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Export robot parts in .stl format

Authors: Kari Karwedsky, PTC, and Colm Prendergast, Mentor for FIRST Robotics Competition Team 1965

3D Printing Step 2: Export Model from PTC Creo in .stl format.

Once your model is completed it needs to be exported from PTC Creo before it can be printed. There are several file formats that can be used for this and PTC Creo supports a wide variety of industry standard file formats. You should check the file formats supported by your printer software before exporting your model. We are using the STL format. STL (Stereo Lithography) is a common file format that is widely used for rapid prototyping and computer-aided manufacturing. STL files describe the surface geometry of a three-dimensional object (http://en.wikipedia.org/wiki/STL_%28file_format%29). STL is supported by the MakerBot printer software (MakerWare).
STL saves the model as a set of three dimensional Cartesian co-ordinates. The accuracy of the model representation will depend on the number of points used. This is important when printing curves as the 3D printer will tend to connect the points in the STL file with lines during the g-code generation for the printer (Step 4). The image below illustrates this.

CAD Model (red curves) Vs STL Mode (black vertices/lines) (Source:http://en.wikipedia.org/wiki/File:The_differences_between_CAD_and_STL_Models.svg) 

From this image you can see that the more points the STL file contains the more accurately the circle will be represented. PTC Creo allows the designer to control the accuracy of the exported model. However, there is a trade-off involved. If the exported model is too dense the exported STL file will be very large and the 3D printer software may have trouble converting the model to g-code for the printer. It is important when exporting the model that there are enough points to accurately represent the model features without creating an overly complex representation.
Once your model is complete and saved, simply use the Save As from in PTC Creo to access the different supported export file formats.

File Save As Menu in PTC Creo

Select .stl from the File Type list and hit the OK button. This will bring up the Export STL form.

Export STL form in PTC Creo

The Export STL Deviation control parameters defined the exported model accuracy and are populate with model specific defaults every time the Form is activated. To view the STL vertices simply hit the Apply Button. 

View of the STL vertices from the Export STL form in PTC Creo

Using the Default Deviation Control Parameters results in a fairly course representation of the model. In this case the circular shaft and the rounded edges are not represented particularly well. The three deviation controls are Chord Height, Angle Control and Step Size. Please refer to PTC Creo Help for a description of what these controls do exactly. After lots of experimenting we found that the following approach worked best:

  1. Divide the Default Chord Height by 10.
  2. Divide the Default Angle Control by 10.
  3. Enable and Divide the Default Step Size by 2.

The Step Size was observed to have a significant impact on how well the MakerBot MakerWare software would handle the exported model. If the Step Size was substantially smaller than this recommendation MakerWare would typically hang for a long time and eventually crash. After applying these setting the following result is obtained.

View of the STL vertices from the Export STL form in PTC Creo with adjusted parameters

The STL export vertices can be adjusted and the model image updated by hitting the Apply button. Once you are happy with your result simply hit OK.At this point you are finished with using PTC Creo.

CAD Files

The above example used our shaft example part. CAD Files for this section can be found on the accompanying PTC Community document (http://communities.ptc.com/docs/DOC-6545) in the attachment, 3D_printing_blog_section4CAD.zip.  The CAD files include