This is a parametric solid coupling. The bores for each end can be set in the openscad file, and the grub screw size can also be set.It can be used to couple a stepper to another sized rod, or any sized rod to any other sized rod.Edit the solid-coupling-grub.scad file to set the bores of each end. Then print, I use 30% infill and 0.3mm layers. It slices well with SFACT, and Slic3r.Two example compiled objects are available, to couple a 6mm shaft to an 8mm shaft, and one to couple a stepper 5mm shaft to a 1/4" shaft.If the shaft sizes are big you may also want to shift the grub screw hole up or down using the setscrew_offset variable.The shaft holes have been good fits for me, as there is a builtin 0.1mm clearance, which you can increase if needed. This requires the myLibs.scad library also attached, that just contains some useful things like a well sized hole etc.