Servo + Cycloidal or Harmonic gears are usually the way to go, but to get them backlash-free is hard (or expensive, if you're buying). Once you got that down challenges include:
* How rigid are the links between my joints? Plastic will wobble, metal is better
* How heavy is my arm and how does that limit its movement? If you go for stiff metal castings, you add weight you need to move. The lever arm relative to the base can get really long
* Motors are heavy! Ideally you can mount them towards the base, but then you need drive shafts or belts, which again add flex. (See KUKA arms which have motors 4, 5 and 6 on the elbow often)
* How much payload do you need to move? 5kg is already challenging in ~1m arms and if you need to move it fast the problem gets even bigger.
* Where do you run your cables? Internal is tricky to build, external can get you tangled.
And so on. When approaching this you get a totally new appreciation for biological arms which arae insane in most aspects except for repeatability. And on the software side you can enjoy inverse kinematics :)
* How rigid are the links between my joints? Plastic will wobble, metal is better
* How heavy is my arm and how does that limit its movement? If you go for stiff metal castings, you add weight you need to move. The lever arm relative to the base can get really long
* Motors are heavy! Ideally you can mount them towards the base, but then you need drive shafts or belts, which again add flex. (See KUKA arms which have motors 4, 5 and 6 on the elbow often)
* How much payload do you need to move? 5kg is already challenging in ~1m arms and if you need to move it fast the problem gets even bigger.
* Where do you run your cables? Internal is tricky to build, external can get you tangled.
And so on. When approaching this you get a totally new appreciation for biological arms which arae insane in most aspects except for repeatability. And on the software side you can enjoy inverse kinematics :)