Yep, the rod is metal and its pitch is 0.8. Originally it is a M5 straight rod but I bended by hand to perfectly match the circumference of a 20cm radius circle (well almost half of it). The big gear is designed to have on its hub an M5 nut and that's how it moves the camera and the upper part of the "hinge". The rod has to be bended that way because if I leave it straight the whole project will have "tangent error" and it will not longer track the stars for a long period of time.
I'll try to do what @gilshultz suggests and see how it works, for how long and if I can decrease the amps delivered to the stepper motor to make it work for longer but still moving the camera.
Once I put everything together I'll share the results in this thread 