To keep it portable I decided to ignore scale for size and distance between Sun, Moon and Earth but focus more on number of rotations of moon around earth in a year as well as alignment of Earth's axis with respect to Sun.
I used 2.2 mm MDF sheet for gears as well as frame which is is laser cut as required. MDF is cheaper than acrylic.
Software
- Geargenerator.com
- FreeCad
- Inkscape
With lots of inputs from existing Orrey built on the net, I used geargenerator.com to come up with the desired ratio 12.37 ( close to 12.368 ) for rotations of the Moon around Earth in one year.
I could manage it with just three gears 33, 24, 8 with module 2.5
To ensure that Earth as well as Moon rotate in anti-clockwise direction, additional idler gears were added.
Once number and sequence for the gears was finalized, I used FreeCad's Gear workbench to model gears with 1 mm hole in the center and exported SVG file for gears.
This SVG file was then opened in Inkscape, which was used to decided alignment for proper meshing.
Sharing assembly sequence here. Some captures are from different versions.
Assembly
Base plate is used for correct placement of gears during assembly. Four 3mm Screw/Nut holds the assembly in place. Pins are used as an axle for the gears. Middle plate separates the two layers of gear train and ensures smooth rotations.
Central first gear is fixed in place to the table with the help of two 3 mm screws placed 10 mm apart.
Circular disc enables assembly to freely move around the central first gear.
Last driven gear has equal number of teeth as that of first gear. This results in exact one rotation of the gear when one year is complete. This gear will drive Earth's axis orientation. Hexagonal pencil profile ensures that axis assembly, circular disc are locked in place with the gear movement without any additional tooling.
Pencil also acts as an axle for outer gear with 8 teeth, This gear has a diameter of 8 mm.
Axial tilt is achieved with the help of angular slot between to plates. Part of the cycle spoke is used to mount the Earth. Small wheel at the end rests on the lower disc which rotates the moon. Resulting friction rotates the Earth. But this interaction resulted in clockwise rotation of the Earth. To rotate it in anti-clockwise direction, I had to introduce pulley like mechanism .
Two discs were used to mount moon. One is attached to the 8 teeth gear while other one rests on the it.
Top disc enable us to adjust position of the Moon as needed. This can be fixed in place with cello tape to the bottom disc.
Circular ring is placed on the Earth's disc which will ensure axial tilt of the Moon's orbit as it rotates.
Sliding of the moon is achieved with the help of rectangular box assembly made from MDF itself.
Circular base disc is also free to rotate in case we need to adjust the position of Constellations as per our viewing angle.
Small outer ring will be used to label the English months on one side and 'Nakshtra' - subset of constellations on the other side.
All dials were designed in Inkscape. Clone tool was handy to work with angles and divisions.
This was 7th version which finally made it to the script.
Do suggest if there are any improvements needed in the current model.
I plan to make to more sturdier by replacing MDF with 3 mm Acrylic and make it more portable for lab use.
Limitations:
- Earth does not rotate around itself exactly 365 days in a year
- Tilt of Moon's Orbit with Earth's Orbit is not 5 degree but much more. This is only to show the up and down movement of the Moon as it rotates around the Earth.
- Size as well as distance of all the three objects is not as per scale.
- Moon is fixed in one place. It does not rotate around itself.
Artifacts
Important files used for this project are available for use at this link. This may help other tinkerers.
Note that only gears, disc and support frames are available in the design. You will have to explore axles and other fitments to keep everything in one place.
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