for planar and radiused faces.
If the face classification box is NOT active, the general tolerances are applied to the complete model.
If the face classification box IS active, additional tolerance parameters become active. This allows the operator to separate the model into 3 tolerance bands – planar faces, radiused faces, and the general faces.
The major advantage of the mesh optimization is that it allows the operator to be more creative with the machining tolerances and still retain a sensible model size.
If you use the face classification option within the piece definition, it is then possible to graphically display and sort by face type when displaying the mesh within the piece manager. Even at this point within the tree structure, it is possible to select an individual face and define a different chordal and angular tolerance. If individual face parameters are applied, the face will be displayed with a different icon within the tree.