The heat that steppers emit is to do with the voltage you are running them on, I have a large router that has a 90 volt AC power supply and has a ton of power and speed, the steppers get to over 150F, too hot to touch, bad enough that I added water cooling jackets to them, cooled off the spindle cooler. I like the power it has, so don't want to compromise motor life for speed. Yet, my lathe has a 42 volt power supply and the motors on that get barely warm, almost cool to the touch, but it isn't very fast either....