When Cecil, my mill-drill, is not set up for milling the spindle is mounted with the Albrecht chuck on an R8 arbor so that it can be used as a drill press on steroids.
When it is set up for milling an ER32 collet chuck is mounted. Handy for holding endmills but, if I want to drill a quick hole it's a pain to demount the collet chuck, mount the Albrecht, drill, and then reverse the procedure to get back to milling. The Albrecht chuck is a bit long so this generally means raising the mill head which means loss of registration on a round column mill.
To avoid this inconvenience I bought a nice 1/4" Jacobs chuck and made a 1/2" diameter straight arbor with a 3/8-24 thread to match the chuck. Most of the time the 1/2" collet is in the collet chuck so this arbor can be plugged in directly and holes drilled.
Wouldn't you just know that the first hole I needed to drill was 5/16"? So I acquired a 1/2" Jacobs chuck and made a 1/2" arbor with a 1/2"-20 thread. Now I can handle any drill up to 1/2".