[kinetic (Steve)]

Ok, enough stalling, finally I tried an ACME shaped thread.

As I thought, it was all down to how well I could shape the
tool.
An M12 x 1.75 ACME tool is a fairly tiny trapezoid to grind
and the errors were soon showing on the result thread that I
cut with it.

It was obvious the flat was too wide because I couldn't go
full depth. The flat at the bottom should match the flat at the
top. As I neared final depth I was sacrificing wall thickness, the
very thing that makes ACME a strong driving thread.
Anyway, I decided to do another hob with it and see how it
went.
Another successful self index.
Then the depth cuts, gradually adding 2 thou each time and
letting it drive around a few turns.

Polished on the clean part for a while then bored the worm and
parted it off.
The last pic shows the ACME worm that can now be swapped
with the other sleeve to try it out on the DEC axis

Steve