Unfortunately, I don't know the solution to your problem, but I found the site where a guy is sharing an original Robot Arm design https://forum.arduino.cc/t/3d-printed-robot-arm/409174. I think he can help you. It’s 3D printed and all parts (STL are also 3d printed on the machine for little details like this one) and an assembly manual can be found on github. It’s the third iteration of the design, and it looks really nice.