In a design that I need to prototype, I have a coaxial, rotational joint between 2 cylinders connected to another coaxial rotational joint between 2 cylinders by a hinge. I have attached the individual part files as well as their assembly where you can use your mouse cursor to check how they move. Click here for the SolidWorks parts and assembly files
Could you tell me an alternative method to get this complicated joint working? I need to 3D print them. I plan to use ABS for the 3D printing.
Thanks for your patience :)
EDIT: Lubrication: I plan to use grease to lubricate the sliding surfaces.
Clearances: I printed them without clearances initially (this is my first ever 3D print and I wanted to know how accurate it would be). I then used Dremel and filing tools to file away about 0.5 mm of both the surfaces. It should have been enough for mating the parts. The gap between the edges of the outer shell while trying to close it around is 3-4 mm which doesn't suggest 3D printing inaccuracy. I suspect that the outer tube isn't circular enough in cross section. It meets the inner tube at the the two ends of the semicircle and not inside its belly (you can see it in the picture). I have already tried the obvious, which is removing material from the surfaces wherever the parts touch. The model seems to be too warped for any success. The parts were printed upwards from the bottom, so it is not the warping due to gravity.
Have you considered to use a service to have the part printed? You can print almost anything in one "pass" using selective laser sintering, typically done in nylon. I am currently un-boxing an SLS printer, so I cannot currently offer to print it, but the capabilities of SLS are substantial for parts of this nature. I've read of entire planetary gearboxes being printed, fully assembled, with appropriate clearances for operation, once removed and "dusted," or cleaned of residual powder.
The printer I hope to be assembling soon has 80 micron powder, so the resolution is quite high, although I forget at the moment the x/y/z figures.
Do you think such a method would suit your purposes?
because FDM printing by nature causes "oozing" especially in the x/y directions, I would not be surprised to learn that your outer piece has a smaller inside diameter than your design has specified. You may have to make adjustments to the design to compensate. Those adjustments would be specific to a particular printer/filament combination as well. Calibration prints would make the task slightly easier.
Depending on your tolerances and how many you need - and how valuable your time is - you can 3D print in ABS with a regular FDM printer, then clear out support material and do an Acetone vapor treatment or vapor wash. It will be smooth enough for most prototypes.
Depending on the requirements for your joint, I would consider making a design that utilizes ball bearings. With ball bearings, your join will not wear down as quickly, and also move smoothly.
The topic of how to achieve dimensional accuracy of FDM printers has already been discussed in this question. You might want have a look at Ryan Carlyle's answer for a specific routine on how to calibrate a Cartesian printer.
From the question linked, a summarized approach for handling dimensional inaccuracy could be:
Generally speaking, the printer settings should handle filament independent dimensional accuracy, while the slicer should be used to account for different characteristics in each filament type. Correcting size through your CAD model is ill-advised, since you should be able to use the same model for multiple printers.
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