Exchanging Data: The Key to the Virtual Network of Resources
CAD data exchange remains a challenge for designers and engineers during the product development stage and in the data turnover to manufacturing. Among the many 3D CAD systems there is limited data transfer capabilities that retain native feature functionalities. Accordingly CAD data is routinely transferred between CAD platforms using IGES (Initial Graphics Exchange Specification) and STEP (Standard for the Exchange of Product model data) formats. While these exchange tools provide for generally good data transfer they do not transfer feature creation data and are thus inflexible for modification. While CAD publishers recognize this as a problem and have tried to address it; to date there has been poor result in translating anything but primitive geometric forms which are comprised of planar, cylindrical, revolved, and spherical surfaces. Parts that require splined curves and surfaces remain problematic to transfer from one CAD platform to another. The reason for this is that primitives are defined mathematical different from those of complex surfaces. While primitive geometries consisting of planar, cylindrical, ruled, spherical, and revolved surfaces and can be defined by unlimited discrete mathematical coordinates, those that are defined using spline surfaces require polynomial equations with interpolation to transition between defined curvature patches within the surface curvature network. The interpolation aspect of surface data translation is the basis of the popular commentary on IGES translation which is that IGES is an acronym for "I guess."
This bit of background brings us to the discussion of the CAD modeler importing complex surface data. More often than not, complex surface data is imported with boundary errors and sometime with surfaces patches missing. The importing CAD modeler rather than proceeding to work with the data must embark on data repair which could take many hours. Among the more robust CAD systems such as Pro/Engineer have very useful Data Repair tools. Import data errors are identified by two conditions: 1.) open boundaries are displayed in a different color and 2.) The solidify commands fail to convert surfaces to solids.
Data Repair follows a hierarchy of tool application wherein the zip gaps function can close many slight boundary gaps. This is the first line of action. If this automated procedure command is not fully successful then manual repair tools are required. If there remains even one tiny gap in the surface boundaries the surfaces can not be converted to solids. It's as if the volume were a vessel from while some liquid or gas can escape.
Manual repair can be tedious. A number of tools can be tried that include modifying edges by moving vertices, straightening crooked boundaries, merging edges, setting tangencies, and dividing boundary curves. New edges can be created by projecting curves onto good surfaces, forcing curves to connect, trimming newly created curves on surfaces, and creating straight lines to replace crooked lines. After these operations are employed zip gaps can be reapplied with probability of greater success. If after all of these tools have failed to remove remaining gaps the next step is to remove the problem surface and create a new one. Removal of a surface can be achieve in the repair tools or by using a surface trim command which involves selecting the boundaries of the problem surface for the trim definition. In the worst case when the data poses excessive boundary repair problems offering at best poor surface data integrity then the last resort is to remodel the part using the imported geometry as data reference only.
In summary, the more robust CAD systems in the hands of experienced professionals produce better quality data export and feature the Data Repair tools that enable effective repair of part data.