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I found my old tracker. Picture attached.
I attached my ballhead to the bolt on the top plate closest to the hinge. (the closer the camera is to the hinge the sturdier it will be and it makes it easier for the stepper to drive the weight up as it follows the sky.)
This was a 3/8 inch bolt in my case as my ball head could take either a 1/4 inch or 3/8.
The threaded rod was bent by printing out the required radius on A4 paper and then bending the rod by hand. I believe a curved rod will stay tracking better for longer than a straight rod.
I used a 1 mtr length and grabbed it by both ends and bent them in to get the curve. If you grab a short length to bend you will just make a a very sharp odd shaped curve instead of a nice round even curve. the middle of the 1 mtr length will have a decent curve (You only need 200-300mm at most in the centre). Everything else either side will still be to straight to be of use.
The gear on the threaded rod was just a kit from Jaycar electronics. Was like 10 bucks for about 6 different gears which all fit on stepper motors. So I used a small spur gear on the stepper (which is missing in the photo).
The larger gear was drilled to slightly under the diameter of the nut to suit your threaded rod. Then I just hammered the nut into place in the gear. Its stuck in there quite nicely just like that. (I put a thrust bearing under this gear to reduce friction against the base board. Probably not required but for an extra dollar or 2 it seemed worthwhile).
The only important thing about the gears is the ratio between them as you need that to calculate the RPM required from the stepper motor. and the larger the ratio the more torque can be applied to the drive rod. (I tested my setup and it was pushing around 2-3kg easily when weight was placed directly over the drive rod. so 2-3kg at the camera mount point was even easier). even though my camera setup weighed somewhere around 1kg at the time.
Hopefully this was helpful. If you need a hand with coding an arduino to drive the stepper, I'm no expert but can bumble my way through it enough to help.
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