With a grinding machine, there's how long the stroke is going back and forth, where it strokes and how many strokes it makes to the rotation of the turntable. Since you are running the overarm off of the main motor, you really don't have much control of the ratio of strokes but you do have the ability to set how far a stroke is (what the radius is on the stroke crank) and where on the turntable it will stroke back and forth (the turnbuckle that sets the length of the rod on the crank to the arm). Those adjustments will set how fast the ROC changes and where the polishing gets done. PS, don't forget to allow for the upper piece to rotate freely or you will get grooves in the work.