These are the pictures of cutting worm gears on a lathe using a high-spiral tap. 
The taps used were a 3/8-16 and an 8mm x 1.25. The gear blanks were made of
half-hard brass. The O.D. of all gear blanks was .995. Pictures are described below:
Worm gear blanks- self explanatory
Worm gear fixture 1- The gear holding fixture parts
Worm gear fixture 2- The fixture assembled
Worm gear blank on fixture- What it says
Worm gear setup- Setting gear blank on center using a wiggler in the chuck
Worm gear cutting 1- One view of the gear being cut
Worm gear cutting 2- Another view of the gear being cut
Worm gear cutting 8mm- The gear being cut with the 8mm x 1.25 tap
Worm gear finished- Three worm gears as they look before being deburred and chamfered.
The gear on the left has finer teeth. It was cut with the 8mm tap
The fixture was made from a 1/2" square tool shank. A hole was drilled and tapped. 
Then counterbored to .3125. A piece of .3125 drill rod was drilled .250 through and
used as the axle. It located in the counterbore. When the tap first touches the gear
blanks and starts to cut it will double track. So there will be the start of two teeth
right next to each other. As the cut deepens this goes away. The tap was fed into the
blank a little at a time and the blank was allowed to make at least one revolution each
time the tap was fed into the work deeper.
Eric R Snow