Step motor current

I have my Grizley MiniMill working. I've taken some photos, will post
them to my web site when I get a chance.

For motors I used some NEMA 23 motors about 4" long that I found
surplus- the only speck I have is that they cost $15. They have no label
or makers name. They are the same size as the ones usually listed at
150-180 oz-in. They are 200 steps/rotation, and measure 2.4 ohm per
coil- ignoring the center tap.

I have a slo-syn 430-th driver module, selected because it was $10 at
the MIT flea market. It is capable of 3.5A, so I started runing the
motors with it at full current, but the motors seem to get very hot-
after 20-30 min they were to hot to hold onto. I lowered the current to
about 3 amps and the motors sounded smoother, still run as fast as my
program will drive them, but still get very warm.

So, how do I select a current for a motor when I dont have a spec?

I also note that at certain speeds, while the motor is rampning up to a
long move, the belts will vibrate a lot. They seem to steady out when it
reaches steady speed. I have measured and I am not loosing steps, so I
dont think its a motor resonance problem. Should I worry about this
vibratiopn?

ron