Introduction: Tiny Planet: a 3D Printed Mechanical Sculpture
I created this small mechanical sculpture using almost entirely 3D printed parts. The axles are steel, and there is some metal hardware as well: shaft collars to hold the axles/gears in place, a couple bronze flanged bearings and some ball bearings to make the central axle and planetary gear parts move smoothly. I decided not to 3D print the entire machine because I wanted the axles to be small (1/8" diameter), and 3D printed parts rubbing against each other produce a lot of friction, and I wanted this machine to move smoothly with minimal force.
I began design with the "petal gears" that surround the machine. All CAD work was done in Solidworks. The Pitch Diameter of the gears was generated simply by arbitrarily picking a circle that would pass around the perimeter of the petal gear train, at the center of each gear. This number was 6.803", and dividing that into 8 sections (for 8 gears), giving me a 2.603" Pitch Diameter.
I tried several different gear tooth profiles, including the classic involute, but decided on this more toy-like profile because it functions and looks more fun. You can see in the image that the profile is created using circles that are tangent to one another, as well as tangent to either the addendum or dedendum circles of the gear, centered along radial lines 20 degrees apart (9 teeth + 9 valleys = 18 lines: 360/18=20 degrees). The "valley" circles are slightly larger than the teeth circles, to add tolerance.
In order to create the domed gear shape, I lofted the flat profiles between two planes outside and within the pitch circle (You can see the reference lines for these planes in the image), and then trimming away everything but the dome shape.
I created a trial chassis just to see how the gears would work with each other. The fact that they are wrapped around a sphere, at a silly angle no less, makes them very difficult to model with anything but a Trial and Error approach.
For the planetary gear train, as well as the bevel gears, I downloaded part files from SDP/SI, and then altered them in Solidworks. I enlarged the bevel gears and turned one into the chassis. I added hubs with flats for the drive gears (the Sun gear, the two bevel gears, and two of the petal gears).
I included void spaces to pop ball bearings into the chassis to smooth the planetary motion, as well as for the flanged bushings where the central axle rotates. Slip fit parts were modeled with about 0.007" clearance (a .257" hole for a 1/4" axle), while the press-fit holes in the chassis (for the planet gears), are 0.002" undersized.
Step 1: Printing
The entire assembly printed in about 18 hours on an Objet Connex 500. A couple hours of cleaning and my parts were ready for assembly.
Step 2: Test Assembly
All holes were reamed to make sure they had the appropriate clearance.
The flanged bushings were added to the chassis.
The planet gears are placed using a 1/4" steel dome-head rivet and a washer for smooth movement.
I did not add the ball bearings yet, as I wanted to be able to remove all hardware for finishing, and they are non-removable.
Step 3: Finishing and Final Assembly
I used an abrasive hand-pad to knock off sharp edges and smooth all exposed surfaces. Then I added the ball bearings and replaced all the hardware.
I used a shaft collar fastened to the central axle to prevent pressure from use from pushing the axle through the bottom of the machine. However, nothing prevents the axle from being pulled up and out of the machine. Future revisions might include space for a retaining clip underneath the machine to prevent this.
I used different shaft collars to fasten the bevel gears for final assembly, but as they were too thick, I ground them down a bit.
I glued some thin silicone sheet to the bottom of the chassis to prevent it from sliding around on the table. For future revisions I will try to have this feature part of the print (a dual material print).
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