How does one determine the center of a worm and a worm wheel? Also, what are the differences between the common worm tooth forms?

While external involute gears are very tolerant of center distance variations, what are the center distance constraints for internal gears?

With the right selection of nonstandard center distance and tool shifting, it may be possible to use standard tools to improve the gear set capacity with a considerable reduction in cost when compared to the use of special tools.