3D Printing for FIRST Teams (5 of 8)

3D Printing for FIRST Teams


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

3D Printing Work Flow

The 3D Printing Work Flow that we use is composed of 7 steps.  

Step 1: Design your part using PTC Creo.
Step 2: Export Model from PTC Creo in .stl format.
Step 3: Prepare Model and Generate g-code file (or equivalent) for the printer.
Step 4: Prepare and Level the 3D Printer Build Plate.
Step 5: Load printer filament.
Step 6: Print Model.
Step 7: Clean up printed model.

In this blog post, we will look more closely at Step 3.

3D Printing Step 3: Prepare Model and Generate g-code file (or equivalent) for the printer.

In Step 1 we introduced two model files demonstrating a 0.5” shaft and bore that are intended to be connected together. We will use these as an example to demonstrate the rest of the 3D Printing process. Since we are using a MakerBot Replicator 2 printer we will be using the MakerBot Desktop software to prepare the models and run the print process.
MakerBot Desktop Software
The Desktop presents a graphical representation of the Replicator 2 Build platform. We will add our models and position them on the Plate. The UI controls can be used to move the parts around the plate as well as scale them as needed. Note that the MakerBot software uses the Metric system. Most of our modeling is Creo is done using Imperial Units. This mismatch must be corrected and is easily done using the convenient inch->mm button on the Desktop Scale Menu.
MakerBot Desktop Software with bore example

Here is the completed print setup with several models ready for printing.
MakerBot Desktop Software with multiple parts for printing
When you are finished with placement and scaling of models it is time to adjust the print settings by opening the Settings Menu. The Settings controls the parameters of the MakerBot Slicer program. The slicer program takes the model information and converts it into a set instructions that will control the printer head and plate movements. This information is often sent to a 3D printer as a g-code file. G-code files are used for to convey machine tool movements in a wide variety of CNC tools. MakerBot uses its own format called a .x3g file. The MakerBot Print Settings (or Slicer) Menu is illustrated below. 
MakerBot Print Settings (or Slicer) Menu
First of all, the Raft and Supports settings should always be on. When Raft is enabled the printer will create a multi-layer plastic base under the model. This serves several useful purposes. It creates a stable base on which to build the model since the model plastic will always do a better adhering to a base of the same material. The 1st layer of the Raft is drawn to make it easier to separate the Raft from the build platform. Finally, although more stable the Raft is also easy to remove from the model making cleanup easier when the print is complete. The Supports are used to supports parts of the model that are not supported by other portions of the model during the build. You can think of Supports like scaffolding that is used to support construction of a bridge. While under construction the bridge is fragile and cannot support itself. Once complete it is self-supporting and the scaffolding/supports can be removed.

The Basic Resolution Controls regulate the print layer height. Most printers on the market today can support heights as low as 0.1mm resolution. The MakerBot Low, Standard and High resolutions offer layer heights of 0.3mm, 0.2mm and 0.1mm respectively. Smaller layer heights will result in better quality models and smoother surfaces. However, the print times can get substantially longer. For our project the print times at the different resolutions are as follows:

After some experimentation we standardized on using the Standard Resolution (0.2mm Layer Height) for our parts. This offers reasonable model accuracy versus print time trade-off.

The “Advanced Options Tab” contains material density controls that allow the user to control the strength of the final model. In addition to a Layer Height adjustment the number of Shells used in the print and the percentage infill can be controlled. The models are never printed to be a solid block. The printed part contains and outer shell and an internal honeycomb pattern that provide structural support. When a model is being printed the perimeter of every layer is drawn as a continuous line. This forms a shell around the model and is its outer layer. More shells give a stronger outer layer and are desirable for making parts stronger. The interior volume is filled with a honeycomb structure. The density of this is controlled by the infill parameter. This adjusts the percentage of the internal volume that is plastic. The following table illustrates the Default (Standard Resolution) and Recommended Settings from printing parts intended for FRC Robots.

In general, structural parts should use the recommended settings. Cosmetic parts can get away with the Default settings. Note that the recommended setting will increase the print time on our example parts to 3Hr, 34mins. When finished adjusting the Settings simply save them or create a profile to save them for future re-use. 

At this point are ready to print. There are two ways to do this. You can print directly to the printer or you can save to a print file. The print file can be used on an SD card in the printer without any computer connection. This is some times more convenient. It can also be more reliable, we have had several long (>12Hr) print jobs crash because the computer goes to sleep or is rebooted by a Windows Update. If you do decide to print directly from a computer you will need to make sure that your printer is setup at this point (complete Steps 4 and 5 before going any further). Hit the PRINT button when ready. The software will then prepare the model for export.
MakerBot Desktop Software Export Window
The export window includes some useful estimates of Print Time and Filament usage. We will now use the Print Preview to examine the print process. This is a convenient feature that allows the user to see how the model will be printed layer by layer and to visualize the tool path movement. Note that you will need a reasonably power CPU/GPU combination to use this interactively. Most Laptops will struggle with medium to large size prints. 

The Print Preview Window is illustrated below.
MakerBot Desktop Software Print Preview Window
The Layer Number can be adjusted to see any part of the print process.
MakerBot Desktop Software Print Preview Window with Layer 49 selected
Finally, we can also see the print head travel moves between the different parts of the print.
MakerBot Desktop Software Print Preview Window with Travel Moves displayed
During a Travel Move the plastic extrusion stops to move the print head between different sections of the print. However, there will be residual plastic stringers left behind by the moves that will need to be removed later.

Once you are done with the Toolpath Visualization tool, close it and hit the Export Now button on the Export Window.  You will be prompted for a file name. After that the design will be verified and saved to the selected file. You can now save the Desktop project as a .thing file and close MakerBot Desktop.

CAD Files

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_section5CAD.zip.  The CAD files include
  • 0.5” Shaft / Bore MakerWare .thing File: hub_shaft_example.thing 
  • 0.5” Shaft / Bore MakerWare ..x3g File: hub_shaft_example.x3g



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