STANDARD TRIANGULATION LANGUAGE (STL) FILE

STANDARD TRIANGULATION LANGUAGE (STL) FILEByD.M.SANTHOSHSARANG13MECD13STL FILESThe STL file has become the Rapid prototyping industrys defacto standard data transmission format and is the format required to interact with quick partsThis format approximates the surface of a solid model triangles.For simple model such as cube, its surface can be approximated in to 12 triangle.Representations simple modelsSimple modelSTL model

STL Model of curved shape

Polygons Divide into TrianglesAn n sided polygon is represented as n-2 triangles

4 sides 25 sides 38 sides 6STL FILE FORMATThere are two formats to the STL file. ASCII format Binary format The size of the ASCII STL file is larger than that of the binary format but is human readable.In a STL file, triangular facets are described by a set of X, Y and Z coordinates for each of the three vertices and a unit normal vector with X, Y and Z to indicate which side of facet is an object.stl File FormatCreate file in any text editorWrite a start line, an end line, and a group of 7 lines for each triangle.Triangles can be listed in any order.solid

endsolid

stl File FormatFor each triangle give 7 lines:facet normal 0 0 0 outer loop vertex X Y Z vertex X Y Z vertex X Y Z endloop endfacetOnly the X Y Z parts need to change for each triangle(Some programs need the normal)Easy to copy and paste in editorA sample STL file

Procedure to create STL fileFile save as .stlSTL export dialog box will appear

STL file problemsSeveral problems plague in STL files and they are due to the nature of STL files as they contain no topological data.Gaps (cracks, holes, punctures) that is, missing facets.Degenerate facets (where all its edges are collinear).Overlapping facets.Non-manifold topology conditions.

STL file problemsThe above stated problems are due to the difficulties encountered in tessellating trimmed surfaces, surface intersections and controlling numerical errors.This inability of the commercial tessellation algorithm to generate valid facet model tessellations makes,it necessary to perform model validity checks before the tessellated model is sent to the Rapid Prototyping equipment for manufacturing.

Missing Facets or Gaps

Tessellation of surfaces with large curvature can result in errors at the intersections between such surfaces, leaving gaps or holes along edges of the part model.

Degenerate Facets

Shell punctures created by unequal tessellation of two adjacent surface patches along their common mating curveOverlapping FacetsOverlapping facets may be generated due to numerical round-off errors occurring during tessellation.

The vertices are represented in 3D space as floating point numbers instead of integers.

Thus the numerical round-off can cause facets to overlap if tolerances are set too liberally.

Non-manifold ConditionsThere are three types of non-manifold conditions, namely:1. A non-manifold edge.2. A non-manifold point.3. A non-manifold face. A non-manifold edge A valid model would be one whose facets have only an adjacent facet each, that is one edge is shared by two facets only.

A non-manifold edge whereby four facets share a common edge after tessellation.

Non-manifold ConditionsA non-manifold faceA non-manifold point

STL file problemsAll problems that have been mentioned previously are difficult for most slicing algorithms to handle and they do cause fabrication problems for RP processes which essentially require valid tessellated solids as input.Moreover, these problems arise because tessellation is a first-order approximation of more complex geometric entities.Thus such problems have become almost inevitable as long as the representation of the solid model is done using the STL format which inherently has these limitations.

A Valid ModelA tessellated model is said to be valid if there are no missing facets, degenerate facets, overlapping facets or any other abnormalities.When a valid tessellated model is used as an input, it will first be sliced into 2D layers.Each layer would then be converted into unidirectional (or 1D) scan lines for the laser or other RP techniques to commence building the model.

An Invalid ModelA solid model is tessellated non-robustly and results in a gap.If this error is not corrected and the model is subsequently sliced.In preparation for it to the missing facet in the geometrical model would cause the system to have no predefined stopping boundary on the particular slice.Thus the building process would continue right to the physical limit of the RP machine, creating a stray physical solid line and ruining the part being produced.

STL FILE REPAIRIn existing RP-system when a punctured shell is encountered, it requires skilled person to manually repair the error.Hence it would negating the advantages of rapid prototyping as the cost and the time taken might be increases.The main problem of repairing the invalid tessellated model is that some necessary designers intended datum may have been lost in the overall process.STL FILE REPAIRGeneric algorithm can be said to have the ability to make an inference from the information contained in the STL file so that the following two conditions can be ensured:(1) The orientation of the generated facet is correct and compatible with the rest of the model.(2) The basis for the working of the algorithm is due to the fact that in a valid tessellated model, there must only be two facets sharing single edge.ANY QUERIES?