uc400eth mach3 threading

[chessie]

i have a uc400etc on my mach3 converted lathe.
all has been good for a few months.
i have just decided to cut some threads and im having issues.
The spindle pick up makes the true rpm counter read ok .

the thread looks like its not starting at the same point all the time.
the mach3 threading pdf said it needs the spindle feedback in sync modes boxed checked.
is this correct? if i do this the lathe doesnt make the threading motion but does unchecked .
are there any other configuration settings that need to be input into mach and or the uc400eth to make good threads
thanks

[Spuds]

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