Why do worm gears need multi-head design?

The first thing to note is that "snail" sometimes chooses bulls instead of worm wheels! Selection principle: 1. If the reduction ratio is too small, resulting in too few teeth of the worm wheel, when "undercutting" occurs in the machining process, "multi-head worm" can be selected to increase the teeth of the worm wheel to avoid undercutting. 2. Sometimes, for high-power transmission, in order to "improve efficiency", multi-head worm (increase spiral angle) transmission will be selected, and the number of heads is generally 2-4 for reference!