Optios on the Key.
One is cut a smaller key in the gear, example, if origami was 1/8 wide and tall, make it only 1/16 tall.
Take your standard key stock and mill it shorter or thinner.
Second option is using the lathe as a shaper/slotter, place gear in chuck and make a cutter that will cut the keyway and place in in your tool post aligned like a drill would be in the tail stock.
Now position it to where it will cut the keyway if the carriage is moved towards the chuck, like a shaper would.
Fix it so adjustment of cross slide effects the depth so you move carriage I and out of the gear then move 0.001 and repeat.
You add a carriage stop so you only cut where the gear is not.
You could do both, with broach cut keyway but only do first cut so it is shallow.
Then if you want more key in part of the gear make the part away from the gear full depth.
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