What are the stepper motor specs, what power supply and controller have you got, do the motors get hot in use? There are probably other questions that will need an answer but we need to start somewhere.

Martin

I've a Novamill with original motors that must be the same vintage. I've fitted modern micro step drivers, with a 36 volt PSU, and it works fine. The motors are size 23 on X and y and 34 on z. New steppers are quite cheap and modern drives at a higher voltage probably work much better. You could try new drivers, maybe just on the z axis first and a higher voltage. If the mill is anything like mine replacing the z stepper motor is a real pain, so worth finding an alternative if you can. I have read that old steppers lose magnetism, but as I say mine seem fine.

If there is a lot of heat build up then the magnets may be degrading. I think I have seen 70° as the maximum working temperature for some stepper motors. This will be the temperature at which either the insulation starts to degrade or the magnets suffer or any adhesive used in the assembly fails. If the step and direction inputs are easy to connect to I would think about replacing the motor and its power supply. You could also consider a hybrid servo stepper as they can be used like an ordinary stepper motor but can give an error signal if steps are lost. The Leadshine three phase ones give a good torque in a small package but unless you can change the value in the software you need to match the steps per rev to what you already have.

Martin