This is mitering for a basic 90 degree joint with 3/8″ tubing.
The tube directly after cutting with a hacksaw. It’s a little messy:
Make a notch in the centerline of the tubing using a file or hacksaw. This keeps the file centered in the next step:
File. A handy hint is that a 12″ Nicholson bastard file has a 3/8″ diameter, so it makes a nice miter into 3/8″ tubing. A 10″ file works for 5/16″, a 8″ for 1/4″ tubing.
Once filed you have a miter, but it’s pretty ugly:
Clean up the inside of the miter with production cloth wrapped around a piece of tubing:
Then clean up the outside with production cloth:
Check the miter. It looks pretty good to me: