Last night I tried two old hard drive platters screwed together
to simulate a thicker test blank.
This will almost be the thickness of an actual blank if/ when
all of this works ok.
I also shaved about 2mm off the diameter to make a fresh
start point to attempt the self index.
Self indexing went beautiful this time.
The start met the finish. By pure luck, I suppose,
the circumference must have been just right.
I noted the diameter!
I gradually gave the cutter more depth until the valleys of the
thread had just the right looking amount of wall left between
them.
Once a nice looking depth was reached I just let it drive itself
around for a good half hour with some cutting compound to
smooth over the roughness.
After a bit of a polish up with fine wet and dry it looks like
quite a good serviceable worm gear!
Apart from the fact that this blank is made of two platters,
(you can see they don't quite sit flat together because of trapped
grit and burrs etc), I think this gives me confidence in
attempting my first real full thickness blank.
Steve