Are You Sure You’re Making Good Parts?

One of the potentially largest issues with additive fabrication (AF) has nothing to do with the machines or processes, but with the data that directs and controls the machines that produce these parts.  This data comes from the computer-aided design (CAD) system or from other software or points of input. In order to make this information useable for the AF machine, an easy to use set of data needs to be created—the STL file.  This STL file is now a major influencing factor for part quality.

An STL file is a triangulated surface file, or a simplified representation of the original CAD geometry.  It does not contain any of the features or design intent originally used to create the model.  These triangles may be defined in either ASCII or Binary file formats.  Files in ASCII format define the points, or vertices, of the triangles as coordinates in a readable text file. These are generally converted to binary files as an ASCII file is many times larger than binary, making it more difficult to process and to e-mail. STL files do not have any specified units and the recipient must guess or inquire about the actual size of the model. With 34 bytes of the file used for the header information and 50 bytes allocated for each triangle, it is likely that a good file will end with 34 or 84 bytes. 

Creating STL Files

When creating STL files from CAD software, there are a few tests that can be done to help ensure the data is being accurately represented as designed.  One of the first steps is to suppress or hide all reference data that should not be in the model.  This includes reference planes or surfaces, construction geometry and/or external constraint models.  With only the model to be built visible, the surface area and volume of the geometry should be checked.  If the surface area or volume is reported as negative, this would suggest that some surfaces or features do not blend with the rest of the geometry correctly. Any open gap in an STL file will require the equipment to “interpret” how to close the model, which may cause the part to not match the intended design.  Providing a ‘water tight’ file is the best way to ensure your end product matches your design.  An assembly can be exported as a single STL with all instances locked in their desired location or as a collection of individual instances.  If exported individually, it is important to make sure that the model is not translated to the software origins unless desired. Many STL file issues arise from “non-standard CAD practices,” or ignoring warnings or errors during the solid modeling phase.

The Origins of STL

The most common origin of STL files are from CAD software, but this is not the only option.  Reverse engineering systems generate Point Cloud data that can easily be converted into STL files and used to build models of products that have no electronic data. Many emerging medical applications obtain their data from Magnetic Resonance Imaging (MRI) or Computer Tomography (CT) scans.  These scans may be used to generate STL files for the purposes of building surgery pre-planning models or patient specific implants.  Considering that the coordinates of the vertices represent the triangles, it is also possible that the STL may be generated by a mathematical equation.

When generating an STL file, there are settings that affect the accuracy of representation to the CAD file.  These are usually shown as the Chord Height and the Angular Control in the file output options.  The chordal deviation is the distance from the chord that represents part of a curved surface and the actual geometry. The angular control is usually the number of degrees a circular section is split into. A setting of 3 degrees will split a circle into 120 sections. A good general setting is a Chord Height of 0.001" and an angular control of 2.5 degrees.

Selecting a Resolution

Some software only references the STL export options as a “resolution” with the choice of coarse, fine and/or custom.  While very low settings will produce the most accurate representation of the CAD file it will also produce the largest number of triangles and thus, the largest file size. Making sure the setting is tight enough to preserve all of the small features but not so tight that it generates a file that becomes unmanageable is very important. In reality, the best way to evaluate the STL file is by viewing it once it is has been created.  Many free viewers are available and can quickly illustrate potential problems with a file even before they are sent out for quotation.

Is it Accurate?

Once an STL file has been generated form the CAD software, it is advised that the file is opened and viewed to ensure it is an accurate representation of the desired geometry. This can be done with a collection of STL viewing software packages. It may also be possible to do so with the native CAD software.  Some of these viewers allow for viewing bad triangles, unshared edges or inverted normal’s.  If any of these complications are observed it is best to fix the CAD geometry, if possible, so the file repair operation does not perform an incorrect repair. Here is a partial list of free STL viewing software packages:

• Magics—www.materialise.com/materialise/view/en/1248872MiniMagics.html

• Solidview—www.solidview.com

• Stlviewer—www.codeproject.com/KB/openGL/stlviewer.aspx

• Geometric e Drawings—http://edrawings.geometricglobal.com/index.aspx?idp=95&idc=109gclid=CPjB3_7aoZUCFQMIswodYSu_jQ

• OpenRP RP Viewer—www.openrp.com

• Deskartes View Expert—www.deskartes.com

• Simpleware ScanIP www.simpleware.com/software/scanip.php

Making sure the parts produced in an AF and manufacturing process are truly representative of what is designed and desired must begin with a good STL file. Providing good, quality files to vendors or machine operators saves considerable time in the handling and processing of additive manufactured parts. Considering that the volume of the part impacts most vendor quotations, good STL files will generate more accurate quotations with less guesswork. They will also reduce the possibility of receiving something that does not closely represent the desired geometry.

Carl K. Dekker is President of Met-L-Flo Inc, a service bureau in the Chicago land area.  He is an active proponent of education of additive technologies. Carl can be reached at (630) 409-9860.

Tim Gornet is the Manager of RPC Operations with the University of Louisville’s Rapid Prototyping Center. His current area of concentration is in Direct Digital Manufacturing of end use parts via additive manufacturing in plastics and metals. Tim may be reached at (502) 852-0714.