I use a G-1550 lathe, an older version of the 9X20s. The plastic gear got set aside when I built a reversing mechanism for the lead screw. To maintain the "weak link" in a gear train is important in that if something jams up, a piece of plastic will "give" before something metal. I do generally light machining with the 1550, often on synthetics such as plastic. For the most part, jamming something is not likely. I also use a 12X36 Craftsman (Atlas), which has no provision for a sacrifical link.
As a general overview, if the plastic gear is intact, leave it. If it's damaged, replace it with plastic. It's there for a reason. You must make the final call based on what you do. The plastic gear can possibly save some more costly part of the machine. If you are making a change to the train, such as mine, there may be no other option. Just do it. If the appropriate plastic gear had been available when I built the reversing toggle, I would have used it. Call it a "judgement" call.
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