Hi Rich, in my opinion the complexity of any pressurised system be it air or liquid is something most would not attempt.
Mounting an air cylinder above the head is a job in itself along with the controls and a compressor that cycles on an off frequently.
I'm currently working on a design that I think will be simple, that works with just a cam on top of the draw bar rotated with a small geared motor, and a windscreen wiper motor, that a friend of mine used to power a wheel chair for a disabled child, seems to have the power to do the job.
The motor driving the wheel chair was further geared down 1:5 with a chain drive, and a 12 volt battery (small car type) lasted for hours between charges so a small transformer will suffice for the needs I envisage.
A lot will depend on the number of tool changes for the work, but even a few repetition operations like drilling and tapping can lead to at least 4 different tools every time.
It just depends on how the design turn out, and air has not been completely ruled out.
One thing's for sure, when the mill arrives it will need to have a power draw bar fitted just to get a tool in the spindle.
I also looked at the solenoid that engages the gear on a starter motor Bendix drive, but that does take a few amps to apply the force, so the transformer would need to be quite big on that design unless a small 12 volt battery was used with a charger to keep it charged..
The starter solenoid has a short throw, and is available from any wreckers yard for a few bob....damaged or burnt out starters are almost throw away.........this could be mounted directly above the draw bar and would be simplicity in itself to push the 'bar down.
There probably won't be a need to have the full power of the solenoid for the push, so some form of current limiting using diodes in series would do the trick to apply just enough force without consuming too many amps.
Now, that option is starting to look good on the drawing board, and with less moving parts in the design..... I'm beginning to like it even more.
I think the force available from a solenoid of that size would be the equivalent to a small air cylinder with 500 PSI in it.
A lot would depend on if the solenoid got hot from frequent switching, but I don't think the frequency would exceed once every 5 minutes between too changes, and it has to be held on while the tools are changed, but if current limiting is used the heat may not be a problem.
Ian.