Its a great project for the electronic enthuziast and so.

I have look every were and never encountered nothing semi/controled and i had to do it myself.  This design ist was inspired from the user Irondmax from Thingiverse.

PARTS NEEDED-

arduino control board i have used LEONARDO but any board will do the job 

push on/off switch and conected to power + 12v of the nema 

poloulu controler stick

100 k  potenciometer to graduade the velocity of the stepper motor

100mf 60v capacitor

protoboard

prototipe wires or any 

THE CODE IS THIS

/*  Simple Stepper Motor coil winding  by Nelujones  */

// Defines pins numbers const int stepPin = 3; const int dirPin = 4;  int customDelay,customDelayMapped; // Defines variables   void setup() {   // Sets the two pins as Outputs   pinMode(stepPin,OUTPUT);   pinMode(dirPin,OUTPUT);     digitalWrite(dirPin,HIGH); //Enables the motor to move in a particular direction } void loop() {      customDelayMapped = speedUp(); // Gets custom delay values from the custom speedUp function   // Makes pules with custom delay, depending on the Potentiometer, from which the speed of the motor depends   digitalWrite(stepPin, HIGH);   delayMicroseconds(customDelayMapped);   digitalWrite(stepPin, LOW);   delayMicroseconds(customDelayMapped); } // Function for reading the Potentiometer int speedUp() {   int customDelay = analogRead(A0); // Reads the potentiometer   int newCustom = map(customDelay, 0, 1023, 300,4000); // Convrests the read values of the potentiometer from 0 to 1023 into desireded delay values (300 to 4000)   return newCustom;   }