There is an area in Sweden where very many of the stones have a natural hole through the centre (easier to tie the string). I have no idea of the postage cost or what the import duty may be but I have arranged for one to be sent to me in the UK for a trial - I will let you know if it works.
Tweakie. Success consists of going from failure to failure without loss of enthusiasm.
I read in one of the posts where someone was attempting to design a spindle that could be rapidly lowered and retracted to allow some sort of half-toning for images. i cannot find that thread but I have a suggestion. You are not doubt familiar with solenoids and that when power is applied to the coil, the moving iron core centers itself in the coil. The same action happens in an electric motor, the rotor tends to center in the coil. So you can emulate a solenoid action by, perhaps, using a spring to retract the rotor until you energize the motor at which time the rotor rotates and pulls down into the stator (coil). You could build a rotor that had two cores which would allow it to be driven up and down in both directions. Mike
It may be possible to use a pair of the electric motors they use for model aircraft. The most powerful motors develop as much as 5 horsepower (or over 3,500 watts). Unfortunately, they will have to be disassembled to make this work and would probably require cooling since they are normally cooled by airflow from the propeller. Perhaps one could make up a fan that would mount on the shaft and blow cooling air over the motors and at the same time, blow the swarf away from the workpiece.