This is v2.

What’s new
I designed the full base assembly: outer shell, swivel bearing interface, and rotating deck. The waist joint (J1) driven by a 2:1 herringbone gear reduction off a STS3215. After research, I chose to use these motors with a 345:1 @12V on each, up to four motors total: one for the yaw, 2 powering the shoulder linkage (J2), and another for an eventual 4th DOF (J4) on the toolhead. This tiny motor already has a built in 12-bit encoder, which eliminates the need for an additional electronic part, aka additional point of failure.

Nailed down the project’s actual design goal (see below — this matters more than it sounds)
Started thinking seriously about tool-head modularity for the business end of the arm
The base / waist joint

I decided on herringbone gears since they self-cancels axial thrust; a straight spur or single-helical gear at this scale wants to walk sideways under load, herringbone doesn’t (that much). Plus, I’m 3D printing it, so machining manufacturing isn’t a concern (yet). I also use a deep groove ball bearing on the outside for smooth rotation and a thrust roller bearing on top to reduce my axial loads. Initially, I was planning to use opposing tapered roller bearing, but this approach is cheaper (and lighter..).

For the 2:1 ratio, my main motivator was to get the least strain (and heat) on the only tiny motor carrying the weight of the entire robot (excluding payload). Using torque calculations, I realized that the 2:1 might be unnecessary and slow, so I already envision converting to a 1:1 or direct drive mechanism after seeing the arm in action. In the next iteration, I’ll tackle the modularity of the bearing base to be able to use an assortment of different sized bearings and sprockets, making it easier for anybody wanting to give the build a shot. My only concern is that this joint could be a silent generator of slop, which can hopefully be mitigated by the servo encoder.

While most hobby arm projects chase 6 DOF because that’s what “real” robots have, I’m deliberately not doing that. It’s kind of overdone/ overrated, but it still amazingly effective. This robot has a different purpose.

The actual goal: a cheap, consistently precise 3-DOF arm waist, shoulder, elbow, amazing to use by itself but increasingly more capable with added attachments/ accessories.. a bit like an Ender 3..lol. That’s the basis of the palletizing topology this guy was inspired by in the first place; those robots don’t need a wrist full of DOF to do useful, precise, and repeatable work, and one robot can be adjusted/ modified to perform a plethora of tasks. Precision, repeatability, and real world practicality is the actual metric I’m chasing, not DOF count. In my eyes, I don’t want to build this guy and be done with him, I want to get and learn the most of the robot, especially since he’ll be mostly plastic.

For modularity, the tool end is the extensibility point

Instead of building DOF into the arm, the plan is to build capability into swappable tool heads at the end effector. Same 3-DOF base arm, different job depending on what’s connected on.
Next up I’m planning to..
Streamline the tool-head mounting standard design (connection pattern + electrical/pneumatic passthrough).
Print the base and validate the herringbone mesh + swivel fit in (preferably in) PETG
Get all 3 servos on the bus talking together and do a first coordinated move. There’s also some more stuff, but I can’t fit it all here lol.. until next time.