im needing a worm gear for the dividing head iv cast so thought id have a go at hobbing a 72T gear with a 3/4 BSw 10TPI tap

after doing a lot of reading online and a lot of calculations I thought I had the method worked out and went with the following calcs (in imperial)

DP = Pi/(1/TPI) =PI*1 = 31.4159

Diameter = T+3/DP = 75/31.4159 = 2.38732414638

I double checked this by using a circular pitch of 10, to give a pitch diameter of 2.291. add 2x tooth depth (0.060?) = 2.411, seems close enough.

so I cut off a lump of bar and turned it to 2.387'' marked out and gashed at 5 degrees with a hacksaw for 72 teeth.

put it on the shaft of the dividing head and with the tap in a collet fired things up

turned nicely and pretty soon I had a nice set of teeth.

problem is there were 4 to many

so I turned the lump down and tried for a 60T, ended up with 66

so I tried for 45 - without gashing - got 44

so I then just kept pushing the blank in to see what would happen, ended up with 35

so before I cut up another bit of ally what am I doing wrong??