Stepper motors do tend to run hot. They are kept under power regardless if they are moving or still. They can easily reach 80°C (176°F) or higher.
For example, one of our 305 oz in motors is rated at 3 amps into 1.4 ohms per coil. Power = I2R, or 32 * 1.4 = 12.6 Watts per coil, 4 coils = 50.4 Watts. That’s a LOT of heat to get rid of.
Any heatsink you can mount the motor to will help. Try a 6 x 6 inch square of aluminum. mount this so it is between the motor and the mill, in intimate contact with the motor. This should help quite a bit. Naturally, the larger the heat sink surface area, the better. Some users add a small fan like for a CPU chip on a PC. Any airflow will help.
I run my steppers without heatsinks and they run too hot to touch for more than a few seconds.
I did a quick-and-dirty test on my test setup, with a fan, the heatsink on the driver chips sat at 115°F (46.1°C). The motors (no heat sink) were 112°F (44.4°C). The system will run all-day-long at these temperatures. Naturally, the lower you can get them, the better!