If you have a dividing head, it would be very easy to make a worm gear. You would have to grind a fly cutter to cut the teeth. It won't be a prefect helix but would get you close enough to having "real" teeth engaging the worm instead of what you have now. I think I have a similar gear on the feed mechanism on my Index 645 mill. If it ever craps out, it will all get replaced with one of many servo drives for sale out there. My dad and I made one way back in time to replace a worn out one on a lathe apron we offered to rebuild for someone. We actually made a hob that we rigged up on the lathe to form the helix on the gear. It worked! Never had the opportunity ever again to make another. And no, I don't have the time to make one for you. Sorry, Ken