For my North Bearing, I glued two layers of 3/4 plywood together. I traced an arc of 17.5", and made a rough cut with a jigsaw. I used a hinge arrangement, with the hinge at the centre of the arc, as a jig to run the curved surface through my router to make a clean cut. I used the same jig to pass the surface past a drum sander on a drill press to smooth out the bumps.
Finally, I cut off the arc with a table saw set at 45 degrees and attached it to my platform with screws and glue. Others have suggested mounting the rough-cut sector to the platform first, and then cutting the arc - this requires a more creative and complicated jig, but does a better job of ensuring the arc actually matches the axis of the finished platform (mine is off slightly).
For my axial south bearing, I used a 4" lazy-Suzan bearing from Lee Valley, mounted on a 45 degree wedge.
The trickest part of the entire project was the roller blocks for the North bearing. I figued out where I wanted them to go, and how big the rollers were (I used cheap rollerblade wheels), and then sort of worked it out diagramatically. Sort of like this:
It still required fine tuning to get it close to right, though it is still not perfect.
The two main methods of driving the platform are through direct drive or tangent drive. In direct drive, a motor turns one of the wheels directly. This has the advantage of ensuring the rate of rotation is constant, but requires significant gearing down. In a tangetnt drive, the motor drives a threaded rod, which pushes a shuttle along, which in turn pushes the platform. Though it requires more parts, I chose the tangent drive as being simpler to construct given the tools and equipment available.
I used a 1/2" threaded rod mounted in 1/2" roller bearings, using a threaded rod connector as the shuttle. I bolted a nailing plate to the connector (using bent threaded rod for U-bolts), and cut a slit in the plate for a pin mounted in the front of the platform. The motor and gear assembly combes from a toy crane. With the gearing I have currently, the platform runs somewhat faster than sidereal rate (about 3-4x faster), so for visual use it requires turning it on when the object begins to drift out of view.
The next step is to get it to run at sidereal rate. I experimented with a DC motor speed controller (pulse width modulation type), but the torque loss with my current motor was too great. I will be experimenting with stepper motors when I have the chance.
Total outlay: ~$25
Now that is budget astronomy!
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