Single Print Differential Gearbox – Second Project Update
Addressal of First Print Problems
The first design iteration and print was unsuccessful – as the gears printed inside of the box structure became attached to the walls. The original clearance called for 0.5 mm between the outside of the wall and the gear stop. This dimension was halved once the model had to be reduced by 50%. The resolution of the printer didn’t allow for this 0.25 mm gap to be realized as shown in Figure 1 below. The 1 mm gap between the back of the gear and inside of the wall that was reduced to 0.5 mm with the scale down allowed for a separation between the two parts – indicating a tolerance that could be used for the next design iteration. Although the gears were not able to rotate, it was still apparent the interface between each gear face was not going to be close enough to transmit torque.
Figure 1 – First design FFF print. The gear could not freely rotate due to the gap specification between the shaft stop and the wall being too small (0.25 mm).
FFF Printer Tolerance Test Design
After the problems due to the first test, it was determined a new print would be created to test how close the print bead could be placed next to another bead while maintaining separation. The first print gave the idea that 0.25 mm separation will most likely form together while specifying a 0.5 mm separation should work. Two test blocks were created to view the effect different gap sizes would have on the separation of print beads within layers. Each layer was spaced between 0.1 – 1.0 mm with 0.1 mm increments. Two test blocks were created so the effects of horizontal and vertical spacing could be studied. Both test blocks are shown below in Figure 2.
Figure 2 – Horizontal (left) and vertical (right) tolerance test pieces. Each test piece had layer spacings that began at 0.1 mm and increased by 0.1 mm until a max thickness of 1.0 mm.
Second Gearbox Design
A new design was created that would incorporate the first print information on clearances – 0.5 mm between each interface. Although the parts should be as close as possible without becoming stuck together, a compromise was made to use the 0.5 mm that was shown to work previously. Instead of just having the simple box with four meshed gears as shown in the first print, the second print incorporated a makeshift drive (or ring) gear. Although a gear was not used to connect the two side gear to reduce print material and time, the drive “gear” is still able to connect the axles both side gears are mounted on, allowing them to rotate together and translate the motion to the driven gears. The assembly was designed to allow for the gears to act like they would in a normal car differential, just instead of using a driveshaft to translate the original motion a user would just rotate the drive gear connector. The actual gear design was also changed to a gear that had less teeth. The reduced amount of teeth led to larger teeth which made alignment in the model easier. The box structure was also reduced to be only a square bracket that held all four gears in place. This material reduction allowed print time and material use to drastically decrease. The second design is shown in Figure 3 below.
Figure 3 – Second design FFF print. The new iteration used less material to house the main gears in the middle which allowed for more features – such as the drive gear – to be added.
Second 3D Print Slicing
An Ultimaker FFF printer was used with PVA and PLA material to print the gearbox and both tolerance test pieces in one go. The gearbox geometry required support structures, and PVA allowed for easy removal of it in a structure that is expected to move and rotate after being post-processed. After importing each geometry in Cura, the print time with 0.2 mm layer thickness totaled 5 hours and 15 minutes for all three parts. The print time allotted for the second project part was 12 hours, so layer thickness was halved to 0.1 mm. The smaller layer thickness allowed for better layer resolution that would hopefully lead to less probability for interfaces between subcomponents to connect with each other. The first part was printed using a 0.2 mm layer thickness which may have contributed to the closure of part gaps. The final print time for all three parts was 11 hours and 35 minutes as shown below in Figure 4.
Figure 4 – Cura slice of all three parts. Since only one iteration would be conducted, all three parts were printed at the same time and kept under 12 hours. Print time could have been save if more parts and tests were desired.
Second 3D Print
When the second design was first printed, the part failed. The PLA tolerance test pieces are shown below in Figure 5. A reprint was conducted and the result for the PVA/PLA gearbox is shown in Figure 6 below. The print was conducted late so the PVA was not able to fully dissolve by the time this update was written. The results of the tolerance tests show that minimal full separation occurs at around 0.5 mm, which confirms the findings of the first print (note: this print was conducted with a layer height of 0.1 mm whereas the first gearbox was printed using 0.2 mm layer thickness). Any smaller gaps were filled and connected due to their close proximity to another polymer bead.
Figure 5 – Horizontal (left) and vertical (right) tolerance prints. The horizontal print first shows edge separation at 0.5 mm, which was the clearance used for the gearbox.
Figure 6 – PLA/PVA print of the second gearbox design. There was not enough time to fully dissolve the PVA due to reprint issues.
Second 3D Print Conclusion
Since the print was done so late after having to be reprinted, no conclusion could be drawn at the moment of the effectiveness of the second design. After the part sits overnight in water, it can be tested to see if it freely moves and can transmit torque. Further research would most likely be conducted on the interfacing between components that are meant to freely move in the assembly. Instead of a connecting bar that holds the two side gears together, a real drive gear could be created that rotated based on a torque input from a driveshaft.