Is there any way to determine the correct voltage for a stepper motor?
If you use a good driver circuit that has current limiting and chopper features, it really won't matter. Basically you can slowly crank up the current on these drivers while checking the stepper motor temperature and usually you can operate them at a voltage MUCH higher then they were intended with the right circuit. I am currently in a CNC build project and I am about to do that very thing. You can follow me on my Blog. WWW.MENDINGTHINGS.COM

Good luck.