Hello, Paul!
There is an easy method of cutting internal keyways that does not require a broach.
Have you considered using your lathe as an internal shaper?
All it requires is a boring bar smaller than the hole in your gear.?
It only requires you to grind a tool bit the width of your intended keyway. Then center the boring bar in the bore. Then you cut the keyway slot with a back and forth motion until the keyway is to depth. Keyway Done! And no expense.
Dick
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OldToolmaker