STL Files and Reverse Engineering

STL Files and Reverse Engineering

Posted on mandag, januar 26, 2009 @ 14:27:33 by jerry

This describes why using STL files may not be the feasible format to use when dealing with reverse engineering.

STL Files and Reverse Engineering

Gerald Francis – CIM Solutions & Networking
01/14/09
 
At the present time there is no fool proof way to use STL (stereolithography) files and import them into any CAD system to recreate surfaces for reverse engineering. The STL format consists of triangulated data (surfaces or sheet bodies on some CAD systems) which are combined to form the surface or solid information. Most STL information is obtained through scanning or rapid prototyping.
 
STL files are one of the worst file formats that one can work with for reverse engineering.  STL was written by 3D Systems nearly 20 years ago to work with Stereolithography machines, and wasn’t intended to be used for reverse engineering by CAD/CAM companies at that time.
 
One reason the STL format has little success is because most files come in ASCII, or man readable information, so the file size may be extremely large to read into most CAD systems. These file sizes take up a lot of system resources that may be required by the CAD system. And obviously, the more triangles included in the STL file, the larger the file size becomes. Binary representations may be a more suitable way to use this format.
 
This also adds to the time for CAD systems to read in larger STL files which may contain thousands of triangles. On a file which contained 47084 sheet bodies (triangles), the file size was 144MB.  Once these sheet bodies were stitched together to form a solid, the file ended up being 250MB. The whole process took nearly 8 hours to convert this STL file into surfaces, and to stitch these surfaces into a solid.  My understanding is the size of the solid is determined from the mathematics that is used from these triangulated sheet bodies.  Basically, this STL information does not create a smooth surface file as one may wish the final output to be.
 
Another disadvantage of using STL files is that most CAD systems can not distinguish what triangle should belong to what surface. A simple example of this would be a solid cube with all the corners filleted. Some of the triangulated information could possibly exist on both a flat surface of the cube, or the fillet of the cube, since all this triangulated data is flat.
 
 
I’ve been in the CAD/CAM field for 22 years and have worked with companies trying to convert STL files over the period of these years.  And none have had any satisfactory results.  The question I ask people using these STL files, “Why doesn’t the company supplying the STL file able to convert the information to surface data or solid data themselves, which can be viably usable by the CAD/CAM industry?”  The answer is that they can’t either.  No matter what you have heard, no company supplying STL data has produced any program capable of supplying usable surface data from these scanned images.  Point Cloud information is available, but this only produces surfaces between the points also, and may not be what the end result is desired.
 
There is only one company that has ever come close to modifying STL data to be usable as surface information and their package runs $10,000-$20,000, and you still have to spend hours manipulating the geometry into a usable format for other systems to use.  You still need to determine which triangles should belong on what surface – what makes up a planar surface, what makes a filleted surface, what makes a hole?  There may be techniques to be used in this package, but not all may work.
 
One way that some CAD systems can read STL files rapidly is to create a Shell entity which contains no surface data - basically, it shows as nothing but a pretty picture that you can’t do anything with.
 
This little blurb came from http://www.stereolithography.com/stlformat.php

STL (stereolithography) Files

The .STL file format has become the Rapid Prototyping industry's defacto standard data transmission format and is the format required to interact with stereolithography machines. This format approximates the surfaces of a solid model with triangles and lacks engineering data! The STL format was designed to give just the amount of data in the form of meshes to operate the Stereolithography machines but not actual engineering data. This format is very difficult to utilize for any purposes other that the 3D building we do in these machine, not good fro reverse engineering.