I used my 10" radial arm saw equipped with an 80 tooth carbide tipped metal cutting blade to rough out the
     gear blank. I then turned and bored the blank to the needed dimensions on the 9x lathe.
     The blank for my 180 tooth gear has an OD of 5.865 ( Pitch diameter is 5.729")
 
 

        This is the tricky part....... I'm not going to go into much detail because I don't know that much about it.
        Possibly someone with some experience in this area can help correct me.
        These calculations are just from my experience hobbing gears which I will remind you is very little.
        There are gear calculators available such as WM Bergs GearSpec V.1.3 ,  This is a free download
        that you can download here.   With this program you can calculate the worm gear dimensions if you
        know the pitch of the worm to be used. This is a problem since Diametral Pitch and TPI ( threads per
        inch ) are not the same. However from what I have come up with a Diametral Pitch of 1/10 worm thread
        it very close to 10 tpi. ( Actually works out that a 1/10 Diametral Pitch is approx 10.2 tpi ) Someone correct
        me if I am wrong here.  It also depends on the pressure angle of the worm thread.
        A 20 tpi bolt thread is very close to a 64 pitch gear ( works out to 20.4 tpi )
        So to sum it up the GearSpec program will not get you exact dimensions unless you use a standard
        pitched worm such as a 16 , 32 , 48 , 64 ,ect Diametral Pitch.  To purchase the worms can get expensive
        and you would have to purchase an extra one to make a cutter.