I have a solution! Thanks for the model. I've attached an image with some pointers and will describe what I did in detail. I'll also email you a modified model with the two pins in place.
The reason you could not add a joint between the pin and the outer tube was because the pin's axis was at a slight angle relative to the hole in the outer tube. I assumed that this hole is where you wanted the pin to be joined. The slight angle was fixed in place by the fact that both the inner and outer tubes were grounded and the inner tube was slightly rotated before being grounded. This meant that there was no way to align the pin's axis with the hole axis. I discovered this slight angle by temporarily adding work axes to the pin and the outer tube hole cylinders and measuring the angle. It was .2 degrees.
I believe you grounded the inner tube to keep it in place as joints were added. When you grounded the tube, a snapshot was created to capture the inner tube's position. This snapshot is needed to make sure that what you see when the ground is added is reproduced by the history when edits are made. You can always revert the current uncaptured positions before adding an operation, like "ground", which is dependent on the uncaptured positions.
Below are the steps I followed with lots of technical detail. I hope this detail is helpful rather than confusing. Fusion is unique in how it adds kinematic constraints (joints) and positions to a modeling history. This is a powerful way to do top-down designs where operations are dependent on captured positions driven by joints. It also allows you to change history and have the model adjust parametrically. This capability takes some getting-used to, and we certainly need to continue working on making it easy to understand.
- I resolved the fixed angle issue by deleting the snapshot and ground features from history. This is enough to provide the freedom to add the pin-outer-tube cylindrical joint. However, I found that adding this joint causes other components to move because of all the interrelated parts. The inner tube slides out of the outer tube. The result is technically correct, but very inconvenient. My guess is that you ran into this and grounded the inner tube, accidentally capturing a slight rotation.
- To avoid this inconvenient movement, I reset the inner-outer-tube cylindrical joint's position to "home" by selecting and right-clicking on Cyl21 and choosing "Go to home position". This sets the joint's values to zero (both slide and rotation). I then locked the rotation of Cyl21. It turns out that this is enough to keep parts in convenient positions as more joints are added. This Cyl21 never needs to rotate anyway. The pins to be added will prevent this rotation. This can be unlocked later. It is always good to minimize redundant constraints on movement, so unlocking it later is a good idea. There are alternative ways to prevent the unwanted movement temporarily. I think I could have grounded the inner tube after setting the Cyl21 to home to remove the slight rotation that was getting in the way. Then I could unground it later. Maybe that is what you were doing but you accidentally captured the slight rotation of the inner tube before it was grounded.
- I then added the cylindrical between the pin and the hole in the outer tube. Since I had moved components and locked a joint before adding this cylindrical in step #2, Fusion prompts to either "revert" these uncaptured changes or to "continue" by automatically saving the changes in a Snapshot. Alternatively, before starting the joint command, I could have explicitly saved the snapshot by clicking on the Snapshot command at the end of the toolbar before starting the joint command. This Snapshot commands appear when there are kinematic changes that you might want to capture in history or revert. As a convenience, Fusion will prompt to either capture or revert kinematic changes when you start a command that could be affected by these changes. Capturing or reverting ensures that what you see as you do modeling operations is what you get when history re-computes the result.
- To constrain the second pin that you copied, I first tried to add the same pin-slot and cylindricals that were added to the first pin. I found this locked up the fourbar. I'm not exactly sure why, but the likely reason is that these joints add lots of redundant constraints to the kinematic system (always good to avoid). In the real world, there is always some minor slop that accomodates redundant constraints imposed by contact between physical parts. Ideally, a model should be perfect enough to allow redundant constraints without slop, but if there are too many redundancies, minor within-tolerance deviations can add up and lock up the system. The solution is to choose joints that fulfill the desired kinematics without adding the redundancy. In this case, a simple revolute between the pin and the hole in the outer tube is sufficient. The fourbar linkage will guarantee that the pin will stay in the link slot. I wanted the pin to be centered between the two walls of the outer tube slot, so I used a little-known capability to specify a position between planes for snapping. You can use the "Between" option to create a "Joint Origin" in the right place before adding the joint, or you can right-click in the view and select "Between" during the joint creation (a hidden short-cut). The first pin did not need to centered because the pin-slot you created results in a centered pin.
So this is a lot of detail to digest! We should make Fusion provide more help when adding a joint fails. This is on our "to do" list. The key is to find what is preventing the alignment that the joint type requires. In this case, the cylindrical joint needs to align two axes to be colinear. The grounded, slightly rotated inner tube and the grounded outer tube prevented this alignment. The solution, ultimately, was to remove the slight rotation of the inner tube while maintaining the temporary constraints on the system that help in the assembly process.
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