Things to keep in mind: DRV8825 boards were configured for Vref = 1.6-1.7 V, which is way too high. This translates into output current of 3.2-3.4 A, as the formula for DRV8825 is
I_TripMax = Vref / (5 * Rs)
Or, as Rs is 0.1 Ohm (resistors are labeled as "R100") -
I_TripMax = Vref / 0.5
which is the same as
I_TripMax = 2 * Vref
See these links -
http://reprap.org/wiki/A4988_vs_DRV8825_Chinese_Stepper_Driver_Boards
https://www.pololu.com/product/2133
This current can damage your motors, so before connecting the motors to RAMPS you must adjust Vref. Thankfully it's not that hard: plug in 12 V power supply into RAMPS (Arduino is powered from RAMPS board), and measure Vref between the ground pin of DRV8825 board and top of adjustment potentiometer, as shown:
I have soldered a wire to the ground pin and wound it onto the ground probe of my multimeter because I only have two hands. With the red probe touching my screwdriver that I used to turn the potentiometer I was able to adjust Vref and check its value on the multimeter.
The first link recommends setting Vref to 0.5 V (which translates into 1 A current) and using that as the starting point to further tweak Vref. However when using puny CD / DVD stepper motors, I think 1 A is waay too high, so I started with ~0.1 V, and went up to 0.16 V. With this Vref my motor works just fine and doesn't seem to overheat. I may end up increasing Vref when I add mechanical load to the motor though.
