It turned out that my idea of how to make the upper platform (Edit: Not the same as a Stewart platform, I have learned. All other instances where that name was used incorrectly has been changed to just "upper platform. Thanks Anrion for pointing that out.), was not so thought through. It's more complicated than it needs to be. I wanted the same tapering connection post for the swivel ball joints, on the upper platform as on the pivot arms. The taper chamfer is there to ensure that the rod end can swivel freely on its ball hinge without hitting the connecting point on the platform or pivot arms. When I choose to make the upper platform in a concave triangular shape, I on the same time made life difficult for me as I initially wanted the upper platform to be machined in one piece including the tapering posts. A concave triangular shape has absolutely no perpendicular flat faces for the part to rest on if I planned to clamp it down in the milling machine to machine the tapering posts directly on the platform piece. So to save me unnecessary headache I "reused" the connecting bar from the pivot arms and added two countersunk screw holes that goes through the cylinder on its radius axis.
I need to know at what diameter (measured from the center of the platform) the arm connection is at, or better yet, design it so the connection happens at a set diameter. As I was abandoning the solid piece upper platform for a simpler version with connecting bars screwed to the outer edges of the platform, I needed to make sure that the bars were parallel to the platform surface and tangential to the theoretical circle that touches the spots where the three arm connections needs to be. Therefore I used the bars unaltered with a full cylindrical shape and made a matching puzzle piece shape for it to sit in with two screws fixating it in its cradle. The holes in this picture are untapped, because I tapped those holes by letting a tap screw through the already tapped holes in the bar to make the resulting thread be continuous through both the bar and the platform. The bars were then screwed down with stainless M4 torx-headed screws and locktite.
Since I had already made one over-complicated swiveling Z-spring anchor with a thrust-bearing on the base, it was easy to make a similar one on the platform.
Now most mechanical parts are machined. A few minor adjustments left to do before I can do a final assembly. I need to add tapped holes for set screws in the torsion spring anchors on the base. I also decided to bead blast all aluminium parts to make them less reflective and shiny. As you see in the pictures most of the parts are bead blasted already. But I was foolish enough to mount the bearings in the bearing blocks before I had time to bead blast them. I don't want to get blasting sand inside the bearings, so I need to figure something out regarding that. I also need to start thinking/worrying about the electronics. I have neglected the electronic part up until now. I have the hall sensors I ordered as free samples from AMS, but I haven't made any pcb for them yet, nor any holder setting the size of the pcb. So there are much left to do before I can test fly anything with my delta throttle. I saw that I have a few duplicates or slightly damaged "spare" parts left over from this build. It would be a waste to just throw them so I might finish the abandoned actobotics sized base and make it mechanically functional even though it's not going to be that pleasing to the eye as this one.