I have some 18-inch long 1-inch diameter ball screw shafts with dual-path ball screw "nuts." I also have some NEMA size 23 stepper motors I'd like to use to drive these ball screw shafts. About 0.65-inch end length of these long shafts is cut to a concentric 0.75-inch diameter, apparently for insertion into a support bearing. NEMA 23 size stepper shafts seem to be either 6mm or ~0.24-inch diameter. Shaft connector blocks add unwanted length, unwanted expense, and reduce heat rejection from the motor's shaft into the driven shaft, another unwanted effect. Heat rejection from stepper motors ultimately determines their duty-cycle electrical current input limit. Colder environments and better air flows enable higher driving currents. Similarly, well thermally-connected aluminum motor mounts with lots of surface area are able to reject higher heat flow rates which enable higher motor-driving currents than comparatively thermally-isolating plastic motor mounts, which would be the opposite polar extreme. Typical steel motor mounts are thermally somewhere between the best and worst. Most motor heat rejection discussions fail to consider dumping heat through the motor's shaft, yet that's a valid but minor consideration.

I've seen motor shaft couplings which include flexible synthetic rubber links. If both shafts are not concentric, flexible links allow much longer bearing service lives. But synthetic rubber is a lousy thermal conductor. Solid steel, brass and aluminum shaft couplers generally limit thermal conduction by their limited pressure contact areas onto both shafts more than they limit thermal conduction within the coupler.

Here's my simple, minimalist plan. Bore and tap a hole 90 degrees to the long screw's axis located within the 0.75-inch diameter end section. Then bore another hole about 1-inch deep into the long screw's axis starting from the 0.75-inch diameter end. Diameter of that concentric hole will be a light press fit to accept the stepper motor's shaft, which will have two flats ground on opposite sides against which two short Allen screws will press. These Allen screws will bind the two shafts together
yet be short enough to allow a flange-type 0.75 inch internal diameter bearing to support the long ball screw shaft axially and pick up single-direction thrust loads into the bearing support frame.

This configuration will be shorter, less expensive, and provide better motor heat rejection than traditional end-to-end shaft coupling configurations.

I keep learning by reading how others have found clever ways to design systems. I don't know if this suggestion rises to that standard. But it seems like it should be considered when coupling a small diameter shaft to a much larger diameter shaft.
John