What is adjustable die?
Adjustable dies-Form A (split) is adjusted either by an integrated screw or by screws on the die holder (termed a "die stock").
Dies with a integrated screw is arranged to work axially, where the movement of the adjusting screw into a threaded hole in the die forces the slit section of the die open, or tangentially where a screw threaded in to one side of the slit bears against the opposite side of the slit.
Dies without integrated screws are adjusted inside the die stock by radially-arranged screws. Two screws in the stock bear into holes on either side of the slit, tending to squeeze the slit closed, whilst a third screw with a tapered tip screws in to the slit forcing it open. Working these three screws against each other adjusts the die.
What is the advantage of the adjustable die?
They allow for cutting a slightly smaller or bigger size, suit for the non-standard sizes or the screws which used on special parts. Also the adjustment happens when the dies were worn slightly.