I'm trying to run a SPS-15RF stepper motor from an old cd rom drive. I'm using a CNC shield and a4988 driver with a 6v power supply. The motion is smooth and fine. I know it has 18 pulse/revolution and works with 5v but It gets quite unexpectedly hot. It feels that it can burn my fingers if I keep it running. What is wrong with it? is it related to micro-stepping or sth else!?