About a year ago, I purchased a used South-Bend heavy 10 lathe. This opened up a whole new world of possibilities in my shop, and provided a means to make other tools and adapters (like the one in the photo above).
The lathe came equipped with a number of attachments and accessories, not the least of which were a pair of 6" chucks (3 and 4 jaw). The lathe is also fitted with a quick change gearbox, allowing the operator to cut a variety of different thread pitch/sizes. Because I wanted to be able to fit the 6" chucks to other bits of machinery (like a diy dividing head, described elsewhere on this site), a suitable adapter was produced. The adapter shown above, is threaded outside to fit both of my 3 and 4 jaw chucks. The inside is a threaded bore that screws onto the output shaft of the gear reduction drive.
Because I built the adapter some time ago, specifically to fit a surplus gear box output shaft, listing any dimensions here would be pointless (plus, I simply have forgotten them). But the concept is simple, and could be altered/modified to fit specific needs...
Thanks for reading, and I hope you find this useful!