I'm using the sweep command to create tubing in this assembly. This is only for visual. I make the tubing in 1 part, and each tube is a feature. I mate the part to the origin of the assembly, then create a 3d sketch and use spline to create the lines.  I then use sweep and click circular profile and use selection manager to select the open loop. I put in the diameter i want and its all good.

But if i use the thin feature command with a wall thickness of 1mm it will not complete the command. A while back, i was able to do this, but now it won't work.

I do have the line constrained with the ends of the spline on the face of the fitting and that first control line concentric with the fitting diameter. Other than that, there is no constraints.