Jerry: Why is a die adjustable?
When you're threading a piece of blank round stock, the die is expanded a few thousandths to make a rough cut, then tightened up a little at a time to sneak up on the final pitch diameter and smooth out the cut. Trying to do a full thread in one pass usually results in a crooked thread, or one that's undersized and rough.
Dies that are intended for repeatedly threading solid round stock are VERY adjustable, but they require the use of a special die stock to allow for the adjustment as a finish thread is completed.
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