The Problem
So what's the big deal with arms anyway? I thought ball joints were the gold standard for articulation?
Well, yes, but a single ball joint can only do so much. The socket it's in limits its range, and there's only so much it can do without popping out and looking strange.
This is a problem I saw most clearly when I made a mistake with my first mermaid's arms.
It looks weird, yes, but even freed from a socket, the forearm still can't move that much. This is where double-jointing comes in.
Double Joints
The most basic description of a ball joint is that you have 1. a ball and 2. a socket. Thys, the ideal articulation is a perfect sphere with a perfect socket that can rotate all the way around it. This isn't possible with a strung joint, but keep it in mind
The way I've been making joints so far is that one section of limb has a ball and the end, and the matching one has a socket. This naturally limits the rotation because no limb is naturally a perfect sphere.
So... What if we did put a sphere in there? And made each limb end in a socket? That's the principle of double ball joints.
Shoulders
My first thought for the shoulders was just to use a cup shape, so not to disrupt the arm silhouette. The way I'm printing these leaves most peices a bit chunky, especially after paint and varnish.
You don't get the full benefit of the ball joint, but I thought it would be enough. In retrospect, I think I should have leaned into the chunkiness and chosen a ball joint anyway, because I'm not sure how much the shoulder cups are giving to the arms at all.
That's for my next project though. My upgraded mermaid works out well anough with these joints, by the most rigorous metric of doll articulation.
Their ability to JoJo pose!
