Voltage, Current, Resistance and Heat?

Hello,

Forgive my ignorance, I am learning all about the electronics on the
fly here.

My CNC Router - phase one, started with a Stepperworld Fet4 driver
board and the Sanyo Denki stepper motors provided by them. They are
old surplus, unipolar, 6 wire, 4.5V, 1.5 Amp, 160 Oz In motors. This
particular driver is an L/R driver. As such, I used the power
resistors as specked by Stepper world for these motors. The power
supply provided by them was a computer power supply, so I used the
12V side of that supply to drive the motors.

That board had a setting for "High Torque" which is full step and I
believe 2 phases on all the time. This setup provided good torque
and moderate speed. However, the motors, especially the Z axis (as
it is not moving much of the time) would get very hot. Almost too
hot to touch.

Recently, I changed out that driver board for the Hobby CNC driver
board which is a chopper driver. I set this unit up at 1/4 step and
it runs very smooth and I could achieve higher RPMS which is cool.
But, in playing around with it for the last 2 days, I noticed
another COOL thing, the motors barely get warm to the touch - they
run very cool.

After some tuning, I believe I have the torque pretty close on the
new board as the old board. But still, the motors run cooler.

This got me to thinking about something I read somewhere.

"Torque is directly related to current"

If this is the case, then, if I have the same current I should have
the same torque. Also, I believe the motors should run at the same
temperature.

So, I am wondering if I could run more current (Amps) into these
motors than what is given on the spec sheet. Eventually, I will be
running them at about 40 volts when I get the parts for the new
power supply. But, I am VERY tempted to crank up the current to see
if I can get more umph out of them. Now I know I am not suppose to
do that, but if the motors are running cool, I am wondering if I
would in fact hurt them by running up the current while keeping a
very close eye on motor temps.

Any thoughts on this?

Again, forgive my ignorance, I know lot's about software and CNC,
but very little about the electronics.

Chris