Would be worth trying to up the spindle speed if you do it otherwise it could take a long time to run any code

Depends what the bearings can take and also what condition they are in. Also what type of things are you thinking of machining? non ferrous materials ideally want a higher spindle speed that steel/iron and fine detailed small parts will need smaller diameter cutters which again work best at a decent speed. I would say 5000min and 7500 if you can get it even then you won't be upto optimum speed for small non ferrous work.

Would almost be easier to fit a high speed spindle on the end of the tubular arm and junk the head, motor and belts.

I have converted a couple of mills to CNC.

Nema 34 is probably the size to use, but you need a high torque motor as they have to hold the work as well as move it.

I would get ball screws and fit them.

I would drive the 3 axis indirectly, with a timing belt on pulleys between the motors and the lead screws.

My first conversion had a top speed of about 1500 rpm, and to be honest this was perfectly ok with hss tools.

To operate I recommend Mach3 or mach4.

For the saddle I suggest you rig up a counter balance to take the weight off. A spring or weight over a pulley would do the trick. Maybe even an old car spring?

Good luck. It looks a useful mill.

John

No, something to counter the weight of the table. A lot of the mills with moving heads have a gas strut or spring so the motor does not have to lift all the weight. Your table is quite likely to be heavier than a typical mill head so need more counterbalance.