i need to rotate cube in z axis or x axis one direction at time

i am rotating cube on touch , if i touch and move on left side then it should rotate from left side , and if drag my touch from right side cube should move from right side , cube is placed on (0,45,0 ) rotation , i want that y axis remain same 45 every time , but other axis should be change based on touch direction here is my code

1. 
   
   
   Code (CSharp):

   1.    
   2.  
   3.   if (touchPos.x < Screen.height / 2)
   4.      {
   5.     transform.Rotate(deltaPosition.y * rotSpeed, 0, 0,Space.Self);
   6.      }
   7.      else if (touchPos.x > Screen.height / 2)
   8.      {
   9.     transform.Rotate(0, 0, -deltaPosition.y * rotSpeed,Space.Self);
   10.     }
   11.  

it works on first side either on left or right , lets suppose i had rotated it from left side first then it changes on z and when z axis become 90 then next time if i will change on right side then it changes y axis and z axis too , if in first time left side will be zero then on right side if works fine , but if z goes to 90 the left side makes issue in rotation alt text


i want to rotate in only in left side or right side , and center should remain same like below