I made a similar crank when a neighbor asked me to turn a 1/2-20 left hand thread . If I recall I used a wedge type of anchor bolt for the expanding piece.
This type of bolt is used for anchoring to concrete. First thing I had to do was make a banjo to hold an extra gear so I could turn left hand threads.
The 20x22 lathe I have does not turn left hand threads without a modification.
The crank was used two or three times and sits in a tool box for now.
mike
I made a similar one as well for having better control of where the thread ends - works great. I make sure to unplug the lathe when using this - you only accidentally leave it in place once before you realize you never want to forget it again.
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