Ultimaker forum user by the name of FoehnSturm presented his approach to toolchanger mechanism on the original Ultimaker machine.
It uses a custom modular direct drive extruders with hot end placed in holders mounted on printer's frame. This extruder is powered by NEMA 8 motor and has a hotend like Merlin, Prometheus, E3D or similar. In the future it will use other tools also.
This system can hack your Ultimaker in true multitool hybrid digital fabricator.
Do keep in mind that this project is still in the development but it looks very interesting. It could be also adapted to other 3d printers. My opinion was always that the multitool machines are the future.
Here is the first prototype:
... more developed version with higher accuracy and repeatability:
Here is the mechanism working well with dual extruders doing two color printing:
Here is the high speed tool change in action:
The mechanism can have magnetic connectors and supports different tools like polishing head:
You can see the entire thread on the UM forum with much more information and insight in the development process:
You can see some of the files on Youmagine. Here is the one for the modular Ultimaker printhead that uses a direct extruder:
... here are some of the files for the magnetic exchangers: