This step-by-step repair guide explains how to replace the worm gear in a stand mixer. Located inside the lower cover, the worm gear turns on the inner edge of the stationary planetary gear to spin the attachment. If the attachment won't turn, the worm gear teeth might be stripped (the teeth are designed to strip if the drive system binds up, to protect other gears and the motor from damage). Replace the worm gear with the manufacturer-approved replacement part if the worm gear teeth are worn or damaged.
These repair steps show how to replace the worm gear in KitchenAid and Whirlpool stand mixers.