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No, right hand thread .. so the force of the cut tightens the chuck, or at least doesn't loosen it.
In fact as far as I'm aware, there should be a short, very accurately machined shoulder at the headstock end of the thread, and a matching perfectly flat, square face on the end .. to precisely locate the chuck on, and square to, the lathe spindle axis.
The thread, although accurate, does not precisely locate the chuck, it merely draws these features together.
My thinking is that machine tools are dense lumps of metal with a very high thermal mass.
If they ever get cold, they take ages to fully warm through and, in the meantime, the condensation just drips off them.
Maybe your machine has experienced such conditions in its past and now has a little corrosion in the threads ....?
The chucks on my Southbend come off with a sharp bash with the flat of my hand on the chuck key, having first locked the spindle gearing ... easy peasy.
I would therefore be a tad wary of using much force .. at least at first.
On the other hand, if you do resort to a bit more welly, it occurred to me that you could grip some sort of bar-and-lever arrangement in the chuck itself, rather than do it via the chuck key .....?
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