It was necessary to drill my coupler out to fit a new stepper motor with a larger shaft.
I wound up with a good solution to the coupler slipping.
I drilled out the coupler on a drill press at the top and bottom, beside each original set screw
and tapped it like the makerslide,
and inserted M5-.80 x 10 hex head machine screws from the home improvement store.
it tightens up much better now, even with the oversized hole for a larger shaft