I am working on a DIY project that requires me to drill and tap a hole in the end of a solid round 6061 aluminum bar 9/16 OD. What would be the best thread maybe 1/4-20 or 1/4-28? The bar will be about 3" long. If I drill and tap at least an inch I'm guessing that would be better than going shorter. Anybody in the Raleigh/Durham area have a lathe that can help a guy out?