Originally Posted by graphworlok
(So, a bit of round metal roughly 15mm diameter, 30mm long, with an (roughly) 8mm hole through the center, and two holes for grub screws through the side to fasten the two shafts in place)
Are you sure about the "roughly" specification?
In most applications of this nature (you do mention guiding as a goal)
you will want to achieve a transition fit between the shafts and the coupler.
Otherwise if the fit is a very loose clearance fit, the mating parts might pirouette
around a little when you tighten the set screws onto the respective shafts. This
will introduce guiding errors.
Ideally you would have the shafts to be mated nearby the machining operation
so that the machinist could check the fit. This may not always be practical if the
mating parts are firmly fastened to the rest of the mount and the assembly is too
large or awkward to bring over the bed of the lathe to check. In which case
the machining would be done on the bases of metrology, the operator would probably
initially undersize the hole by a tiny amount and then possibly use an adjustable
hand reamer to widen one end at a time a little at a time, the whole time continually
re-checking the fits for both respective shafts.
In a nutshell, the "roughly" specification might provide enough scope for disappointment
and might be an obstacle for someone putting up their hand to volunteer unless they
happen to live near you.