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.
Step 2: Start Lathe-ing!
Hold the hacksaw blade on the point you wish to cut.
The hacksaw blade's teeth will need to be facing into the feed or rotation direction. The vertical - not angled - face of the teeth should face into the tube material as it is rotating in order to cut.
Start the drill slowly to get a line or indentation that will help "hold" the blade in place on the tube before getting all Western and pulling the trigger fully.
Step 3: Cut.
Run the drill at a moderate speed. The hacksaw is not considered a "high-speed" cutting tool. Keeping it slow will also allow you better control. Once a line is established you should only need to control the drill speed and keep the hacksaw blade straight and level. There should be little need to bear down and put pressure on the blade, it should dig in itself. Move the blade back and forth slowly so the cutting is done along the blade, not in one area. Allow the tools to do the work!