Found an interesting video for a retracting threading tool

Seems like it would be easier to make a carriage disengage mechanism, assuming there's an existing lever for that. Not exactly the same thing, but would do 90% with 10% of the effort.

Or, as has been discussed elsewhere here, thread away from the chuck and avoid the entire drama.
 
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Very neat video. Not sure how one would failsafe the retraction mechanism. I'm going to keep doing it the old way, but at least I know there are options...
 
There's always more than one approach to solving a problem. I enjoy seeing different ways people have done it. Not commenting on how good or bad it is, just interesting.
 
This mechanism still requires disengaging the half nuts or, if threading metric on an Imperial lathe (or vice versa ) stopping the lathe, to avoid crashing into the chuck or shoulder. At 1000 rpm and 8 tpi, the carriage is moving at 2"/sec. The roughly .4" of retraction in the z direction will require a reaction time of less than .2 sec. in order to avoid a crash.
 
The mechanism is basically the same as is used to release a crossbow bolt or a rifle firing pin. Failure causes could be the ram sticking and failing to withdraw completely or the trigger mechanism sticking, delaying tripping the ram.

However,every time we use some automated process, we introduce a probability of failure. When machining, the failure modes can have disastrous consequences. I would feel safer with relying on this mechanical device than I do with my CNC mill.

I would wonder about rigidity as the mechanism must have enough clearance for free movement of the ram.
 
I wondered that as well, about the needed freedom of movement for the cutter to be retracted. Presumably because it's always loaded in the same direction, it "probably" isn't an issue.
 
You essentially are using a "sear" for all intents and purposes. Unless it was heat treated, I can see it becoming imprecise or worse.
 
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