The project is to make a worm gear set to drive an instrument, a telescope. I have settled on a worm gear design. I have a piece of Acme rod for the worm drive and am planning on cutting the gear with a tap. I am ready to cut the first test gear and after watching several videos I realized the effective diameter of the gear is not the raw outside diameter of the blank, but rather the resulting diameter after the tap as removed material sufficient to seat the threads. I am using a 1/2 inch drive and tap. The gear is 1/4 inch thick. I believe the tap will reduce the effective diameter of the gear blank as it cuts teeth. Does anyone have experience with designing this? Is there a reference?