I think hardly have problem at J1, sure you make mistake. try watch videos from here. There is 40 videos there regarding AR3. J1 issue in first few video. The walk through very detail and clear.

https://youtu.be/Sjv8fxr5LJc

Thank you for your reply. I saw the videos as well. Also, I assembled this several times. However, it is locked when I screwed the (8) M6 X 14 socket head cap screws. I think the spindle dimensions, especially its height and J1 turret platform are not very accurate. I bought the aluminum kits from Annin Robotics. The only way is to put the (4) M4 x 10 set at a special distance from the bearing and not tighten (8) M6 X 14 socket head cap screws. However, in this way, the (8) M6 X 14 socket head cap screws can be rotated by hand. What is your opinion?

Can shoot video or photo put here?