There comes a time when your Direct Digital Manufacturing (DDM) project involves specific functionality, in addition to static end use parts. One function may involve the use of ball bearings captured in FDM parts.
Traditionally, ball bearings sound and feel smooth, and when that sound or feel are missing, we perceive the part is damaged. When ball bearings are used on FDM or DDM parts, the raster produced from printing will give a rough finish and the perception the assembly is damaged.
The real challenge is to make an assembly housing of ball bearings in FDM sound and feel as smooth as though they were machined, and to accomplish this without secondary finish operations.
The floor, where the bearing will ride, has rasters that are inherent of what I call traditional printing, or commonly known as Green Flag.
The parts above were digitally manufactured on a Fortus 380mc in ASA, and the STLs were processed in Insight software, which allows for advanced editing features, such as Custom Groups. Custom Groups provide powerful capabilities that help transform the part’s floor rasters into contours, which help make a smooth surface for the ball bearings. Note: Contours are printed material that follow the outline of the region, while rasters fill the interior from edge to edge.
Custom Groups are accessed from Toolpaths on the main menu
When selected, the Custom Group menu appears on the right side.
Click New and rename Group1 to a name which is meaningful to you.
Under Contour Parameters, I changed the Contour Style from the default “Single Contour” to “Multiple contours.” I set the contour width to the minimum that my .010 slice parameter would allow. Finally, I set the number of contours to 30.
Next, select the layer, or a group of layers, and click Add. Your layer(s) will turn magenta and have no toolpath. Click the Z icon on the bottom right to preview toolpath.
As you can see below, there are some problems caused by the design, which uses a flat internal face to keep the base from spinning. There is an air gap right in the middle of the floor where the bearing will ride which will cause a slight wobble and a single raster that will act as brake every 90 degrees.
I attempted numerous combinations of the settings for contour width and number of contours to correct the issues. Sticking to my guns to have the smallest contour width (believing this would provide the smoothest floor for ball bearings), I needed to remove this air gap .
Another factor in overcoming the issues shown above is adhering to traditional wall thickness. Remember that this is a DDM part and the rule of even wall thickness does not apply. Therefore, the best solution to solve this problem would be to make revisions on the CAD model rather than edit in Insight.
After making the revision in SOLIDWORKS, the layer has a solid raster and ready to apply a new Custom Group. I will keep the desired contour width and increase the number of contours.
The results of these revisions are now satisfactory. The contours are at a minimum for my ten thousandths (.010) extruder tip and I will have smoother and quieter ball bearing rotation. The center has rasters which are okay, because the boss is going to start there and cover it.
While not as perfect as machined or cast parts, our processed ball bearing floor turned out successful. Sound is greatly improved; the rough sound produced from the original rasters should satisfy customer concerns of a damaged assembly. The feel is smoother; the ball bearings will encounter much less resistance. When weight is added, sound, feel and function give this DDM project the green light.
Without Custom Groups, this part of the product could never reach its full potential. FDM parts would remain in the realm of “Traditional” printing or processing. One of the benefits of Insight Software is that we don’t have to settle. We can dare to do better. It isn’t perfect, but it is a process we can create and control.
The field of DDM has great potential with huge reward. There is much for us to learn and improve upon. Still to come: I plan to finish these parts and will post the results when complete.