Equipment
Equipment : Import-STL : Control the Process
Control the Process
Translation Process
Within the STL format all objects are represented by triangles, the triangle being the shape with the minimum number of sides that can be used to represent a three-dimensional object. Unfortunately, reducing a shape to triangles increases the quantity of electronic data necessary to define the shape.
For example, a square drawn in a CAD system is defined by its four corners or vertices. The same square in STL format is represented by two triangles, each triangle having three vertices, totalling six vertices, representing a 50% increase in data needed to define the square.
Taking this analogy a step further, a cube within a CAD system is defined by its eight vertices. The surface of the same cube in STL format is represented by 12 triangles, two for each of the six faces of the cube, each triangle having three vertices, totalling 36 vertices. A 450% increase in data is needed to define the cube.
Shapes within a CAD system more complex than a cube result in even greater data increase factors. It can therefore be seen that STL files can be very large in terms of data volume or file size. There are two main effects:
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Large STL files may take several hours to map into MODEL.
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Highly detailed models in MODEL may drag down the performance of the computer and are slow to manipulate.
It is very important that the detail in the source CAD file used to generate the STL file and the amount of detail required in the MODEL should be considered very carefully.
It is impossible to give any definite instructions as to what should be done as there are too many possible combinations of source CAD systems and MODEL uses to define parameters for. The most suitable combination of parameters and settings is best determined by trial and error, however, the following guidelines may be useful:
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control the quantity of STL data
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control the quality of STL data
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control the translation
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review and improve the DESIGN model.
Control the Quantity of STL Data
Import-STL processes STL data intelligently, attempting to recognise groups of triangular facets that equate to particular standard shapes defined in MODEL as primitives (box/cylinder/pyramid/extrusions and negative versions of these). Facet data that cannot be mapped to any of these primitives, or a solid polyhedron is dealt with by creating the facets as POHE (polyhedron) primitives, a less efficient form of handling the data.
A good indicator of a successful operation using ImPlant - STL is the quantity of POHE primitives within the resultant model. The fewer the number of POHEs the easier it is to manipulate the data in MODEL and drafting modules.
There are therefore two factors that directly affect the performance of model manipulation in MODEL. These are:
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The ability of Import-STL to recognise and map triangular facet groups to MODEL primitives.
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The quantity of data to be translated.
It is a good working practice to minimise the amount of data that needs to be translated; this can be done by applying the following guidelines:
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Remove all non-essential aspects of the source model, for export in the case of a gearbox, export the gearbox casing but not the gearbox internals.
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Remove or disable features in the originating Mechanical CAD System that automatically generate many curved surfaces, such as fillets and chamfers. Some Mechanical CAD Systems allow these features to be disabled temporarily, while the export to STL file operation is in progress.
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Use an appropriate tolerance setting when exporting to the STL file. It is impossible to stipulate an exact tolerance setting since this depends very much upon the users requirements for the model. The looser the tolerance factor setting the less accurate the resultant model and vice versa. The recommended approach is to start with a loose tolerance setting, inspect the resultant model, and re-import all or part of the model at a tighter tolerance setting, as necessary.
Control the Quality of STL Data
The integrity of the source model has a direct bearing on the quality of the data produced during an STL export operation. For example, if a solid in the original model is not properly closed prior to being exported, then Import-STL does not recognise the resultant triangular facet group as a single primitive and maps the solid inefficiently with POHE primitives. Most Mechanical CAD Systems have a healing facility that processes the model and check it for inconsistencies such as solids that are not properly closed. It is therefore strongly recommended that source models are healed in their originating Mechanical CAD System prior to the data being exported in STL format. Where the Mechanical CAD System does not have this facility, another application such as CADfix may be used to provide the healing function and to export to STL format.
The STL format is an approximation of a curved surface model, the accuracy and level of detail of which may be controlled through tolerance setting. A balance must be struck between too great and too low tolerance settings. These can be reviewed and where necessary to improve the created model.
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Too great a tolerance setting may result in errors in the STL file.
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Too low a tolerance setting may result in the STL translation operation failing, for example, where holes are located close to the edge of a solid.
Control the Translation
If holes below a certain diameter need not be transferred to the model then the option to remove these holes should be selected. For example, choose to remove bolt-holes, enabling Import-STL to recognise and map more parts to primitives, making it easier to manipulate the model. There may be occasions, however, when Import-STL cannot remove the holes in a part because in doing so it would invalidate the part.
Obtain the value of the minimum hole size that has been exported by viewing the Import-STL report. Use a higher value than this to remove these holes and re-run the export operation through Import-STL to review the changes this makes to the model.
The Wrapper option should be used only when most or all solid parts are inside other parts and can therefore be removed without affecting the appearance of the model. If the Wrapper option is used when this is not the case, then the resultant model may be too difficult to manipulate, due to the fact that Wrapping prevents any primitive recognition, increasing the amount of data that has to be transferred.
Review and Improve the AVEVA E3D™ Model
The extent to which the resultant imported model may require to be modified depends entirely upon the end requirements. A crude representation may be sufficient for space management or clash checking purposes, whereas a highly detailed model is required for realistic visualisation. Alternatively, if the intention is to actively work on and manipulate the imported data, then an efficient and compact data structure is of paramount importance. Whatever the intention it is best to review the AVEVA E3D™ model. The review can range from a superficial visual check to a detailed scrutiny of the graphics and database hierarchy.
Reviewing the model allows:
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verification that the level of detail in the model is sufficient for their purposes
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identification of areas where the data structure can be improved
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identification of any problem areas.
If the model is generated through Import-STL with a loose tolerance setting, the model needs to be checked visually to locate errors due to failings in the STL export process. Identified problem areas can then be re-exported at a higher tolerance setting. If this does not correct the errors or there is geometry missing from the model, then there may be faults with the source model. In this case check that the source model has been healed prior to export.
If the model is visually acceptable but is slow to manipulate it should be checked to see if there are any areas where geometry can be remodelled more efficiently using fewer primitives. Viewing the model in wireline mode effectively displays areas that are densely packed with POHE primitives, for example, areas where it has not been possible for Import-STL to generate a more efficient representation based on primitives.
Typical STL Files and Resultant Models
The following images are examples of typical STL files and the models resulting from their import into MODEL with various option changes in Import-STL.
Note:
Model generated from a Pro/ENGINEER source file.
Model generated from a CATIA source file.
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