Repairing STL data

The increasing use of STL data in the programming process (e.g. as a specification of an actual state of the workpiece in roughing path calculation or description of a cast blank or for determining safety distances in drilling/deep hole machining) also raises a number of questions. STL data are not always of good or acceptable quality. In principle, an STL file consists of a more or less well sorted pile of more or less large triangles without deeper topological or geometrical information. The data in this application context are generated e.g. by calculations of the workpiece actual state, by analytical material removal simulation calculations or optical data acquisition.

Typical quirks of these data are 1. wrong or erroneous or incomplete normal vectors, 2. wrongly oriented triangles or double triangles, or 3. disconnected regions.

These quirks and errors become clearly visible when the SHADE-2-SIDE mode is activated. Besides the unattractive appearance, it must also be stated that this can have negative effects on calculations by producing unexpected results.

Figure 1: STL data after import without normals

Figure 2: STL data with completely calculated normal vectors

When importing the STL data, the IO option 'approximate normal vectors' can be used to repair the defect from image 1. The result is shown in figure 2.

Figure 3: STL data with topological defects before (left) and after repair

The repair of topological defects or incorrectly oriented sub-areas is somewhat more complex. After the data import (necessarily with activated normal vector approximation), the SURFACE OBJECT (SO) must be divided by means of <split>. Optimally, the result is a 'correct' SO and one or more 'splitters'. These incorrectly oriented parts are to be interactively corrected or - if necessary - deleted by means of <face orientation>. In the end, all partial SO's are to be exported into a new STL file - in doing so, they are automatically reassembled.

Figure 4: STL data with 'unqualified sub-areas'

Figure 5: repaired SO

Figure 4 and Figure 5 show a frequently encountered situation that is typical for STL data originating from a digitising process. Impurities, external partial areas or the like can usually be eliminated step by step with the <splitting> procedure.

Repairing STL data

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