I had a milled carbon steel tube that I wanted to modify by cutting a "cone" off the end, but wasn't sure how to make an accurate, square cut as I could not use a tubing cutter. I also did not want to pay someone to cut the cone. My "on-the-fly" method using a hacksaw and cordless drill worked surprisingly well and quickly, with very little flash and few burs.
Step 1: Chuck It Up!
Find "something" to use to chuck your tube/pipe/etc. in your drill.
In my case, a 7/16-inch socket with a hex base made for hand drills had the perfect exterior diameter for a really tight friction fit with the interior diameter of the tube.
Sorry for the mixed-up photo timeline. I really came up with this quickly and was surprised it worked as well as it did and I didn't think to take pics until well into the project.