Need some guide

i am facing problems about running my stepper motors . I am using CNC shield, nema 17 stepper motor (SL42STH34-1334A),servo motor (sg-90),power adapter of 12v,2amp .I set the vref on A4988 equals to 0.952 (using formula found somewhere on internet that is vref=(Imax8Rs*0.7)).I am not sure about this formula .i run my motor using a4988 making connection on bread board whis this set ref voltage.but when i used cnc shield only servo motors responds stepper motor did not respond. please guide me about my issue .by the way i am working on cnc plotter that will work on arduino uno,cnc shield ,servo motor .


The formula depends on the current sensing resistors on the board and we have two different versions of it. Can you post pictures that show the boards so we can see which version you are using?