Even though the underlying modeling technology is similar among various CAD systems, there are a number of significant differences in how 3-D objects are defined. CAD systems must select various tolerances that best suit their mathematical algorithms for generating generally shaped surfaces, surface-surface intersection curves and positions in space. This must be done because computers are machines with limited floating-point precision.

For example, an IEEE sixty-four-bit floating-point (real number) representation gives fifteen digits of precision. In addition, many mathematical algorithms are numerical and iterative in nature, for which exact solutions simply don’t exist. Therefore, appropriate tolerances must be used as stopping criteria to decide when a numerical solution is “good enough.” This works fine within each CAD system as long as the selected modeling tolerances are smaller than those used in the manufacturing processes.

However, if a part needs to be exchanged between CAD system A and B, then the differences in modeling tolerances come into play and may cause geometric inconsistencies, affecting the translation success rate.
The following are examples of problems that can arise during translation between two CAD systems due to different tolerances:

A curve start and end points are not coincident with edge terminal vertices to within the receiving CAD system tolerance.

A curve representing an edge between two faces does not lie on both surfaces to within the receiving CAD system tolerance.

An edge may be considered too small in the receiving system and should be represented by a vertex instead.

A face may be considered too small in the receiving system and should be represented by an edge or a vertex instead.

A different set of translation problems can arise due to topological differences. For example, closed surfaces represented differently among CAD systems. Another example involves surfaces with degenerate boundaries . By definition, all NURBS surfaces are four-sided.

Degenerate surface boundaries arise when a NURBS surface is used to represent a sphere (two degenerate boundaries: north and south poles) or a triangular-shaped patch (one degenerate boundary), for example. Topology may have to be modified to be compatible with the receiving system. This really is a topology that is a result of the limitations in the NURBS technology, so it can be called an artifact topology.

It is clear that CAD data translation requires a high level of knowledge and experience in both modeling technology and the specifics of the sending and receiving CAD systems. Reliable data translation cannot be accomplished by a casual development effort. In addition to the technical know-how, the maturity and comprehensive exposure to a wide range of large models of different industries is a trademark of good software