Update: I have settled on an arrangement for the ATC and completed the preliminary design.
So far, I have built one prototype tool storage 'pod' (for lack of a better term) for testing and while it works as expected, there *might* be a tendency for the release mechanism to jamb in the long term, so I have made some changes, adding to reliability but also to the cost, as is usually the case.
After reviewing a lot of existing ATC configurations, both DIY and commercial, I chose to not move the entire tool storage device and instead use a single swing arm to transfer tools from storage to the spindle. here are some design goals which are met with the design thus far:
1) safety - accommodate sensors at each step in the process for monitoring purposes.
2) configurable - choice of 6 tool turret or unlimited number of tools via chain belt
3) self contained - optionally the ATC (with turret only) can be mounted entirely on the mill head (credit to Hood for this goal),
4) Tool tapers are completely covered and protected by the tool storage 'pods'.
5) positive mechanical retention of the tools in the storage device - no reliance on material spring - i.e. nylon 'fingers' or equiv.
6) tool release from storage pod is initiated by the swing arm 'claw', and then only after the claw has firm control of the tool.
7) fast - have not quantified this yet, but definitely not 10 to 20 seconds as typical DIY changers require.
Feedback always appreciated, especially if you have a favorite ATC arrangement and can cite the advantages. I will post a photo of the prototype storage pod a little later today perhaps.